BIOLOGY 
UBRARV 

G 


AN    INTRODUCTION    TO    ZOOLOGY 


AN     INTRODUCTION     TO 

ZOOLOGY 


BY 

C.   H.  O'DONOGHUE,    D.Sc.,    F.Z.S. 
If  • 

PROFESSOR   OF  ZOOLOGY,    UNIVERSITY   OF   MANITOBA;    FELLOW   OK  UNIVERSITY 

COLLEGE,   LONDON  ;   ASSOCIATE  OF   KING'S   COLLEGE,    LONDON  ;    FORMERI.Y 

BEIT   MEMORIAL   FELLOW    FOR   MEDICAL   RESEARCH, 

UNIVERSITY    COLLEGE,    LONDON 


D.    APPLETON    AND    COMPANY 
NEW  YORK  MCMXXI 


03T 

BIOLOGY 

UBRARY 

G 


PREFACE 

THE  object  of  this  volume  is  to  provide  a  text-book  for  the  zoological 
portion  of  the  syllabus  in  Biology  for  the  First  Examination  for 
Medical  Degrees  of  the  University  of  London,  and  the  First  Examina- 
tion for  the  Conjoint  Examining  Board  in  England  of  the  Royal 
College  of  Physicians  of  London  and  the  Royal  College  of  Surgeons 
of  England.  Some  years'  experience  in  preparing  fairly  large  classes 
of  students  for  the  above  examinations  made  quite  clear  the  need 
for  such  a  text  in  spite  of  the  text-books  of  Zoology  available.  It 
is  based  on  the  lectures  then  given,  and  its  method  of  treatment  is 
that  found  to  suit  the  needs  of  the  students.  It  is  hoped  that  it  will 
also  prove  useful  to  students  preparing  for  similar  examinations  of 
other  British  Universities,  and  for  those  who  are  taking  classes, 
like  the  premedical  courses  in  American  Universities,  requiring  a 
knowledge  more  particularly  of  Vertebrate  Zoology. 

The  above  requirements  have  naturally  determined  in  the  main 
the  types  studied,  but  the  syllabus  has  not  been  slavishly  followed 
in  places  where  it  was  considered  the  student  would  benefit  by 
considering  examples  outside  it.  Types  like  the  Crayfish,  Astacus, 
the  Fresh-water  mussel,  Anodon,  and  the  Cockroach,  Periplaneta, 
often  included  in  elementary  books,  have  been  omitted.  This  was 
done  because  it  was  felt  that  the  knowledge  of  such  specialised  types, 
while  essential  to  a  student  proceeding  with  the  more  advanced 
study  of  Zoology,  involve  a  great  deal  of  detail  unnecessary  from  the 
point  of  view  of  students  proceeding  with  medical  studies,  and, 
moreover,  they  do  not  illustrate  any  fundamental  principles  that 
cannot  also  be  dealt  with  in  connection  with  the  types  chosen.  The 
space  so  gained  has  been  utilised  to  describe  fairly  fully  the  structure 
of  a  mammalian  skull,  that  of  the  dog,  of  a  mammalian  heart,  that 
of  the  pig  or  sheep,  and  of  a  mammalian  brain,  that  of  the  sheep. 
While  the  detail  in  these  cases  is  perhaps  rather  more  than  required 
for  examination  purposes,  it  is  hoped  that  their  study  will  be  of 
value  in  the  subsequent  work  in  Anatomy.  Then,  too,  Cytology, 
Histology,  and  Embryology  have  received  fuller  treatment  than  is 
usually  the  case,  since  they  are  subjects  of  importance  in  future 

V 

513264 


vi  PREFACE 

studies.  Finally,  the  last  chapter  has  been  devoted  to  an  intro- 
ductory outline  of  the  principal  theories  concerning  Heredity, 
Variation,  and  Evolution.  This  chapter  does  not  pretend  to  com- 
pleteness, for  the  treatment  of  this  even  in  an  elementary  way  would 
require  a  large  volume  in  itself.  It  is  treated  in  a  historical  manner 
and  intended  as  a  preliminary  to  wider  reading,  much  of  which  is  of 
importance  in  the  practice  of  medicine  and  should  be  included  in  any 
curriculum  of  education.  The  many  excellent  books  now  available 
on  these  topics  almost  all  assume  implicitly  or  explicitly  such  infor- 
mation as  this  chapter  contains. 

In  arranging  the  book  the  plan  has  been  followed  of  first  treating 
the  frog  fully  as  an  introduction  to  morphology,  physiology,  and 
histology,  and  then  starting  with  the  Protozoa  and  working  up  to  the 
higher  forms.  This  has  been  done  not  because  it  is  perhaps  the 
traditional  method  in '  England  since  Huxley's  time,  but  because 
actual  experience  of  other  ways  has  shown  it  to  be  the  most  satis- 
factory both  from  the  point  of  view  of  the  teacher  and  the  student. 
The  arrangement  of  the  book,  however,  allows  of  almost  any  altera- 
tion in  the  order  in  which  the  forms  are  studied.  As  the  medical 
student  is  going  on  to  what  is  and  must  be  a  highly  technical  training, 
no  attempt  has  been  made  to  avoid  technical  terminology,  although 
in  all  cases  where  such  terms  are  used  they  have  been  defined  and 
also  printed  in  thicker  type  to  facilitate  ready  reference.  No  course 
in  Zoology  can  be  considered  as  adequate  unless  the  reading  is 
accompanied  by  a  satisfactory  practical  course,  and  this  has  been 
borne  in  mind  in  choosing  the  illustrations.  Certain  portions  are 
better  illustrated  by  the  practical  work  itself,  while  others  need 
fuller  illustration  in  the  text. 

I  have  to  thank  the  following  publishers  for  permission  to  use 
the  following  illustrations  :— 

Messrs.  Bell  and  Sons, for  13  figures  from  Bourne's  "Comparative 
Anatomy  of  Animals  "  ;  Messrs.  Constable  &  Co.,  for  4  figures  from 
•Dendy's  "  Evolutionary  Biology  "  ;  Messrs.  H.  Frowde,  for  6  figures 
from  Borradaile's  "  Manual  of  Zoology  "  ;  Messrs.  H.  Holt,  for 
5  figures  from  Calkin's  "  Biology,"  for  3  figures  from  Kellicott's 
"  Textbook  of  General  Embryology,"  for  6  figures  from  Kellicott's 
"  Outlines  of  Chordate  Development,"  and  for  6  figures  from  Lillie's 
"  Development  of  the  Chick  "  ;  Messrs.  Longmans,  Green  &  Co.,  for 

5  figures  from  Furneaux's  "  Elementary  Physiology,"  for  6  figures 
from  Quain's  "  Anatomy,"  for  2  figures  from  Gray's  "  Anatomy," 
and  for  i  figure    from  Owen's  "  Anatomy "  ;    Messrs.    Macmillan 

6  Co.,  for  20  figures  from  Lull's  "  Organic  Evolution,"  and  for  9 
figures  from  Marshall  and  Gamble's  "  The  Frog  "  ;  The  Zoological 
Society  of  London,  for  4  figures. 


PREFACE  vii 

To  my  colleague,  Mr.  William  Rowan,  I  am  indebted  for  the 
originals-of  figures  3,  4,  6,  7,  91,  and  93.  The  remaining  figures  have 
been  drawn  for  this  work  from  preparations  or  from  the  original 
papers.  My  former  colleague,  Dr.  Katherine  M.  Parker,  kindly 
read  through  much  of  the  manuscript  of  the  early  chapters.  Messrs. 
Bell  and  Sons,  the  publishers,  have  given  me  much  assistance  and 
consideration.  Lastly,  my  wife  has  helped  me  in  many  ways,  not  the 
least  of  which  was  the  preparation  of  the  index.  To  all  of  these  I 
wish  to  express  my  sincere  gratitude  and  thanks. 

In  spite  of  good  intentions,  errors  have  doubtless  crept  in,  and 
I  should  be  glad  if  these  are  brought  to  my  notice. 

The  book  is  for  the  use  of  students,  most  of  whom  will  be  pursuing 
courses  in  allied  sciences,  and  I  trust  that  they  will,  be  able  to  obtain 
from  it  a  sound  introduction  to  Vertebrate  Zoology,  and  some  know- 
ledge of  and  interest  in  the  general  principles  of  that  science.  Much 
of  any  good  it  may  contain  is  due  indirectly  to  the  excellent  courses 
given  by  Professor  A.  Dendy,  F.R.S.,  and  Professor  J.  P.  Hill,  F.R.S., 
of  the  University  of  London,  to  both  of  whom  the  author  is  deeply 
indebted.  In  the  case  of  Professor  Hill,  moreover,  my  indebted- 
ness is  more  direct,  since,  for  several  years,  I  was  in  charge  of  the 
practical  classes  accompanying  his  lectures,  and  so  had  an 
opportunity  of  becoming  familiar  with  them.  This  was  of  con- 
siderable advantage  to  me  in  preparing  the  book,  parts  of  which  are 
more  or  less  directly  based  upon  his  lecture-notes  ;  and  I  desire, 
therefore,  to  express  to  him  my  sincere  thanks. 

CHAS.   H.   O'DONOGHUE. 

UNIVERSITY  OF  MANITOBA, 

WINNIPEG, 

September^  1920. 


CONTENTS 

PAGE 

CHAPTER   I 
INTRODUCTION i 

CHAPTER    II 

THE    FROG— RAN  A    TEMPORAR1A 

Introduction— General  Internal  Structure — Skeletal  System — Muscular  System 

and  Integument  .  .          .          .          .          .          .          ,          .          .12 

CHAPTER   III 

THE    FROG— RANA     TEMPORAR1A   (continued 
Alimentary     System — Respiratory    System — Circulatory    System — Urogenital 

System       ............       46 

CHAPTER    IV 
THE    FROG—  RANA     TEMPORARIA    (continued) 

Nervous  System  and  Sense  Organs — Ductless  Glands — Life  History — 'Animals 

and  Plants — Classification 76 

CHAPTER   V 

THE    PROTOZOA 

Free-living  Protozoa,  Amoeba  and  Paranuxcium — Parasitic  Protozoa,  Monocystis 

and  Plasmodium  .          .          .          .          .          .          .          .          .          .118 

CHAPTER   VI 

THE   COZLENTERATA 
A  Simple  Ccelenterate,  Hydra — A  Compound  Ccelenterate,  Obelia  .          .          .148 

CHAPTER   VII 

THE   CCELOMATA    INVERTEBRATA 

The   Earthworm,    Lumbricus   sp.,    a    Free-living   Annelid — Tcenia  solium,   a 

parasitic  flat  worm       .          .          ,         .         •   .      •          .          .         .          .      170 


CONTENTS 


CHAPTER    VIII 

VERTEBRATE    ANIMALS— SCYLLIUM    CAN1CULA 

Introduction  to  Craniata — External  features,  Scy Ilium— Integument — Muscular 

System  — Endoskeleton         .     ' .          .     204 

CHAPTER   IX 

SCYLLIUM   CANICULA   (continued} 

Alimentary    System— Respiratory    System— Circulatory    System  —  Urogenital 

System       .         .          .          .          .          .         .         .         .          .  .     228 

CHAPTER   X 

SCYLLIUM    CANICULA   (continued} 
Nervous  System  and  Sense  Organs       .          .          .         .       •  .          .         .         .251 

CHAPTER   XI 

LEPUS    CUNICULUS 

A  Mammal  —  Lepus  cttnictdus^  the  Rabbit — Introduction — External  characters 

— Skin — Muscular  System— Skeleton  and  the  Skull  of  the  Dog         .         .     266 

CHAPTER   XII 

LEPUS   CUNICULUS   (continued) 
Digestive  System — Respiratory  System — Circulatory  System — The  Mammalian 

Heart— Urogenital  System — Ductless  Glands          •'      >  f         •         •          •     3°2 

CHAPTER    XIII 

LEPUS   CUNICULUS   (continued} 
The  Nervous  System  and  Sense  Organs — The  Mammalian  Brain    .          .          .     327 

CHAPTER   XIV 
HISTOLOGY   AND   CYTOLOGY          .         .         .355 

CHAPTER   XV 

EMBRYOLOGY 

Fertilisation,  Segmentation  and  Germ  Layer  Formation          *          .         .          .     376 

CHAPTER    XVI 
LATER   DEVELOPMENT   OF   CHICK    AND    RABBIT        .     413 

CHAPTER  XVII 
EVOLUTION,   VARIATION,    AND    HEREDITY     .         .     442 

INDEX 485 


AN  INTRODUCTION  TO  ZOOLOGY 
FOR  MEDICAL  STUDENTS 

CHAPTER   I 

INTRODUCTION 

THE  many  things  that  we  encounter  in  our  everyday  life 
we  soon  learn  to  divide  into  two  more  or  less  sharply  contrasted 
groups,  the  living  and  the  non-living.  This  classification  is  a 
fundamental  one,  and  when  we  come  to  the  Sciences,  that  is,  the 
ordered  and  intensive  study  of  these  things  with  a  view  to  under- 
standing them  and  explaining  them,  we  find  that  they  also  fall 
into  two  corresponding  fairly  distinct  categories,  the  Physical 
Sciences,  treating  of  non-living  things,  and  the  Biological  Sciences, 
concerned  with  living  things.  The  two  groups  differ  not  only  in 
their  content,  but,  as  the  nature  of  this  is  dissimilar,  the  methods 
in  which  they  can  be  investigated,  that  is  to  say,  the  technique,  is 
also  different  in  the  two  cases. 

Biology  is  the  branch  of  science  that  treats  of  living  things  in 
their  manifold  relations  with  one  another  and  with  non-living  things. 
Living  beings  are  generally  recognised  as  being  divided  into  Plants 
and  Animals,  and  while,  as  a  matter  of  fact,  we  find  a  certain 
amount  of  overlapping  between  the  two  when  we  study  them  in 
detail,  this  is  a  moderately  clear  cut  division,  to  correspond  with 
which  Biology  may  be  subdivided  into  Botany,  the  study  of  plants, 
and  Zoology,  the  study  of  animals.  The  present  book  is  concerned 
with  the  latter,  namely  Zoology,  or  more  accurately  with  some 
aspects  of  it  as  illustrated  by  certain  type  forms  treated  in  an 
elementary  manner. 

The  field  covered  by  Zoology  is  so  vast  that  it  is  necessary  to 
split  it  up  into  a  number  of  daughter  sciences.  These  fall  under 
two  fairly  general  headings,  according  to  the  point  of  view  adopted 
in  making  our  study.  In  the  first  place,  we  may  be  concerned 
chiefly  with  the  questions  of  the  structure  of  animals,  this  is  termed 
Morphology,  and  in  the  second  we  may  consider  in  the  main  the 
various  activities  manifested  by  animals,  and  this  is  Physiology. 
The  two  require  different  techniques,  but,  for  the  satisfactory 

i  B 


2  AN   INTRODUCTION"-  TO  ZOOLOGY 

investigation  of  either  one,  it  is  necessary  to  take  into  consideration 
the  other,  and  so  they  are,  or  should  be,  to  a  considerable  extent 
complementary. 

These  two  main  divisions  are  sub-divided,  as  has  been  indicated. 
In  Morphology  we  may  consider  the  general  external  form,  number 
of  parts,  their  relation,  size,  etc.,  and  this  is  distinguished  as  gross 
Morphology.  We  may  also  examine  the  actual  parts,  both  internal 
and  external,  of  an  animal,  their  general  structure,  relation  to  one 
another,  and  to  the  body  as  a  whole.  This  we  term  Anatomy,  and 
as  a  rule  it  necessitates  the  use  of  scalpels,  scissors,  etc.,  to  dissect 
the  various  structures.  When  this  is  done  we  may  pass  on  to 
consider  in  detail  the  manner  in  which  the  various  parts  are  built 
up,  a  branch  of  enquiry  termed  Histology,  and  requiring  for  its 
pursuit  a  microscope,  microtome,  etc.  This  reveals  the  fact  that 
the  whole  of  an  animal's  body  is  composed  of,  or  has  been  derived 
from,  a  series  of  tiny  vital  units  termed  cells,  and  the  further  more 
intensive  study  of  these  minute  particles  is  termed  Cytology.  Again , 
all  animals  commence  life  as  a  small  portion  of  living  matter  known 
as  an  egg,  a  fact  that  is  common  knowledge  in  the  case  of  birds, 
butterflies,  flies,  etc.,  and  we  are  confronted  with  the  problem  as 
to  how  this  comparatively  simple  body  becomes  a  complex  adult 
animal.  This  is  a  branch  of  enquiry  known  as  Embryology,  the 
study  of  development. 

Underlying  all  sciences  are  certain  fundamental  beliefs,  and  the 
two  most  important  of  these,  in  the  case  of  Zoology,  are  the  Uni- 
formity of  Nature  and  Evolution.  The  first  of  these  implies  that  the 
various  forces  that  we  can  see  operating  in  Nature  to-day  have  been 
in  action  in  times  past,  and  will  go  on  in  the  future,  although  it 
may  be  that  individual  forces  have  varied  somewhat  in  intensity  or 
point  of  application  at  different  times,  and  may  do  so  again.  By 
Evolution  we  mean  that  the  various  living  forms  that  we  know  to-day 
are  directly  derived  from  those  that  have  gone  before,  and  that  in 
turn  they  will  give  rise  to  those  that  are  to  come.  Their  diversity 
is  due  to  the  continuous  interplay  of  the  forces  included  in  the 
first,  acting  upon  and  modifying  successive  generations  of  living 
beings.  It  should  be  borne  in  mind,  however,  that  these  beliefs 
do  not  imply  an  act  of  faith,  but  are  simply  generalised  statements 
of  observations  that  have  not  been  contradicted  by  experience. 
As  an  outcome  of  the  last,  it  follows  that  the  various  animals  are 
related  to  one  another  in  different  degrees.  The  last  branch  of 
Morphology  concerns  itself  with  expressing  these  relationships  as 
accurately  as  our  knowledge  allows,  by  classifying  and  arranging 
animals  in  groups  and  systems  of  groups,  and  when  this  is  done 
they  are  given  names  indicating,  to  a  certain  extent,  their  positions 


INTRODUCTION  3 

in  such  a  scheme.    These  two  branches  are  closely  allied,  and  are 
termed  Classification  and  Taxonomy. 

Physiology,  as  has  been  noted,  deals  with  the  activities 
or  functions  of  animals,  and  falls  into  three  main  divisions.  The 
first  of  these  is  Ecology  or  Bionomics,  that  is  to  say,  the  study  of 
the  activities  and  responses  of  an  animal  in  its  natural  surroundings., 
and  its  relation  to  other  animals,  inanimate  things,  and  conditions. 
This  in  a  way  resembles  gross  morphology,  since  it  requires  the 
use  of  no  special  instruments  or  technical  appliances,  and  as  it  is 
not  suited  to  laboratory  study  is  unfortunately  omitted  from  most 
courses. 

Secondly,  we  can  investigate  the  activities  of  an  animal  or  its 
parts  under  experimental  conditions,  testing  and  measuring  them 
in  various  ways,  and  so  employing  apparatus  of  a  physical  nature  ; 
this  we  usually  term  Experimental  Physiology.  A  further  interesting 
branch  of  biology  most  closely  connected  with  this  is  Experimental 
Psychology,  which  is  the  study  of  the  responses  that  an  animal  is 
able  to  make  by  reason  of  its  possessing  a  distinct  nervous  system, 
and  hence  in  the  higher  forms  often  described  as  a  study  of  the  mind. 

Thirdly,  the  functions  of  an  animal  may  be  approached  from 
the  chemical  point  of  view,  and  their  chemical  antecedents  and 
results  investigated  ;  this  is  termed  Chemical  Physiology,  and,  as 
will  be  seen,  requires  the  apparatus  and  methods  of  the  chemist. 

A  last  branch,  which  although  sometimes  considered  as  separate, 
is  nevertheless  closely  bound  up  with  the  first  of  these,  is  Distribu- 
tion. It  is  obvious,  of  course,  that  the  reactions  of  an  animal,  to  a 
large  extent,  determine  where  it  is  possible  for  it  to  live  and  its 
activity,  whether  or  not  it  can  spread  widely.  Distribution  itself 
has  two  distinct  aspects :  Geological  Distribution,  i.e.  distribution 
in  time,  and  Geographical  Distribution,  i.e.  that  in  space. 

All  these  various  branches  of  zoology  have  been  concerned 
with  the  acquisition  and  verification  by  various  ways  and  means  of 
all  the  many  facts  relating  to  animals.  Finally,  we  have  left  the 
branch  of  that  science  that  deals  with  the  correlation  of  these  facts, 
their  reduction  to  a  state  of  order  and  arrangement  in  systems,  and 
further  endeavours  to  ascertain  the  causes  responsible  for  their 
production,  and  to  give  explanations  of  the  manifold  phenomena. 
This  branch,  which  rounds  off  the  others  and  borrows  extensively 
from  all  other  sciences,  is  termed  Aetiology,  and  obviously  it  is 
closely  concerned  with  evolution. 

We  may  divide  up  the  study  of  Zoology  into  these  various 
aspects  for  the  sake  of  clearness  in  arranging  our  ideas,  but  any 
piece  of  zoological  work  cannot  be  confined  to  just  one  of  them,  it 
must  involve  two  or  three,  at  any  rate,  and  probably  more. 


4  AN   INTRODUCTION   TO  ZOOLOGY 

Before  leaving  this  question  of  classification  we  may 
glance  at  the  somewhat  misleading  way  in  which  the  terms  are 
ordinarily  applied  in  Universities  and  Colleges.  Owing  to  the  import- 
ance of  one  animal,  Man,  and  the  fact  that  a  medical  training  is 
intended  to  give  the  student  some  detailed  knowledge  of  this  species 
in  particular,  it  is  customary  to  take  the  general  terms  Anatomy, 
Histology,  Physiology,  etc.,  and  apply  them  to  Man.  They  would 
be  more  accurate  if  they  had  prefixed  to  them  the  word  Human, 
and  so  read  Human  Anatomy,  etc.,  as  is  sometimes  done.  Then, 
too,  in  the  case  of  the  terms  Biology,  Zoology  and  Botany,  it  is 
obvious  that  a  junior  student  cannot  be  expected  to  know  the 
whole  of  the  ground  covered  by  any  one  of  these.  So  the  terms 
come  to  be  used  as  convenient  ones  for  indicating  courses  of  study 
that  endeavour  to  give,  often  by  the  utilisation  of  certain  types, 
an  introduction  to  the  elementary  fundamental  ideas  underlying 
the  Morphology,  Physiology  and  Aetiology  of  animals  or  plants, 
or  both. 

Life. 

We  have  spoken  of  the  division  of  material  objects  into 
living  or  animate  and  non-living  or  inanimate,  and  it  is  necessary 
to  consider  further  what  we  mean  by  the  word  "  living."  Living 
things  are  characterised  by  the  possession  of  "  life/'  and,  furthermore, 
we  only  know  of  "  life  "  as  a  manifestation  of  such  beings.  It  is 
a  matter  of  great  difficulty  to  give  a  concise  and  satisfactory  defini- 
tion of  life,  although  we  all  of  us  know  more  or  less  clearly  what  it 
implies,  and  it  is  brought  most  strikingly  to  our  notice  when  it 
ceases,  and  a  living  being  becomes  dead. 

It  is  unnecessary  to  attempt  to  define  life,  since  for  our  purposes 
it  will  be  sufficient  to  become  familiar  with  its  more  important 
manifestations  which  we  distinguish  as  vital  phenomena,  some  of 
the  most  obvious  of  which  we  make  use  of  in  determining  whether 
a  thing  is  alive  or  lifeless.  The  most  striking  are  those  that  concern 
the  activities  of  living  things,  and  so  fall  under  the  heading  of 
physiology. 

Irritability. 

If  we  poke  a  stone  that  is  firmly  seated  nothing  happens, 
yet,  on  the  other  hand,  if  we  perform  the  same  experiment  on  a 
living  animal  it  would  most  probably  -make  some  movement  In 
reply,  the  nature  of  the  reply  depending  on  the  animal.  Here,  in 
a  crude  way,  we  have  utilised  one  of  the  characteristic  vital  activities 
to  determine  whether  a  thing  is  living  or  not,  and  we  shall  consider 
briefly  what  it  involves.  In  the  case  of  the  stone  nothing  happens, 


INTRODUCTION  5 

because,  as  we  put  it  in  everyday  language,  the  stone  cannot 
"  feel."  Any  force,  mechanical,  electrical,  or  heat  change,  light 
change,  gravity,  and  so  on,  that  can  in  any  way  effect  an  organism 
we  speak  of  as  a  stimulus.  The  animal,  then,  has  a  means  of 
appreciating  stimuli,  and  this  is  true  also  of  plants.  We  can  test 
this  very  readily  if  we  put  a  living  plant  near  a  window,  when  we 
shall  find  that  the  growing  parts  turn  towards  the  light,  the  plant 
then  can  appreciate  the  stimulus  of  light.  A  second  factor  is  also 
involved  in  the  initial  experiment,  and  that  is  that  not  merely  is 
the  stimulus  received,  but  a  reply  is  made  to  it,  and  still  further, 
the  reply  is  generally  a  fitting  one.  Here,  then,  we  have  had  an 
illustration  of  a  fundamental  vital  phenomenon,  namely,  the  power 
of  making  a  response,  usually  a  suitable  one,  to  a  stimulus,  and  we 
term  this  attribute  Irritability  or  Sensitivity. 

Any  particular  living  being  occupies  a  characteristic  situation 
or,  as  we  say,  habitat,  where  it  is  constantly  subjected  to  a  stream 
of  typical  stimuli  reaching  it  both  from  within  and  without.  The 
whole  of  the  stimuli  that  affects  an  organism  from  birth  to  death 
we  can  conveniently  include  in  the  one  term  Environment.  It  is 
obvious,  on  reflection,  that  the  nature  of  the  habitat  determines 
to  a  large  extent  the  environment.  Thus,  for  example,  it  is  clear 
that  the  stimuli  affecting  an  animal  like  a  fish  living  in  the  water 
are  different  from  those  reaching  an  air-dwelling  form  like  a  bird. 
Moreover,  while  both  may  have  a  number  of  stimuli  in  common, 
such  as  external  temperature  changes,  or  internal  feelings  of  hunger, 
and  so  on,  each  is  subjected  to  stimuli  peculiar  to  its  habitat,  and  not 
playing  upon  the  other,  so  that  each  comes  under  the  influence  of 
a  different  and  characteristic  environment.  .Again,  when  we 
consider  the  response  that  is  to  be  made  to  these  manifold  stimuli, 
we  shall  see  that  in  order  to  live  an  animal  or  plant  must  reply  in 
such  a  way  as  to  preserve  its  life  in  its  environment.  Hence,  this 
power  of  irritability  is  not  only  a  fundamental  characteristic  of 
organisms,  but  within  certain  limits  an  absolutely  essential  one 
for  self-preservation. 

Metabolism. 

It  is  a  matter  of  common  knowledge  that  living  beings 
require  food  in  order  to  live,  and  the  nature  of  this  food  and  the 
manner  in  which  it  is  dealt  with  constitute  the  second -of  the  vital 
phenomena.  While  an  inorganic  body,  such  as  a  crystal,  is  able 
to  increase  in  size  or  grow,  it  can  only  do  so  when  it  is  provided 
with  a  substance  that  is  chemically  similar  to  itself.  On  the  other 
hand,  an  organism  is  able  to  utilise  materials  unlike  itself  for  food, 
and  from-  them  to  build  up  its  own  substance.  This  is  a  capability 


6  AN   INTRODUCTION  TO  ZOOLOGY 

that  we  term  the  power  of  nutrition,  and  it  always  involves  certain 
subsidiary  processes.  Just  as  a  fire  needs  a  constant  supply  of 
fuel,  so  an  organism  must  have  food  to  maintain  its  vitality.  Plants 
in  general  grow  in  such  situations  that  they  can  obtain  their  food 
from  the  water,  the  soil  and  the  air  surrounding  them,  and  so  are 
fixed,  whereas  animals  usually  have  to  seek  out  their  food,  and  are, 
in  consequence,  able  to  move  from  place  to  place.  The  food,  when 
it  reaches  the  organism,  is  taken  in  or  ingested  in  some  way  or 
other,  and  this  is  the  first  process  of  nutrition.  Many  of  the  sub- 
stances ingested  can  be  dealt  with  straight  away,  because  they  are 
soluble,  but  a  number  of  others  are  insoluble  or  unable  to  pass 
into  the  living  substance  until  they  are  altered  in  some  way  or 
other,  usually,  in  the  case  of  animals,  reduced  to  more  simple 
compounds.  This,  then,  is  the  second  process  of  nutrition,  namely, 
digestion,  or  the  changing  of  the  chemical  nature  of  the  food  in 
such  a  way  that  it  can  be  taken  up  into  the  organism.  The  third 
step  consists  of  the  building  up  of  these  substances,  chemically 
speaking,  relatively  simple,  into  the  complex  compounds  character- 
istic of  living  beings,  and  this  is  termed  assimilation.  Lastly,  in 
order  to  obtain  the  substances  required  as  food,  it  is  nearly  always 
necessary  to  take  in  certain  other  materials  that  are  not  wanted, 
and  often,  too,  materials  that  cannot  be  rendered  soluble  by  the 
process  of  digestion.  This  is  noticeably  the  case  in  animals,  and 
we  find  this  waste,  or  better,  not-utilisable  matter,  voided  from  the 
body  as  faeces,  a  process  termed  egestion. 

This  is,  as  it  were,  the  "  receipts  "  side  of  the  account  of  living 
matter,  and  it  results  in  the  replacement  of  used-up  material,  and 
perhaps  an  increase  in  size  or  growth  or  a  storage  of  certain  reserve 
food  substances  until  such  time  as  they  may  be  required.  To  one 
part  of  this,  namely,  the  building  up  of  the  living  matter  or  Anabo- 
lism,  we  shall  return  later  when  we  have  considered  this  substance 
from  the  physical  and  chemical  points  of  view. 

On  the  other  hand,  we  have  also  to  consider  the  "  expendi- 
ture "  side  of  the  account.  In  order  that  these  complicated  chemical 
changes  may  go  on,  it  is  necessary  for  the  organism  to  obtain 
Oxygen,  and  this  it  gets,  as  a  rule,  from  the  air  or  water  surrounding 
it  by  a  process  termed  Respiration.  Again,  just  as  a  fire  requires 
Oxygen  to  continue  burning,  so  also  oxygen  is  a  necessity  to  living 
matter,  and  a  further  parallel  between  the  two  can  also  be  drawn. 
The  burning  of  the  fire  results  in  the  formation  of  various  waste 
matters,  taking  the  form  of  gases  and  ash.  The  activities  of  an 
organism  also  produce  waste  products.  One  of  these  is  the  gas 
Carbon  Dioxide,  and  quite  frequently  the  same  mechanism  that 
serves  for  obtaining  the  oxygen  supply  is  also  utilised  to  get  rid  of 


INTRODUCTION  7 

the  carbon  dioxide.  This  is  particularly  the  case  in  animals,  and 
the  term  respiration  is  most  frequently  used  to  imply  this  exchange 
of  carbon  dioxide  for  oxygen.  The  other  waste  products,  corre- 
sponding superficially  to  the  ash  of  the  fire,  cannot  be  eliminated 
in  this  way,  and  the  process  of  removing  them  is  termed  excretion. 
This  getting  rid  of  the  substances  resulting  from  the  break  down 
of  living  matter,  i.e.  excretion,  must  be  carefully  distinguished 
from  the  removal  of  the  insoluble  material  taken  in  with  the  food, 
but  never  at  any  time  forming  an  actual  part  of  the  organism. 
These  breaking  down  processes  are  collectively  termed  Katabolisni. 
The  power  to  carry  out  the  constructive  and  destructive  changes, 
together  included  in  the  term  Metabolism,  constitutes  the  second  of 
the  fundamental  vital  phenomena,  and  we  shall  investigate  this 
matter  more  fully  later  after  a  fuller  examination  of  the  nature  of 
the  living  substance. 

Growth  and  Reproduction. 

The  organism,  then,  is  the  seat  of  two  antagonistic  processes, 
anabolism  and  katabolism.  If  the  two  are  about  balanced  a  con- 
dition of  relative  stability  is  attained,  whereas  if  the  former  is  in 
excess  of  the  latter,  then  an  increase  in  size  or,  in  other  words, 
growth  takes  place.  The  growth  is  of  a  peculiar  form,  for  the  new 
material  is  added  in  extremely  minute  particles,  throughout  the 
whole  of  the  living  substance,  a  process  termed  growth  by  intussuscep- 
tion. A  crystal,  on  the  other  hand,  even  if  it  does  increase  in  size, 
does  so  by  the  deposition  of  layers  upon  its  surface  ;  this  is  growth 
by  accretion.  The  former  variety  is  definitely  characteristic  of 
living  things. 

In  lower  forms  of  animals,  when  they  reach  a  certain  size, 
determined  probably  by  physico-chemical  requirements,  they 
divide  into  two  separate  daughter  organisms,  each  of  which,  in  the 
presence  of  an  adequate  food  supply,  proceeds  to  grow  until  it 
reaches  a  maximum  size.  Thus  it  will  be  seen  that  from  the  one 
original  animal  two  have  been  produced,  in  other  words,  Reproduc- 
tion or  Multiplication  has  taken  place,  and,  further,  it  is  clear  that 
in  such  cases  we  can  regard  it  as  discontinuous  growth.  All  organisms 
possess  this  power  of  reproducing  their  like,  although  it  is  not 
always  so  clearly  a  case  of  discontinuous  growth.  In  higher  forms 
two  parent  individuals  are  concerned,  termed  respectively  the 
male  and  the  female,  each  of  which  produces  as  a  part  of  itself  a 
tiny  particle  of  living  matter,  the  male  or  the  female  germ  cell. 
These  two  germs  unite,  a  phenomenon  known  as  fertilisation,  and 
from  their  union  a  single  body  results  that  will  grow  up  like  its 
parents  in  all  essential  respects.  Such  a  process  is  not  met  with 


8  AN   INTRODUCTION   TO   ZOOLOGY 

in  the  inorganic  world,  so  that  in  this  power  of  being  able  to  reproduce 
their  like  living  things  differ  from  the  non-living,  and  so  it  constitutes 
a  further  vital  characteristic. 

It  has  just  been  noted  that  in  higher  forms  the  union  of 
a  male  and  female  germ  cell  gives  rise  to  a  new  individual  which 
undergoes  a  series  of  changes,  often  quite  complicated,  whereby  it 
grows  more  and  more  like  its  parent.  This  process  we  term  develop- 
ment, and  it  is  usually  accompanied  by  a  marked  increase  in  size 
or  growth.  It  is  then  a  period  when  anabolism  is  in  excess  of 
katabolism.  'At  the  conclusion  of  this  period  the  animal  is  full 
grown,  and  the  two  processes  approximately  balance.  Somewhere 
about  the  same  time  the  animal  becomes  adult,  that  is  to  say,  it 
is  able  in  its  turn  to  produce  germ  cells,  and  so  in  conjunction  with 
another  animal  of  the  opposite  sex  give  rise  to  number  of  new 
individuals.  After  a  longer  or  shorter  period  of  adult  life  the 
katabolic  processes  exceed  the  anabolic,  with  the  result  that,  even 
if  no  accident  or  illness  supervenes,  there  finally  comes  a  time  when 
some  part  of  the  organism  that  is  of  vital  importance  gives  way, 
and  the  animal  ceases  to  live,  or  as  we  say,  dies.  Death  is  the  normal 
termination  of  this  series  of  events,  just  as  much  as  birth  is  its 
beginning.  We  see,  then,  that  an  animal  goes  through  a  succession 
of  changes,  beginning  in  the  higher  forms  at  fertilisation,  including 
growth,  and  ending  with  death,  and  somewhere  between  the  middle 
and  the  end  possesses  the  power  of  reproduction.  All  these  pheno- 
mena together  constitute  an  ordered  whole  termed  the  life  history 
or  life  cycle  of  the  animal,  and  in  exhibiting  this  organisms  differ 
from  non-living  things.  We  have  now  considered  the  main  pheno- 
mena characteristic  of  life,  and  seen  that  they  are  expressed  as  the 
functions  of  living  beings.  They  are  in  the  main  four :  (i)  Irrita- 
bility ;  (2)  Metabolism  ;  (3)  and  (4)  Growth  and  Reproduction, 
which  are  closely  linked  and  imply  a  life  cycle.  It  is  now  necessary 
to  examine  organisms  from  the  point  of  view  of  their  structure. 

Protoplasm. 

The  above  considerations  have  led  us  to  see  that  there 
are  a  number  of  points  in  which  the  behaviour  of  living  and  non- 
living things  differ,  and  the  powers  possessed  by  the  former  are  not 
shared  by  the  latter.  In  considering  the  structure,  too,  we  shall 
find  a  striking  and  absolute  difference.  When  we  press  our  enquiries 
to  the  limit  we  shall  find  that  all  organisms  or,  at  any  rate,  the  living 
portions  of  them,  are  composed  of  a  highly  complex  substance  that 
is  termed  Protoplasm,  that  is  never  found  in  the  inorganic  world. 
This  material  is  found  wherever  we  find  evidences  of  life,  and  it  is 
remarkably  similar  in  chemical  and  physical  properties  in  all 


INTRODUCTION  9 

animals  and  plants.  So  closely  is  it  bound  up  with  the  vita! 
phenomena  that  wherever  we  find  life  we  find  protoplasm,  and 
vice  versa  wherever  that  substance  is  found  we  find  manifestations 
of  vitality.  This  being  the  case,  we  sometimes  say  the  protoplasm 
is  the  physical  basis  of  life,  or  the  same  idea  is  expressed  by  saying 
that  life  is  a  property  of  protoplasm.  From  the  universality  of  its 
occurrence  in  organisms,  and  the  fact  that  it  is  indissolubly  bound 
up  with  vital  activities,  it  is  obvious  that  protoplasm  is  a  substance 
of  extreme  importance. 

It  is  a  very  difficult  material  to  deal  with  chemically,  because 
when  we  place  it  in  a  test  tube,  or  submit  it  to  the  ordinary  methods 
of  chemical  analysis,  we  at  once  kill  it,  and  so  are  no  longer  dealing 
with  living  matter.  Ultimate  analysis  shows  that  there  is  no 
chemical  element  to  be  found  in  protoplasm  that  we  cannot  also 
find  in  the  inorganic  world,  so  that  we  cannot  regard  living  and 
•non-living  as  two  essentially  different  sorts  of  matter,  but  only 
as  the  same  matter  in  different  stages. 

While  this  is  true  of  the  elements  contained  in  the  living  sub- 
stance, it  does  not  hold  for  the  compounds.  Chemical  analysis 
shows  us  that  protoplasm  is  an  intimate  mixture  of  a  number  of 
different  classes  of  compounds  mainly  falling  under  five  headings, 
viz.  :  water,  inorganic  salts,  fats,  carbohydrates  and  proteins. 
Of  these  the  first  two  are  also  met  with  in  inorganic  matter,  but 
the  last  three  are  only  found  as  a  part  of,  or  as  the  products  of, 
protoplasm,  and  encountered  nowhere  else  in  nature.  They  are 
for  this  reason  spoken  of  as  organic  compounds.  Fats  and  Carbo- 
hydrates (e.g.  starches  and  sugars)  are  composed  of  the  elements 
Carbon,  Hydrogen  and  Oxygen  in  various  proportions.  Proteins, 
the  most  complicated  compounds  that  we  know,  always  contain 
in  addition  to  these  elements  Nitrogen,  and  usually  several  others, 
such  as  Sulphur,  Phosphorus,  Sodium,  Magnesium,  Iron,  etc. 

In  addition  to  this  we  can  ascertain  by  experiment  that  when  a 
living  being  dies  there  is  no  loss  of  weight ;  a  very  important  point, 
since  it  demonstrates  that  the  cessation  of  life  is  not  accompanied 
by  the  loss  of  any  material  substance.  There  is,  however,  a  remark- 
able change  in  both  its  chemical  and  physical  properties,  and  we 
must  assume  that  this  is  due  to  a  rearrangement  of  the  composition 
and  minute  physical  structure  of  the  protoplasm,  as  it  is  not  due 
to  the  withdrawal  of  anything  tangible. 

Physics  teaches  us  that  energy,  which  is  necessary  for  the 
performance  of  any  work,  exists  in  two  forms.  Firstly,  we  have 
kinetic  energy,  the  variety  of  energy  that  expresses  itself  in  the 
form  of  motion,  heat,  light  and  electricity,  and  is  brought  promi- 
nently to  our  notice  by  its  results.  Secondly,  potential  energy  is 


io  AN    INTRODUCTION    TO   ZOOLOGY 

energy  that  is,  as  it  were,  stored  up  waiting  for  some  stimulus  to 
release  it.  A  body  possesses  potential  energy  by  virtue  of  the 
previous  expenditure  of  kinetic  energy  upon  it.  Thus,  to  take  a 
simple  example,  if  we  take  a  weight  from  the  floor  and  place  it  on 
a  high  shelf,  kinetic  energy  has  to  be  utilised  to  do  this.  While 
on  the  shelf  the  weight  manifests  no  activity,  but  it  possesses 
potential  energy  as  a  result  of  its  position,  and  if  we  remove  the 
shelf  from  under  it  this  stored  energy  becomes  released  as  potential 
energy,  and  the  weight  starts  to  move.  The  work  it  is  capable  of 
performing  may  be  utilised  by  connecting  it  with  a  machine,  or 
else  it  will  be  transformed  into  heat  when  it  strikes  the  ground. 
In  a  somewhat  analogous  way,  the  complex  compounds  have  used 
up  a  supply  of  kinetic  energy  in  their  formation,  and  under  suitable 
conditions  they  can  be  induced  to  break  down  into  simpler  substances 
and  release  a  certain  amount  of  kinetic  energy. 

The  maintenance  of  life  and  the  exhibition  of  vital  activities 
require  the  expenditure  of  a  certain  amount  of  energy,  and  this  is 
obtained  by  the  oxidation  of  certain  constituent  parts  of  the  living 
organism  itself.  These  compounds  under  suitable  conditions 
combine  directly  or  indirectly  with  the  oxygen  supplied  by  respira- 
tion and  break  down  into  simpler  compounds,  releasing,  as  they  do 
so,  the  necessary  kinetic  energy.  In  many  cases  these  break-down 
products  are  substances  that  are  of  no  further  use,  and  so  need  to 
be  eliminated.  The  production  of  kinetic  energy  therefore  is  the 
result  of  katabolic  changes,  and  it  takes  place  throughout  every 
minute  particle  of  the  whole  of  the  protoplasm.  This  wastage 
needs  to  be  made  good  and  stores  of  potential  energy  built  up  for 
future  use,  and  this  is  brought  about  by  anabolism.  Thus  we  see 
that,  from  the  chemical  point  of  view,  protoplasm  is  an  intensely 
active  substance,  the  seat  of  unceasing  destructive  and  constructive 
changes  ;  it  is  never  quite  the  same  for  two  consecutive  moments. 

When  viewed  under  the  microscope  living  protoplasm  is 
found  to  be  a  semi-viscous,  almost  transparent,  granular  substance 
usually  in  a  state  of  motion.  When  examined  under  the  highest 
powers  a  certain  amount  of  structural  organisation  can  be  made 
out,  but  this  is  relatively  simple,  and  certainly  gives  no  hint  of  its 
wonderful  complexity. 

All  animals  are  composed  of  this  substance.  At  the  com- 
mencement of  their  existence  they  consist  practically  exclusively  of 
protoplasm,  but  in  the  higher  animals  other  non-living  products 
appear  as  a  result  of  its  activities  ;  in  ourselves,  for  example,  hair, 
nails,  and  the  inorganic  matter  of  bone  and  teeth.  Such  animals 
do  not  consist  of  protoplasm  alone,  but  the  other  lifeless  substances 
in  them  have  been  formed  by  it.  Then,  too,  we  find  that  the 


INTRODUCTION  n 

protoplasm  of  a  complex  animal  is  not  homogeneous,  but  is  built 
up  of  an  enormous  number  of  tiny  vital  units,  just  as  a  building 
may  be  composed  of  bricks.  Closer  examination  shows  each  unit 
to  consist  of  a  certain  amount  of  protoplasm  containing  within  it  a 
denser  portion.  These  tiny  masses  we  call  cells,  a  somewhat 
misleading  name.  A  cell  then  is  a  small  portion  of  protoplasm 
surrounding  a  denser  and  more  opaque  nodule,  the  nucleus. 

As  we  pass  down  in  the  animal  scale  we  find  that  these  cells 
become  relatively  fewer  in  each  individual,  until  finally,  when  we 
come  to  the  lowest  forms,  the  whole  creature  is  composed  of  just 
one  cell.  This  single  cell,  however,  is  capable  of  exhibiting  all  the 
vital  phenomena.  We  therefore  find  that  from  this  point  of  view 
we  may  divide  animals  into  two  distinct  groups :  one  in  which  the 
individual  is  composed  of  only  one  cell,  these  we  term  the  Protozoa 
or  unicellular  animals,  and  the  other,  in  which  the  animal  is  built 
up  of  a  large  number  of  cells  often  differing  enormously  among 
themselves  in  size  and  shape,  and  these  we  designate  the  Metazoa 
or  multicellular  animals. 

Although  we  have  not  actually  defined  life  we  have 
enumerated  certain  of  the  functional  and  structural  characters  that 
mark  off  the  organic  from  the  inorganic  world,  and  this  in  itself 
enables  us  to  get  a  clearer  idea  of  what  is  implied  when  we  say  that 
a  certain  thing  is  alive. 

The  world  of  living  beings,  of  which  we  ourselves  form  an 
integral  part,  presents  an  infinite  number  of  problems  to  the  enquiring 
mind.  So  diverse  and  so  complex  are  they  that  no  one  person  can 
ever  hope  to  master  more  than  a  small  group  of  them.  We  have 
indicated  above  the  various  ways  of  approaching  these  questions, 
and  also  endeavoured  to  set  out  those  fundamental  attributes  of 
organisms,  more  particularly  animals,  that  are  more  or  less  common 
knowledge,  and  often  taken  for  granted  without  further  thought. 
The  purpose  of  the  succeeding  chapters  is  to  elaborate  these  two 
points  in  a  more  thorough  and  systematic  way,  by  the  examination 
of  the  main  problems  and  the  methods  of  treating  them.  To  do 
this  we  shall  consider  in  a  certain  amount  of  detail  a  selected  number 
of  animals  differing  both  in  structure  and  mode  of  life,  and  chosen 
with  a  view  to  emphasising  the  course  of  evolution  in  the  animal 
kingdom. 


CHAPTER   II 
THE   FROG— RANA   TEMPORARIA 

Introduction — General  Internal  Structure — Skeletal   System — Muscular 
System  and  Integument. 

Introduction. 

Few  four-legged  animals  are  more  common  or  more  readily 
obtained  than  the  common  grass  frog,  Rana  temporaria.  It  is  to  be 
found  in  the  damp  grass  in  the  neighbourhood  of  ponds  and  ditches 
in  all  parts  of  the  British  Isles,  and  is  probably  the  only  native  frog. 
A  larger  form.  Rana  esculenta,  the  edible  frog,  is  very  abundant  on 
the  continent,  and  although  rare  in  this  country  is  not  infrequently 
imported  by  dealers,  and  finds  its  way  thence  into  the  laboratory. 
For  our  purposes  the  differences,  in  the  main  of  size  and  colour, 
between  the  two  species  need  not  be  taken  into  account. 

The  colour  of  the  grass  frog  is  subject  to  great  variation,  according 
to  the  nature  of  the  surroundings  in  which  the  animal  lives,  but 
generally  the  back  is  mottled  dark  green,  brown  and  black,  and  the 
under  surface  is  pale  yellow  with  a  few  dark  spots.  The  coloration 
is  due  to  the  presence  in  the  skin  of  a  large  number  of  deeply 
pigmented  cells,  and  the  colour  of  an  individual  can  be  changed  to 
suit  its  habitat  by  an  alteration  in  the  size  of  the  coloured  cells. 
They  can  each  be  contracted  into  quite  a  tiny  spot  or,  again,  spread 
out  over  a  comparatively  large  area.  In  this  way  the  frog  is 
protected  from  its  enemies,  for  it  is  able  to  harmonise  with  the  ground 
and  so  escape  detection  as  long  as  it  remains  still.  It  is  almost 
equally  at  home  on  the  land  or  in  the  water,  and  it  belongs  to  that 
class  of  animals  termed  the  AMPHIBIA,  in  order  to  indicate  this 
capability  of  living  either  aquatic  or  terrestrial  lives.  Although 
unable  to  walk  easily  it  can  still  get  about  well  on  land,  and  in  the 
water  is  a  powerful  swimmer.  The  long  hind  legs  serve  as  efficient 
swimming  organs,  and  also  for  locomotion  on  land  by  means  of  a 
series  of  jumps.  It  moves  freely  from  place  to  place,  and  so  we  say 
it  exhibits  the  power  of  voluntary  movement.  The  food  of  the  frog 

12 


THE   FROG  13 

consists  of  small  insects,  their  grubs  or  caterpillars,  worms  and  slugs, 
which  it  catches  by  means  of  its  tongue.  The  tongue  is  capable  of 
being  shot  out  suddenly  to  a  considerable  distance,  and  has  a  sticky 
extremity  to  which  the  prey  adheres.  During  the  winter  frogs 
hide  away  in  holes  in  the  ground  in  order  to  escape  the  cold,  not  only 
to  avoid  being  frozen,  but  also  because  it  would  be  practically 
impossible  for  them  to  obtain  food  during  this  part  of  the  year. 
They  pass  the  time  in  a  torpid  state,  in  almost  a  lifeless  condition, 
requiring  no  food  and  but  little  air.  Such  a  winter  sleep,  in  which 
animation  is  suspended,  is  met  with  in  a  number  of  different  animals, 
and  is  spoken  of  as  hibernation.  In  early  spring  they  awake,  and 


V   -\.   '?• 

FIG.  i. — The  common  frog. — .From  Borradaile. 

during  February  and  March  are  always  to  be  found  near  water  for 
the  purposes  of  reproduction.  The  females  lay  a  large  number  oi 
eggs  which  are  fertilised  by  the  male,  and  adhere  together  in  jelly- 
like  masses,  the  spawn,  and  are  common  enough  in  our  ponds  and 
ditches.  When  laid,  the  eggs  are  left  absolutely  alone,  and  the 
young  frog  or  tadpole  has  to  fend  for  itself  During  the  summer 
frogs  often  wander  quite  considerable  distances  from  water  in  search 
of  food,  but  always  return  again  in  the  succeeding  spring. 

The  general  features  of  shape  and  size,  the  number  and 
arrangement  of  limbs  and  openings,  etc.,  that  can  be  made  out 
without  resorting  to  dissection  or  opening  the  animal  up  are  spoken 


14  AN   INTRODUCTION   TO   ZOOLOGY 

of  as  the  external  characters.  You  will  readily  observe  that  the 
frog  as  a  whole  is  divisible  into  head,  trunk  and  limbs.  The  head, 
unlike  our  own,  is  not  joined  to  the  body  by  means  of  a  flexible  neck, 
but  the  two  are  continuous  with  one  another,  a  condition  that  we 
frequently  meet  with  in  water-dwelling  animals.  For  the  purposes 
of  description  we  speak  of  the  back  as  the  dorsal  surface,  the  under- 
neath as  the  ventral  surface,  the  end  that  is  foremost  in  moving, 
i.e.  the  head,  as  anterior,  and  the  after  part  of  the  body  as  posterior. 
A  little  closer  observation  will  show  that  the  skin  is  quite  smooth, 
and  has  no  structures  resembling  scales,  feathers,  hairs  or  nails  upon 
it.  Such  structures,  when  present,  as  in  many  animals,  we  term 
collectively  the  exoskeleton. 

Turning  first  to  the  head,  we  find  it  possesses  a  bluntly  rounded 
snout,  and  that  the  mouth  is  a  long  slit  running  the  greater  part  of 
the  way  round  its  edge.  Two  small  openings,  the  nostrils  or  external 
nares,  are  situated  on  the  top  of  the  front  end  of  the  snout.  If  the 
living  animal  be  watched  carefully  it  will  be  noted  that  the  neck  is 
constantly  falling  and  rising,  and  at  the  same  time  a  little  flap  or 
valve  just  inside  the  external  nares  is  opening  and  shutting.  These 
movements  are  concerned  with  breathing  or  respiration,  and  the 
air  is  not  drawn  into  the  lungs  by  expanding  the  chest,  as  in  our- 
selves, but  pumped  in  by  the  action  of  the  throat.  A  short  distance 
behind  the  external  nares  are  situated  the  large  protruding  eyes, 
whose  prominence  in  some  degree  compensates  for  the  absence  of  a 
neck  by  allowing  a  wide  field  of  vision.  In  each  we  can  make  out, 
as  in  ourselves,  an  eyeball,  in  the  middle  of  which  is  a  circular  black 
space,  the  pupil,  through  which  the  light  enters,  surrounded  by  a 
wide  coloured  band,  the  iris.  .The  upper  eye  lid  is  fairly  well  marked 
and  slightly  movable,  but  the  lower  one  is  represented  by  a  very 
transparent  skin,  the  nictitating  membrane,  which  can  be  drawn 
right  up  over  the  eye.  An  external  ear  or  pinna,  such  as  our  own. 
is  absent,  but  just  behind  the  eye  and  above  and  behind  the  mouth 
is  a  circular  patch  of  black,  thin,  lightly  stretched  skin.  This  is 
the  ear  drum  or  tympanic  membrane. 

The  trunk  is  not  marked  by  any  striking  characters,  save  that  in 
the  sitting  position  a  hump  appears  in  the  middle  of  the  back  ;  the 
meaning  of  this  will  be  made  clear  when  the  internal  structure  is 
examined.  It  ends  bluntly  without  any  tail.  At  the  posterior  end 
of  the  body,  between  the  hind  legs  and  slightly  on  the  dorsal  side, 
is  an  opening,  the  cloaca,  through  which  the  waste  matters  and 
reproductive  products  are  expelled  from  the  body.  Attached  to  the 
trunk  are  the  limbs  or  appendages,  consisting  of  a  pair  of  fore  limbs 
or  arms,  and  a  pair  of  hind  limbs  or  legs.  The  arm  is  composed,  as 
in  ourselves,  of  an  upper  arm  or  brachium,  a  fore-arm  or  ante- 


THE  FROG  15 

brachium,  a  small  indistinct  wrist  or  carpus,  and  a  hand  or  manus. 
Unlike  our  own  the  hand  has  only  four  fingers  or  digits,  a  thumb  or 
pollex  being  absent.  The  leg  similarly  is  composed  of  a  thigh  or 
femur,  a  shank  or  cms,  a  very  long  ankle  or  tarsus,  and  a  foot  or 
pes.  The  foot  appears  to  have  six  digits,  five, large  and  one  small, 
but  we  shall  see  later  that  the  small  inner  one,  the  calcar,  is  not  a 
true  toe,  and  the  first  big  one  is  the  great  toe  or  hallux.  The  five 
large  toes  are  united  by  a  thin  fold  of  the  skin,  the  so-called  web,  and 
in  this  way  a  large  surface  for  swimming  is  obtained.  The  form  of 
the  animal  is  maintained  by  a  number  of  hard  structures  or  bones, 
that  are  felt  when  handling  it,  and  which  together  constitute  the 
internal  skeleton  or  endoskeleton,  to  distinguish  it  from  the  exo- 
skeleton. 

Among  the  amphibia  in  general  it  is  difficult  to  tell  the  male  from 
the  female  externally,  but  this  may  be  done  quite  easily  in  the  frog 
by  an  examination  of  the  hand.  The  under  side  of  the  inner  digit 
in  the  male  possesses  a  brownish-black  swelling,  somewhat  like  the 
ball  of  our  limb,  but  still  more  marked,  while  the  same  digit  of  the 
female  is  not  swollen.  This  enlargement  in  the  male  varies  at 
different  times  of  the  year,  and  is  particularly  well  developed  during 
the  breeding  season.  Further,  at  the  same  period  the  male  when 
croaking  exhibits  two  bladder-like  swellings,  the  vocal  sacs,  under 
its  throat,  these  serve  as  resonating  chambers  to  increase  the  volume 
of  sound. 

General  Internal  Structure. 

Having  considered  the  external  characters,  out  next  duty 
is  to  examine  the  internal  structure.  If  the  mouth  be  opened 
widely  and  the  joint  between  the  jaws  snipped  through  with  the 
scissors  a  number  of  structures  can  be  made  out.  The  roof  of  the 
mouth,  or  buccal  cavity,  is  surrounded  by  the  upper  jaw,  which  is 
immovably  fixed  to  the  skull,  as  in  ourselves,  and  contains  a  large 
number  of  small  pointed  teeth,  the  maxillary  teeth.  The  roof  itself 
is  formed  by  the  base  of  the  skull  and  the  orbits  or  cavities  in  which 
the  eyes  are  lodged,  and  is  covered  by  a  very  soft  moist  skin,  the 
mucous  membrane.  Two  very  prominent  swellings  caused  by  the 
eyes  are  readily  seen.  Just  in  front  of  these,  nearer  the  middle  line, 
are  two  small  groups  of  teeth,  the  vomerine  teeth,  and  in  front  of 
these  again  two  small  openings,  the  internal  nostrils  or  nares.  These 
communicate  with  the  internal  nares,  as  may  be  seen  by  inserting 
a  bristle  into  the  latter,  by  means  of  a  short  tube,  the  narial  passage. 
This  channel  is  dilated  to  form  a  large  narial  chamber  containing  the 
olfactory  organ,  by  means  of  which  the  frog  is  able  to  smell.  Behind 
and  outside  the  eye  swellings  at  the  back  of  the  upper  jaw  are  two 


16  AN   INTRODUCTION   TO  ZOOLOGY 

small  holes,  the  openings  of  the  Eustachian  tubes,  and  if  a  seeker 
be  pushed  into  them  it  will  be  found  to  come  to  the  outside  of  the 
head  at  the  tympanic  membrane. 

The  floor  of  the  buccal  cavity  is  bounded  by  the  lower  jaw, 
which  contains  no  teeth,  and  is  covered  by  the  same  soft  mucous 
membrane.  This  jaw  is  jointed  or  articulated  to  the  upper  jaw  in 
such  a  way  that  it  is  free  to  move  in  a  vertical  direction.  The  floor 
itself  is  soft  but  supported  by  a  flat  plate  of  gristle  or  cartilage,  the 
hyoid  plate.  Most  of  its  space  is  occupied  by  the  comparatively 
large  fleshy  tongue.  This,  unlike  our  own,  is  bifid  at  the  end,  and 
is  attached  in  front  and  free  at  the  back,  so  that  it  is  able  to  be 
projected  a  good  distance  from  the  mouth.  Behind  the  tongue  is  a 
median  slit,  the  glottis,  situated  on  a  small  oval  elevation,  the  larynx, 
through  which  the  air  gains  access  to  the  lungs.  Right  at  the  back, 
where  floor  and  roof  come  together,  the  cavity  narrows  somewhat, 
and  is  termed  the  pharynx.  It  communicates  with  a  wide  slit, 
the  opening  of  the  oesophagus,  which  leads  to  the  stomach.  If  the 
frog  is  a  male,  two  small  openings  lying  slightly  in  front  and  to  the 
side  of  the  larynx  will  be  seen.  These  lead  into  the  collapsed  vocal 
sacs,  which  can  be  inflated  while  the  animal  is  croaking,  and  are 
particularly  marked  during  the  breeding  season. 

If  the  frog  be  pinned  out  on  its  back  under  water  the  skin 
can  be  slit  up  from  the  posterior  end  to  the  front  of  the  snout,  and 
also  a  little  way  along  the  base  of  each  limb.  In  doing  this  it  will 
be  noticed  that  the  skin  is  not  attached  to  the  underlying  parts,  save 
by  one  or  two  bands  of  thin  white  substance,  the  connective  tissue 
septa.  These  divide  up  the  fairly  extensive  cavity  into  a  number  of 
smaller  cavities  which,  since  they  are  filled  with  a  watery  fluid, 
lymph,  and  are  situated  under  the  skin,  are  termed  the  sub-cutaneous 
lymph  sinuses.  A  large  blood-vessel,  the  musculo-cutaneous  vein, 
will  be  seen  running  in  the  skin  in  the  region  of  the  arm. 

Beneath  the  skin  will  be  seen  the  flesh  or  muscles  forming  the 
outer  wall  of  the  body,  arranged  in  sheets  and  covered  by  a  very 
thin  layer  of  semi-transparent  substance,  the  fascia.  Passing  across 
the  underneath  side  of  the  head  from  jaw  to  jaw  is  a  broad  muscle, 
the  Mylo-hyoideus.  Posterior  to  this  is  a  fan-shaped  muscle  com- 
posed of  five  parts  on  each  side  running  together  at  the  base  of  the 
arm  ;  this  is  the  pectoralis  muscle.  In  the  posterior  half  of  the  body 
in  the  middle  line  is  a  thin  clear  line  of  connective  tissue,  the  linea 
alba,  through  which  a  blood-vessel,  the  anterior  abdominal  vein, 
shows  clearly.  On  each  side  of  this  line  up  to  the  pectoralis  the 
muscles  are  arranged  in  a  series  of  small  rectangles,  the  recti 
abdominis.  Between  the  middle  parts  of  the  pectoralis  muscles  and 
the  linea  alba  is  a  small  plate  of  cartilage,  the  sternum  or  heart  bone, 


THE   FROG  17 

underneath  which  lies  the  heart.  The  two  hindermost  portions  of 
the  pectoralis  muscle  appear  to  form  the  body  wall,  but  it  will  be 
seen,  if  they  are  carefully  removed  with  the  scissors,  that  the  true 
body  wall  is  beneath.  If  a  small  portion  of  the  body  wall  be 
removed,  exposing  the  internal  or  body  cavity,  placed  on  a  glass 
slide  and  examined  with  a  lens  against  the  light,  it  will  be  seen  to 
consist  of  two  layers  of  muscles.  The  outer  of  these  is  composed  of 
a  thin  layer  of  fibres,  the  obliquus  externus,  the  grain  of  which  runs 
obliquely  upwards  and  outwards.  The  external  oblique  has  under- 
neath it  an  obliquus  interims,  whose  grain  runs  outwards  and  down- 
wards, and  so  the  two-  together  form  the  wall  of  the  body. 

An  incision  may  now  be  made  in  the  body  wall  on  one  side 
close  to  the  linea  alba  from  the  hinder  end  forwards.  In  the  region 
of  the  middle  division  of  the  pectoralis  muscle  a  bone  will  be  en- 
countered. This  must  be  cut  through  with  a  strong  pair  of  scissors, 
care  being  taken  not  to  go  too  deeply  and  damage  the  underlying 
structures.  A  short  distance  in  front  of  this  is  another  bone  that 
may  be  cut  through  in  the  same  way.  These  two  bones  form  part 
of  a  bony  framework,  the  pectoral  girdle,  that  supports  the  fore- 
limb.  A  more  lateral  cut  may  now  be  made,  great  care  being  taken 
not  to  injure  any  of  the  large  blood-vessels  connected  with  the  arm, 
and  so  a  piece  of  the  ventral  body  wall  will  Ue  removed.  A  similar 
operation  can  be  performed  on  the  opposite  side,  thus  leaving  only 
a  narrow  strip  of  tissue  in  the  middle  line.  On  turning  aside  this 
strip  in  the  region  of  the  front  end  of  the  linea  alba  you  will  see  that 
the  anterior  abdominal  vein  leaves  it  and  goes  down  into  the  organs 
of  the  body.  By  cutting  across  the  strip  just  in  front  of  this  point 
the  anterior  part  of  it  may  be  removed.* 

By  thus  removing  strips  of  the  body  wall  a  large  hollow,  the  body 
cavity,  coelom  or  pleuro-peritoneal  cavity,  will  have  been  opened  up. 
In  it  are  situated  the  various  internal  organs  or,  as  they  are  com- 
prehensively termed,  the  viscera.  In  ourselves  it  is  completely 
divided  into  two  cavities,  a  pleural  and  peritoneal,  by  a  transverse 
sheet  of  muscles,  the  diaphragm.  The  purple-coloured  heart  will  be 
seen  situated  close  to  the  sternum  in  a  fairly  tight  fitting  membranous 
bag,  the  pericardium.  Within  this  bag  is  a  hollow,  the  pericardial 
cavity,  actually  a  part  of  the  general  ccelom  that  has  been  cut  off. 
The  heart  is  composed  of  three  separate  chambers,  a  thick-walled 
posterior  pointed  chamber,  the  ventricle,  and  two  more  anterior  thin- 
walled  auricles.  From  the  top  right-hand  side  of  the  ventricle  a 

*  To  obtain  more  freedom  in  the  examination  of  the  various  structures,  the 
linea  alba  with  its  accompanying  vein  and  small  strip  of  body  wall  may  be 
tied,  i.e.  ligatured,  in  two  places  fairly  close  together  and  then  cut  between. 
By  this  means  loss  of  blood  is  prevented. 

C 


i8  AN   INTRODUCTION  TO  ZOOLOGY 

stout  tube  runs  forwards  between  the  auricles  and  then  divides  up 
into  two  trunks,  passing  right  and  left.  This  is  the  truncus  arteriosus, 
and  from  it  spring  the  arteries  or  blood-vessels  that  carry  the  blood 
from  the  heart  all  over  the  body.  If  the  heart  be  turned  to  one  side 
a  triangular  very  thin-walled  sac,  the  sinus  venosus,  will  be  seen. 
Into  this  open  large  blood-vessels,  the  veins,  bringing  the  blood  back 
from  the  whole  of  the  various  parts  of  the  body.  A  network  of  blood- 
vessels will  be  seen  ramifying  all  over  the  viscera,  and  they  are. 
termed  arteries  or  veins,  according  to  whether  they  convey  blood 
away  from  or  towards  the  heart. 

Just  behind  the  heart  are  two  large  reddish-brown  masses,  one 
on  each  side.  These  are  the  two  parts  or  lobes  of  the  liver,  and  are 
joined  together  in  the  middle  line  at  the  front.  Between  them  will 
be  seen  a  small  thin-walled  sac,  the  gall-bladder,  usually  filled  with 
a  dark  green  liquid,  the  gall  or  bile,  which  is  made  by  the  liver  and 
utilised  in  digestion.  On  the  dorsal  side  of  the  liver  and  heart  will 
be  seen  two  bright  pink  sacs  with  honeycombed  walls  ;  these  are  the 
lungs.  By  inserting  a  blowpipe  into  the  glottis  they  can  quite 
easily  be  inflated  and  rendered  conspicuous. 

A  large  whitish  tube  will  be  seen  on  the  left  behind  the  liver. 
This  is  the  stomach,  and  if  traced  forward  dorsal  to  the  liver  it 
will  be  found  to  merge  into  a  slightly  narrower  tube,  the  gullet  or 
oesophagus,  which  in  its  turn  opens  into  the  pharynx.  The  line 
of  demarcation  between  the  stomach  and  oesophagus  is  not  nearly 
so  sharp  in  Rana  as  it  is  in  the  rabbit  or  ourselves.  The  posterior 
end  of  the  stomach  is  marked  off  by  a  slight  constriction,  the  pylorus, 
from  a  very  long  narrow  tube,  the  small  intestine,  the  first  part  of 
which,  lying  more  or  less  parallel  with  the  stomach,  is  called  the 
duodenum,  and  the  remainder  the  ileum.  This  pursues  a  twisted 
course,  and  finally  expands  to  form  a  wide  tube,  the  large  intestine 
or  rectum,  which  communicates  with  the  exterior  through  the  cloaca. 
Thus  we  find  that  the  food  is  passed  from  the  mouth  into  a  tube,  the 
enteric  or  alimentary  canal,  which  has  no  openings  save  the  mouth 
and  cloaca.  In  order  that  the  food  may  be  distributed  to  the 
various  organs  it  is  necessary  for  it  to  pass  through  the  wall  of  this 
canal.  This  is  accomplished  by  its  being  made  soluble  or  digested. 

A  small  yellow  elongated  mass  is  situated  between  the  duodenum 
and  the  stomach,  this  is  the  pancreas,  and  it  also  is  concerned  with 
digestion. 

Near  the  front  end  of  the  rectum  is  the  spleen,  a  globular  structure 
about  the  size  of  a  small  pea  and  dark  red  in  colour. 

The  urinary  bladder  is  a  fairly  large  bilobed  sac  with  very  thin 
walls,  and  is  to  be  found  opening  into  the  ventral  wall  of  the  cloaca 
and  lying  in  the  posterior  part  of  the  body  cavity. 


THE   FROG 


When  we  push  aside  the  alimentary  canal  still  other  organs  are 
brought  into  view,  these  are  the  reproductive  organs,  which,  as  they 
differ  in  the  two  sexes,  need  separate  description.  In  the  male  a 


hy.n. 


ly. 


FIG.  2. — A  male  frog  dissected  from  the  ventral  side. — From  Borradaile. 

a.ab.v.,  anterior  abdominal  vein,  cut  short,  ligatured,  and  turned  back ;  a.musc.,  cut  edge 
of  abdominal  muscles  ;  bl.,  urinary  bladder  ;  c.d.,  common  duct  of  gall  bladder  and  pancreas 
d.ao.,  dorsal  aorta  ;  du.,  duodenum  ;  f.b.,  fat  body  ;   fem.v.,  femoral  vein  ;   g.b.,   gall-bladder 
ht.,  heart  ;    hy.n.,  hypoglossal  nerve  ;  int.,  ileum  ;  i.v.c.,  inferior  vena  cava  ;  k.,  kidney  ;  k.d. 
kidney  duct  with  vesicula  seminalis  ;  lr..  liver  ;    o.,  point  at  which  c.d.  enters  the  duodenum 
pcs.,   pancreas  ;  pl.v.,  pelvic  vein  ;    r.l.,   right   lung  ;    rm.,  rectum  ;  r.p.v.,   renal  portal  vein  ; 
sar.,  sartorius  muscle  ;  sm.,  mylohyoid  muscle  ;  sp.t  spleen  ;  St.,  stomach  ;  t.,  testis  ;  v.v.,  vesical 
vein. 


20  AN    INTRODUCTION  TO  ZOOLOGY 

whitish  ovoid  body,  the  testis,  will  be  seen  on  each  side.  It  is 
covered  by  a  thin  skin  in  which  are  a  number  of  black  pigment 
cells.  A  bright  yellow  finger-shaped  structure,  the  fat  body,  is 
attached  to  the  front  end  of  each  testis. 

The  primary  reproductive  organs  in  the  female  consist  of  the 
ovaries,  which  vary  greatly  at  different  times  of  the  year.  In  the 
early  spring,  before  the  egg  laying  has  taken  place,  they  are  very 
large  indeed,  and  contain  a  large  number  of  spheres,  each  composed 
of  black  and  white  halves.  These  are  the  ova  or  eggs.  After  the 
egg-laying  period  the  ovaries  are  much  smaller,  but  still  of  fair 
size,  and  are  long  lobed  structures  of  a  greyish  colour.  A  long 
white  coiled  tube  will  be  found  dorso-laterally  to  the  ovary, 
extending  from  the  anterior  to  the  posterior  end  of  the  body  cavity. 
The  oviduct,  as  this  tube  is  named,  conveys  the  eggs  to  the  exterior, 
and  is  much  larger  during  the  breeding  season. 

Dorsal  to  the  reproductive  organs,  and  attached  fairly  closely 
to  the  back  of  the  coelom,  are  two  irregularly  oval  bodies  of  a  red 
colour,  the  kidneys.  These  are  concerned  with  the  elimination  of 
the  fluid  waste  material  from  the  body.  From  the  outer  posterior 
end  of  each  a  white  tube,  the  ureter,  passes  backwards  to  open  into 
the  dorsal  wall  of  the  cloaca,  and  this  conveys  the  waste  to  the 
exterior.  In  the  male  this  tube  swells  out  to  form  a  sac,  the 
vesicula  seminalis,  in  which  the  male  reproductive  elements  are 
stored  until  required. 

The  whole  of  the  pleuro-peritoneal  cavity  is  lined  by  a 
thin  glistening  membrane,  the  peritoneum,  more  or  less  well  supplied 
with  pigment  cells.  In  the  mid-dorsal  line  the  membrane  on  each 
side  passes  ventrally  and  forms  a  series  of  folds,  the  mesenteries, 
each  composed  of  two  layers,  which  hold  the  various  viscera  in  their 
places.  The  layer  on  the  body  wall  is  termed  the  parietal,  and  that 
around  the  various  organs  the  visceral  layer  of  the  peritoneum. 
Where  the  two  sheets  of  parietal  peritoneum  are  reflected  ventrally 
to  form  the  mesentery  a  space  is  left,  the  sub-vertebral  lymph  sinus. 
In  this  are  situated  the  main  artery  of  the  body,  the  dorsal  aorta, 
and  a  large  vein  that  has  its  origin  between  the  kidneys,  the  post-caval 
vein. 

On  the  dorsal  side  of  this  sinus  in  the  middle  line  will  be  found  a 
jointed  bony  rod,  the  backbone  or  vertebral  column. 

If  the  frog  now  be  turned  on  its  back  and  the  skin  removed, 
the  vertebral  column  will  be  seen  in  the  middle  line  almost  com- 
pletely hidden  by  muscles.  These  can  be  scraped  off  and  the  dorsal 
part  of  the  backbone  carefully  snipped  away  with  a  strong  pair  of 
scissors.  It  will  be  found  that  the  column  possesses  a  distinct 
cavity,  the  neural  canal,  in  which  is  situated  a  soft  rod-like  structure, 


THE   FROG  21 

the  spinal  cord.  This  is  really  a  tube,  but  its  walls  are  very  thick 
and  the  bore  very  small.  If  the  neural  canal  be  followed  backwards 
it  will  be  found  that  the  spinal  cord  is  reduced  to  a  mere  thread 
before  it  reaches  the  level  of  the  hump  in  the  back  where  the  back- 
bone ends.  The  reduced  part  passes  back  in  the  neural  canal 
accompanied  by  other  similar  threads,  which  also  come  off  from  the 
cord.  Sooner  or  later  these  pass  outwards  through  the  vertebral 
column  to  form  the  nerves.  At  the  anterior  end  the  spinal  cord  is 
continued  into  the  head,  where  it  lies  in  a  bony  case,  the  skull. 
Here,  however,  it  no  longer  remains  simple,  but  swells  out  in  various 
parts  to  form  a  complex  structure,  the  brain,  which  we  shall  have  to 
consider  in  detail  later  on.  The  whole  of  this  soft  structure,  the 
brain  and  spinal  cord,  is  known  as  the  central  nervous  system,  and 
it  is  the  great  controlling  and  co-ordinating  organ  of  the  body.  It 
is  connected  by  nerves  with  the  eyes,  ears,  nose,  etc.,  whence  it 
receives  messages  from  the  outside  world,  and  it  is  also  connected 
with  the  various  muscles,  and  so  is  able  to  regulate  the  activities  of 
the  animal  as  a  whole. 

The  frog  will  serve  to  illustrate  the  general  plan  of  structure 
common  to  those  animals  with  backbones,  i.e.  Vertebrate  animals. 
An  elongated  cavity,  situated  partly  in  the  skull  and  partly  in  the 
vertebral  column,  extends  along  the  head  and  trunk  in  the  mid- 
dorsal  line,  and  contains  the  central  nervous  system.  The  nervous 
system  itself  is  hollow.  The  central  canal,  as  the  cavity  is  called, 
in  the  spinal  cord  enlarges  to  form  a  series  of  spaces  or  ventricles 
in  the  brain.  On  the  ventral  side,  in  the  trunk  only,  is  another 
much  larger  cavity,  the  pleuro-peritoneal  cavity,  completely 
separated  from  the  neural  cavity  by  parts  of  the  backbone  and  also 
by  the  dorsal  muscles.  A  small  portion  of  this  is  separated  off  to 
form  the  pericardium,  which  is  situated  below  the  gut.  A  long 
coiled  tube,  the  alimentary  canal,  in  which  a  number  of  different 
parts  are  distinguishable,  runs  through  the  remaining  portion  of 
this  cavity,  in  which  are  situated  the  various  structures  which  we 
term  the  viscera. 

Before  leaving  the  frog  its  blood  should  be  examined  under 
the  microscope.  This  may  be  done  by  putting  a  drop  of  it  on  a 
clean  slide  and  adding  to  it  a  drop  of  physiological  salt  solution 
(i.e.  '5  gram  of  salt  dissolved  in  100  c.c.  of  distilled  water)  to 
prevent  it  clotting,  and  then  covering  it  carefully  with  a  clean  cover- 
slip.  It  will  be  found  to  consist  of  a  fluid  or  plasma,  in  which 
float  an  enormous  number  of  small  solid  bodies  or  corpuscles. 
Closer  examination  will  show  that  these  are  not  all  alike,  but  are 
of  two  different  sorts.  Some  of  them  are  oval,  constant  in  form, 


22  AN   INTRODUCTION   TO  ZOOLOGY 

and  flattened  with  a  bulged  central  portion  ;  these  are  a  pale 
yellowish-red  colour,  and  are  called  the  red  corpuscles.  The  others 
are  less  numerous,  smaller  and  colourless.  They  also  differ  from  the 
red  in  that  they  are  not  of  a  definite  shape,  and  if  watched  carefully 
will  be  found  to  change  their  shape  slightly.  They  are  termed  the 
white  corpuscles  or  leucocytes.  If  a  drop  of  weak  acetic  acid  and 
some  dye,  such  as  methyl-blue  or  methyl-green,  be  added  to  the 
slide,*  an  alteration  will  be  seen  in  both  sorts  of  corpuscles.  They 
both  become  faintly  tinged  with  the  dye,  but  within  them  will 
appear  a  definite  circumscribed  part  that  becomes  much  more 
deeply  stained.  This  is  the  nucleus,  and  all  the  corpuscles  will 
contain  one  such  body,  while  certain  of  the  leucocytes  may  contain 
more  than  one. 

As  we  have  learned  already,  living  matter  is  composed  of  a 
substance  called  protoplasm,  and  now  we  have  seen  that  in  the  blood 
the  protoplasm  is  in  the  form  of  small  pieces,  the  corpuscles.  VHthin 
the  corpuscle  itself,  however,  we  find  that  at  least  two  parts  are  to 
be  distinguished  ;  a  central  more  dense  portion  that  stains  deeply 
with  certain  dyes,  and,  secondly,  a  surrounding  zone,  less  dense, 
that  stains  more  lightly.  Such  a  small  living  mass  we  designate  a 
cell,  and  it  may  be  defined  as  a  mass  of  protoplasm,  usually  minute, 
consisting  of  a  central  dense  deeply-staining  nucleus  surrounded  by  a 
clearer,  more  lightly  staining  material,  usually  termed  the  cytoplasm. 
The  corpuscles  are  very  simple  types  of  cells,  and  do  not  exhibit  the 
modifications  that  we  find  in  the  cells  which  with  their  products 
constitute  the  entire  body. 

This  preliminary  investigation  of  the  frog  has  served  to 
enlarge  our  conception  of  a  complex  animal.  We  have  seen  that  it 
is  capable  of  swimming,  jumping,  and  performing  a  number  of 
voluntary  actions  that  represent  a  certain  amount  of  work,  and  need 
the  expenditure  of  a  corresponding  amount  of  energy.  Moreover, 
a  very  casual  examination  of  the  living  frog  is  sufficient  to  show 
that  the  movements  of  the  various  parts  of  the  body  are  not 
spasmodic,  but  are  co-ordinated  in  such  a  way  as  to  produce  a 
definite  result.  The  frog,  like  all  other  animals,  increases  in  size, 
and  by  means  of  eggs  is  able  to  produce  a  number  of  animals,  tad- 
poles, which,  after  undergoing  a  series  of  changes,  become  similar 
to  their  parents.  The  energy  for  these  various  activities  is  obtained 
in  a  potential  form  from  the  food,  which  is  first  digested  and  then 
assimilated,  or  made  part  of  the  body  of  the  animal.  The  energy 
thus  stored  is  .released  by  slow  combustion  or  oxidation,  and  the 

*  This  process  will  be  hastened  if  the  acetic  acid  and  dye  be  placed  on  one 
side  of  the  cover-slip  and  the  excess  of  fluid  drawn  off  from  the  oppos  ite  side 
by  means  of  a  piece  of  blotting-paper,  thus  causing  the  two  fluids  to  be  drawn 
under  the  cover-slip  much  more  rapidly. 


THE   FROG  23 

oxygen  necessary  for  this  process  is  obtained  from  the  atmosphere 
by  respiration.  The  oxidation  of  the  various  substances  results  in 
the  production  of  waste  materials,  which  must  be  removed  from  the 
body  or  excreted.  For  the  carrying  on  of  these  processes  we  find 
a  collecting  and  distributing  agent  in  the  blood,  which  carries  oxygen 
and  food  to  the  places  where  they  are  wanted,  and  also  collects  up  the 
waste  matter  so  that  it  can  be  eliminated  by  the  kidneys  and  lungs. 

In  order  to  perform  all  the  various  operations  in  as  efficient  a 
manner  as  possible,  we  find  that  the  body  is  divided  up  into  a  large 
number  of  separate  parts,  each  specially  modified  to  perform  one  or 
two  functions  ;  such  parts  are  termed  organs.  Thus  the  ovaries 
and  testes  are  reproductive  organs,  the  pancreas  and  liver  digestive 
organs,  and  so  on.  The  maintenance  of  life  depends  on  the  correla- 
tion of  the  activities  of  the  various  organs,  and  when  an  important 
one  of  them  is  put  out  of  action  by  disease  the  other  vital  phenomena 
of  the  animal  are  brought  to  a  standstill,  or,  as  we  say,  the 
animal  dies.  As  a  rule  an  organ  is  not  homogeneous  throughout, 
but  is  composed  of  various  kinds  of  living  material ;  each  separate 
kind  is  spoken  of  as  a  tissue.  Again,  a  number  of  organs  are  often 
linked  together,  and  their  functions  are  contributory  to  one  main  end. 
A  group  of  such  organs  constitutes  a  system.  The  alimentary  canal 
with  its  various  parts,  the  liver  and  pancreas,  form  the  digestive 
system  for  example. 

We  can  thus  recognise  in  the  higher  animals  nine  such  systems, 
as  follows  :  (i)  the  skeletal,  (2)  the  muscular,  (3)  the  integumentary, 
(4)  the  alimentary  or  digestive,  (5)  the -respiratory,  (6)  the  circulatory, 
(7)  the  excretory,  (8)  the  reproductive,  and  (9)  the  nervous  and 
sensory.  In  addition  to  these  there  are  a  number  of  more  or  less 
isolated  organs  collectively  termed  the  ductless  glands,  whose 
functions  in  some  cases  are  but  imperfectly  understood,  and  they 
may  conveniently  be  dealt  with  together,  although  they  do  not 
constitute  a  system.  We  shall  now  pass  on  to  consider  the  various 
systems  in  Rana  from  the  point  of  view  of  their  structure,  both 
gross  and  minute,  and  their  physiology. 

Skeletal  System. 

Certain  portions  of  the  hard  framework,  or  skeleton,  within 
the  body  have  already  been  noticed,  and  it  is  now  necessary  to  make 
a  more  detailed  study  of  its  constituent  parts.  This  may  be  done 
on  prepared  skeletons,  or,  preferably,  on  one  you  have  made  for 
yourself.* 

*  A  skeleton  may  easily  be  prepared  by  removing  as  much  as  possible  of 
the  flesh  from  a  frog  and  then  leaving  it  to  go  bad  or  macerate  in  water.  The 
muscles  can  then  be  picked  off  or  removed  with  a  stiff  brush.  It  may  be  done 
more  quickly,  but  not  so  well,  by  dipping  the  frog  repeatedly  into  hot  water. 


24  AN   INTRODUCTION   TO  ZOOLOGY 

The  skeleton  of  the  frog  consists  of  two  different  tissues  :  a  softer, 
more  elastic  one,  cartilage  ;  and  a  harder,  firmer  one,  bone.  In  the 
tadpole,  and  even  in  the  young  frog,  the  entire  skeleton  is  formed  of 
cartilage,  but  as  it  grows  up,  although  some  cartilage  persists,  the 
greater  part  of  it  is  changed.  It  is  replaced  by  bone,  and,  in  addition, 
other  bones  are  formed  where  no  cartilages  existed  previously. 
These  latter  are  formed  by  bone  tissue  being  laid  down  in  a  membrane, 
and  hence  are  termed  membrane  bones,  to  distinguish  them  from 
cartilage  bones,  which  were  preceded  by  cartilage.  This  is  an 
important  distinction  that  must  be  borne  in  mind  when  considering 
the  vertebrate  skeleton. 

The  skeleton  forms  a  framework,  giving  support,  rigidity  and 
shape  to  the  whole  body.  It  serves  in  the  main  for  the  attachment 
of  muscles  by  providing  a  series  of  solid  structures,  which  at  the 
same  time  act  as  levers  and  form  joints.  Although  principally 
concerned  with  muscular  activity,  indeed,  mainly  with  locomotion, 
the  skeleton  also  serves  as  a  protection  for  the  underlying  organs 
in  some  parts,  e,g.  the  sternum  protects  the  heart,  and  the  neural 
canal  and  skull  protect  the  spinal  cord  and  the  brain. 

Like  the  whole  animal  the  skeleton  may  be  divided  into  an  axial 
portion,  consisting  of  the  vertebral  column  and  the  skull,  and  an 
appendicular  portion,  composed  of  the  limbs  and  the  girdles,  by  means 
of  which  they  are  attached  to  the  body. 


Vertebral  Column. 

The  backbone  consists  of  nine  short  separate  parts  or 
vertebrae,  and  a  much  longer  posterior  one,  the  urostyle.  All  the 
vertebrae,  except  the  first  one  and  the  last  two/are  more  or  less  similar, 
and  any  one  can  be  taken  as  typical.  A  single  vertebra  forms  an 
entire  bony  ring  surrounding  the  neural  canal,  with  a  thickened 
ventral  part,  the  centrum,  upon  which  is  supported, the  hoop-shaped 
neural  arch.  The  front  end  of  the  centrum  is  concave  and  the 
hinder  end  convex,  a  condition  which  we  describe  as  pro-ccelus. 
The  two  adjacent  ends  of  succeeding  vertebrae  fit  into  one  another, 
and  so  form  a  shallow  ball  and  socket  joint.  To  ensure  ease  of 
motion  they  are  capped  with  smooth  articular  cartilage,  and  between 
them  is  a  fluid-filled  space,  the  synovia!  cavity.  This  is  surrounded 
at  its  periphery  by  a  tough  tissue,  the  intervertebral  capsular  liga- 
ment. In  the  preparation  of  the  skeleton  both  cavity  and  ligament 
are  destroyed. 

The  neural  arch  on  each  side  is  composed  of  a  vertical  pedicle 
and  a  more  expanded  horizontal  lamina,  forming  the  roof  of  the 
neural  canal.  In  the  mid-dorsal  line  it  bears  a  short  backwardly- 


THE   FROG  25 

directed  process,  the  neural  spine,  serving  for  the  attachment  of 
muscles.     A  flattened  rod  of  bone,  the  transverse  process,  passes 


poi 


P0.2 


FIG.  3. — Vertebrae  Rana.  A,  5th  vertebra  dorsal  aspect ;  B,  5th  vertebra 
posterior  aspect ;  C,  Atlas  anterior  aspect  ;  D,  Atlas  posterior  aspect ; 
E,  F,  G,  7th,  8th  and  gth  vertebrae  ventral  aspect. 

c.,  centrum  ;  f.c.,  facet  for  articulation  with  exoccipital  bone  ;  /.«.,  facet  for  articulation 
with  urostyle  ;  /.,  lamina  ;  n.a..  neural  arch  ;  n.c.,  neural  canal ;  n.s.,  neural  spine  ;  p.t., 
pre-zygapophysis  ;  po.z.,  post-zygapopbysis  ;  t.p.,  transverse  process. 


outwards  from  the  side  of  the  pedicle  near  its  base,  and  is  tipped  with 
cartilage  in  the  second,  third  and  fourth  vertebrae. 

A  pair  of  processes,  the  zygapophyses,  is  given  off  from  each  end 


26  AN   INTRODUCTION   TO   ZOOLOGY 

from  the  junctions  of  the  pedicles  and  laminae.  The  front  or  pre- 
zygapophyses  have  smooth  cartilage-covered  surfaces,  the  articular 
facets,  facing  upwards  and  a  little  inwards,  while  the  post-zygapo- 
physes  have  similar  facets  directed  downwards  and  outwards. 
Thus,  in  addition  to  the  intercentral  joint  or  articulation,  a  pair  of 
zygapophyseal  articulations  is  present  between  successive  vertebrae. 
It  is  characteristic  of  the  vertebrates  in  general  that  when  zygapo- 
physes  are  present  articulating  surfaces  of  the  anterior  face  upwards 
or  inwards,  or  both,  and  the  posterior  downwards  or  outwards,  or 
both. 

The  first  vertebra,  or  atlas,  differs  markedly  from  the  rest.  The 
centrum  and  neural  spine  are  reduced,  and  the  transverse  processes 
and  pre-zygapophyses  are  entirely  absent.  Its  anterior  end  is 
provided  with  two  concavities,  by  means  of  which  it  articulates 
with  the  skull.  The  third  vertebra  has  very  well-developed  trans- 
verse processes,  serving  for  the  attachment  of  muscles  connected 
with  the  girdle  bearing  the  fore-limbs.  The  centrum  of  the  eighth 
vertebra  is  hollow  in  front  and  behind,  a  condition  that  we  describe 
as  amphicoelous.  The  ninth  vertebra  is  also  modified,  and  since  its 
long  somewhat  backwardly  directed  transverse  processes  articulate 
with  the  girdle  of  the  hind  limbs,  it  is  called  the  sacrum.  The  front 
end  of  its  centrum  is  convex,  the  posterior  end  bears  two  small 
knobs  and  has  no  zygapophyses. 

Apart  from  the  differences  in  detail,  the  nine  vertebrae  form  a 
series  of  similar  structures  repeated  one  behind  the  other.  Structures 
that  are  alike,  made  of  the  same  parts,  and  developed  in  the  same 
way,  are  said  to  be  homologous,  and  when  repeated,  as  in  the 
vertebral  column,  are  described  as  serially  homologous.  When 
placed  side  by  side,  as  in  the  living  frog,  a  gap  is  left  between  the 
pedicle  of  one  vertebra  and  the  next.  This  is  the  intervertebral 
foramen,  and  through  it  the  nerves  from  the  spinal  cord  pass  into 
the  body. 

The  urostyle  is  a  long  unsegmented  rod  of  bone,  bearing 
at  its  anterior  end  two  facets,  which  articulate  with  the  knob-like 
projections  from  the  centrum  of  the  sacrum.  A  short  distance 
behind  these  is  a  pair  of  small  foramina,  to  allow  the  last  pair  of 
spinal  nerves  to  leave  the  neural  canal.  The  urostyle  is  to  be 
regarded  as  representing  a  series  of  fused  and  reduced  vertebrae,  and 
faint  indications  of  segmentation,  or  even  traces  of  one  or  two 
vertebrae,  are  sometimes  to  be  seen  at  the  anterior  end. 

The  frog  is  markedly  different  from  ourselves  in  not  possess- 
ing any  ribs  nor  any  indications  of  them. 


THE  FROG  27 

Skull. 

The  skull  of  the  frog,  as  of  all  higher  vertebrates,  consists 
of  a  case  for  the  brain,  the  cranium,  with  which  are  connected  the 
sense  capsules,  the  jaws  and  the  hyoid  apparatus.  The  sense 
capsules  comprise  three  pairs  :  the  auditory,  lodging  the  organ  of 
hearingand  attached  to  the  hinder  end  of  the  cranium ;  the  olfactory, 
lodging  the  organ  of  smell  and  attached  to  the  anterior  end  ;  and 
lastly,  the  optic  capsule  or  eye,  between  the  former  two.  In  the 
frog  the  optic  capsule  is  not  attached  to  the  skull,  but  the  other 
bones  are  so  arranged  that  a  space,  the  orbit,  is  left  for  it.  Two  jaws 
are  present,  the  upper,  firmly,  although  indirectly,  attached  to  the 
cranium,  and  consequently  not  capable  of  independent  movement, 
and  the  lower,  connected  with  the  upper  by  means  of  a  joint  at  the 
hinder  end,  and  free  to  move  in  a  vertical  direction.  The  hyoid 


po 


FIG.  4.— The  skull  of  Ran  a.     A,  Dorsal  aspect ;  B,  Ventral  aspect . 

e.o,,   exoccipital  ;  f.m.,  foramen  magnum  ;   f.o.  fenestra  ovalis ;    /.p.,  fronto-parietal ;    mx.> 
maxilla  ;    na.,  nasal  ;    o.f.,  optic  foramen  ;    pal.,  palatine  ;    par.,  parietal ;    p.mx.,  pre-maxilla 
P.O.,  pro-otic  ;    pt.  pterygoid  ;    qu.,  quadrato-jugal  ;    s.m.,  septo-maxillary  ;    sp.  sphenethmoid  ; 
si).,  squamosal  ;  vo.,  vomer  ;  v.t.,  vomerine  teeth. 

apparatus  consists  of  the  flat  hyoid  plate,  situated  as  already  noted, 
in  the  floor  of  the  mouth,  and  its  projecting  horns. 

In  the  middle  of  the  back  of  the  skull  is  a  large  hole,  the 
foramen  magnum,  through  which  brain  and  spinal  cord  are  con- 
tinuous. On  each  side  of  this,  and  forming  its  entire  margin,  save 
for  small  pieces  of  cartilage  in  the  mid-dorsal  and  ventral  lines,  is  a 
cartilage  bone,  the  exoccipital,  which  is  produced  backwards  into  a 
rounded  knob,  the  occipital  condyle.  The  condyle  fits  into  and 
articulates  with  the  corresponding  concavity  on  the  front  of  the 
atlas  vertebra.  If  the  skull  be  placed  in  its  natural  position  with 
regard  to  the  atlas,  it  will  be  seen  that  on  the  dorsal  surface  a  con 
siderable  space  is  left  between  it  and  the  neural  arch  of  the  atlas. 


28  AN   INTRODUCTION  TO  ZOOLOGY 

In  life  this  space  is  covered  by  a  tough  skin,  the  atlanto-occipital 
membrane.  The  exoccipital  not  only  forms  the  back  of  the  cranium, 
but  also  the  back  and  part  of  the  floor  of  the  auditory  capsule,  which 
forms  a  prominent  structure  fixed  to  each  side  of  the  hinder  end  of 
the  cranium.  The  front  wall  of  the  capsule  is  formed  by  another 
cartilage  bone,  the  pro-otic,  at  whose  lower  inner  corner  is  situated 
a  foramen,  through  which  the  fifth,  sixth  and  seventh  nerves  given 
off  from  the  brain  leave  the  skull.  The  lateral  wall  of  the  auditory 
capsule,  its  roof  and  floor,  are  formed  of  cartilage  more  or  less 
covered  by  superposed  membrane  bones.  In  the  posterior  part  of 
the  lateral  wall  is  a  small  opening,  the  fenestra  ovalis,  leading  to  the 
sensory  part  of  the  ear.  This  is  closed  by  a  small  block  of  cartilage, 
the  stapes,  to  which  is  attached  a  short  thin  bony  rod,  the  columella, 
and  this  in  its  turn  is  connected  with  the  tympanum. 

From  the  auditory  capsule  the  cranium  is  continued  forward  as  a 
cartilaginous  tube  to  the  front  end  of  the  orbit.  Half-way  along 
its  side  it  is  perforated  by  a  large  circular  foramen,  through  which 
the  second  nerve  from  the  brain  passes  to  the  eye.  The  dorsal 
surface  is  also  incomplete,  and  is  perforated  at  the  front  end  by  a  large 
hole,  the  anterior  fontanelle,  and  at  the  hinder  end  by  two  smaller 
holes,  the  posterior  fontanelles.  The  fontanelles  are  completely 
hidden  by  the  paired  overlying  membrane  bones,  the  fronto-parietals, 
which  spread  out  slightly  at  the  posterior  end  and  help  to  cover  the 
inner  part  of  the  roof  of  the  auditory  capsule.  The  cartilaginous 
floor  of  the  cranium  is  similarly  covered  by  a  membrane  bone,  the 
unpaired  parasphenoid,  a  T-shaped  bone,  whose  arms  underly  the 
whole  of  the  bases  of  the  auditory  capsules. 

The  anterior  end  of  the  cranium  is  formed  by  the  girdle  bone, 
or  sphenethmoid.  This  is  a  cartilage  bone  completely  encircling 
the  cranium,  and  it  has  a  deep  hollow  at  each  end.  The  partition 
between  the  two  cavities  is  perforated  for  the  passage  of  the  nerves 
of  smell.  It  is  a  bone  that  is  peculiar  to  the  Amphibia.  The  cavity 
at  the  front  end  is  divided  into  two,  a  right  and  a  left,  by  means  of 
the  cartilaginous  internasal  septum,  which  passes  forwards  and 
completely  separates  the  two  olfactory  cavities  from  one  another. 

The  front  end  of  the  cranium  bears  the  two  olfactory  capsules, 
two  incomplete  cartilaginous  structures.  Over  the  dorsal  part  of  the 
capsule  is  situated  a  triangular  membrane  bone,  the  nasal,  and  just 
in  front  of  this  is  a  very  small  bone,  the  septomaxillary.  A  triangular 
membrane  bone,  the  vomer,  lies  beneath  the  capsule  and  bears  at 
its  hinder  inner  corner  a  small  group  of  pointed  teeth,  the  vomerine 
teeth,  while  its  hinder  outer  corner  is  deeply  notched  to  form  the 
boundary  of  the  internal  nostril. 

The  outline  of  the  lower  part  of  the  skull  is  arch-shaped,  and 


THE   FROG  29 

is  formed  by  the  two  jaws,  both  of  which  are  first  laid  down  in  carti- 
lage.    The  upper  jaw  in  the  adult,  however,  is  covered  by  three 
membrane  bones.     The  quadrato-jugal  is  a  short  bone  sheathing  the 
jaw  at  its  hinder  end  and  bearing  no  teeth.  !  From  this  the  much 
longer  maxilla  runs  forward,  and  lastly,  the  front  part  is  completed 
by  a  smaller  bone,  the  pre-maxilla,  which  joins  its  fellow  in  the  middle 
line  and  also  sends  a  small  dorsal  process  backwards  to  form  the 
inner  boundary  of  the  external  nostril.     Both  the  pre-maxilla  and 
maxilla  bear  a  row  of  small  sharp  conical  teeth.     The  upper  jaw 
is  immovably  fixed  to  the  cranium  at  the  posterior  end  and  again 
near  the  anterior  end.     The  hinder  connection  is  made  by  the 
quadrate  cartilage,  or,  as  it  is  also  termed,  the  suspensorium.    This 
is  a  cartilaginous  rod,  forked  at  its  inner  end,  that  runs  outwards 
from  the  side  of  the  auditory  capsule  and  bears  at  its  outer  end  a 
hollow,  glenoid  cavity,  with  which  the  lower  jaw  articulates.    The 
dorsal  side  of  the  suspensorium  is  covered  by  one  limb  of  a  large 
tri-radiate  membrane  bone,  the  squamosal,  of  whose  other  limbs  one 
runs  to  the  pro-otic  bone  and  the  other  forwards  towards  the  second 
point  of  attachment  of  the  upper  jaw.     A  similar  triradiate  bone, 
the  pterygoid,  in  part  underlies  the  quadrate  cartilage.     Its  three 
limbs  are  similarly  distributed,  and  the  anterior  one  reaches  and 
joins  with  the  upper  jaw.    The  anterior  attachment  of  the  jaw  is  also 
formed  by  a  bar  of  cartilage,  the  palatine  cartilage,  passing  outwards 
from  the  sphenethmoid  bone.    This  is  underneath  the  hinder  edge  of 
the  nasal  bone  and  encased  on  the  ventral  side  by  a  slender  mem- 
brane bone,  the  palatine  bone.    Like  the  pterygoid,  the  palatine  bone 
in  the  higher  animals  is  a  cartilage  bone,  but  this  is  not  so  in  Rana. 
The  lower  jaw  or  mandible  is  similarly  a  cartilaginous  arch, 
Meckel's  cartilage,  almost  entirely  covered  by  membrane  bones, 
none  of  which  bears  teeth.     The  posterior  covering  bone  is  the  short 
angulo-splenial,  which  possesses  a  small  articular  knob,  and  in  front 
of  it  is  situated  the  long  dentary.     At  the  anterior  end  in  young 
frogs  a  small  piece  of  cartilage  is  left,  but  in  older  skulls  this  ossifies 
to  form  a  small  bone,  the  mento-meckelian,  and  the  two  halves  of  the 
jaw  are  bound  together  by  a  tough  ligament. 

The  hyoid  cartilage,  or  body  of  the  hyoid,  situated  in  the 
floor  of  the  mouth,  is  a  thin  shield-shaped  plate,  giving  off  processes 
at  its  fore  and  hind  ends.  The  anterior  processes,  or  cornua,  are 
very  slight  and  cartilaginous,  and  pass  first  forwards,  then  backwards 
and  upwards.  They  are  finally  attached  to  the  back  of  the  auditory 
capsules,  and  the  columellae  are  formed  from  small  pieces  separated 
off  from  their  anterior  ends.  The  posterior  cornua  are  short  some- 
what stout  rods  of  cartilage  bone,  that  pass  back  one  on  each  side  of 
the  glottis. 


AN    INTRODUCTION   TO   ZOOLOGY 


Appendicular  Skeleton. 

The  skeleton  of  the  fore-limb  consists  of  a  number  of  bones, 
which  may  be  divided  up  in  the  same  way  as  the  limb  itself.  Inside 
the  brachium  is  a  single  bone,  the  humerus,  which 
serves  as  a  good  example  of  what  is  known  as  a 
long  bone,  and  is  divisible  into  a  proximal  part, 
i.e.  the  end  near  the  body  called  the  head,  a  long 
middle  part  the  shank,  and  a  distal  portion  the 
condyle.  Like  all  the  bones  of  the  limbs  it  is  a 
cartilage  bone,  and  its  two  ends,  termed  epiphyses, 
ossify  separately  from  the  shank,  with  which  they 
afterwards  fuse.  The  head  of  the  humerus  is 
rounded  and  covered  with  articular  cartilage. 
From  its  head  a  strong  keel,  the  deltoid  ridge, 
runs  along  the  front  side  of  the  shank.  The 
distal  end  has  two  prominent  lateral  condylar 
ridges,  between  which  is  a  depression,  the  tro- 
chanter,  in  part  filled  with  a  knob  covered  with 
articular  cartilage.  The  skeleton  of  the  anti- 
brachium  consists  of  a  bone,  the  radio-ulna, 
formed  by  the  union  of  two  separate  bones,  the 
radius  and  ulna,  showing  distinct  signs  of  its 
double  origin.  Its  proximal  end  is  hollowed  to 
receive  the  humerus,  and  it  projects  backwards 
beyond  this  as  the  olecranon  process.  At  its 
distal  end  it  articulates  with  a  series  of  six  wrist 
bones,  or  carpalia.  The  first  row  consists  of 
three  bones  called  the  radiale,  the  intermedium, 
and  the  ulnare.  The  first  and  last  are  situated 
at  the  ends  of  the  radial  and  ulnar  parts  of  the 
combined  bone,  and  the  intermedium  lies  between 
FIG.  5. — Longitu-  them.  The  second  row  als'o  consists  of  three 

dmal  section  of    bones,  two  small  ones  and  a  larger  posterior  one, 
the  femur,  show-  ..  ,,  .        ,    .        ., 

ing  the  compact    representing  three  smaller  ones  fused  together. 

and    cancellous    Following  this  comes  the  skeleton  of  the  manus. 

tissues,  and  the    jne  proximai  part  consists  of  a  row  of  five  small 

medullary    cav-  . 

ity. From  Fur-    bony  rods,  the    metacarpals.     The    anterior    of 

neaux. "  these  is  smaller  than  the  rest,  and  is  all  that  re- 

mains of  the  first  digit,  the  pollex  or  thumb.  The 
other  metacarpals  are  followed  by  a  varying  number  of  small  rods, 
the  phalanges.  Two  are  borne  on  the  first  or  fore-finger,  two  on  the 
second,  and  three  each  on  the  third  and  fourth.  When  a  limb  is 
stretched  out  the  side  on  which  the  big  toe  or  thumb  is  situated 


THE   FROG  31 

is  termed  the  anterior  or  pre-axial  border  of  the  limb,  and  the  other 
the  posterior  or  post-axial  border. 

The  fore-limb  is  attached  to  the  body  by  means  of  the  pectoral 
girdle,  which  consists  of  a  bony  framework,  almost  completely 
encircling  the  body,  but  incomplete  on  the  dorsal  side.  Each  half 
is  divided  into  two  portions  by  the  glenoid  cavity,  a  hollow  depression 
lined  by  articular  cartilage,  into  which  the  head  of  the  humerus  fits. 
The  dorsal,  scapular  part,  consists  of  two  elements,  a  fairly  stout 
cartilage  bone,  the  scapula,  which  forms  part  of  the  glenoid  cavity, 
and  a  thinner  wider  bone,  the  supra-scapula,  situated  more  dorsally. 
The  supra-scapula  never  completely  ossifies,  and  its  dorsal  edge  is 


hh 
FIG.  6. — A,  Left  fore-limb  Rana'.   B,  Left  half  pectoral  girdle  Rana. 

a.,  articular  knob  of  humerus  ;  c.f.,  coracoid  foramen  ;  cl.,  clavicle  ;  co.,  coracoid  :  c.r.,  con- 
dylar  ridge  ;  d.c.,  distal  carpalia  ;  d.r.,  deltoid  ridge  ;  ec.,  epicoracoid  ;  es,,  episternum  ;  /.&., 
facet  for  articulation  with  head  of  humerus  ;  h.,  humerus  ;  h.h.,  head  oi  humerus  ;  m.,  meta- 
carpalia  ;  ms.,  mesosternum  ;  os.,  omosternum  ;  ^p.,  phalanges  ;  r .,  radiale  ;  r.u.,  radio-ulna  ; 
sc.,  scapula  ;  s.s.,  supra-scapula  ;  «.,  ulnare  ;  xs.,  xiphisternum  ;  /.,  ist  metacarpal ;  II. -V ., 
digits. 

always  formed  of  cartilage.  The  ventral  part  of  the  girdle  is  known 
as  the  coracoid  portion.  From  the  anterior  corner  of  the  glenoid 
cavity  a  narrow  rod,  the  pre-coracoid,  runs  inwards  toward  the 
middle  line,  not  quite  meeting  its  fellow  from  the  opposite  side. 
Actually  only  a  very  narrow  strip  of  this  is  visible,  because  a  thin 
bone,  the  clavicle,  is  wrapped  round  it.  The  clavicle  is  the  only 
membrane  bone  in  the  appendicular  skeleton,  and  may  or  may  not 
be  represented  in  ourselves  by  the  collar  bone.  The  posterior  ventral 
part  of  the  glenoid  cavity  is  formed  by  the  coracoid  bone,  which 
passes  inwards  and  backwards,  leaving  a  space,  the  coracoid  foramen, 
between  it  and  the  clavicle.  Its  inner  end  is  expanded,  but,  like 
the  pre-coracoid,  does  not  touch  its  fellow  of  the  other  side.  The 


32  AN   INTRODUCTION   TO  ZOOLOGY 

inner  ends  of  the  coracoid  and  pre-coracoid  are  joined  by  a  narrow 
strip  of  cartilage,  the  epicoracoid,  and  the  two  epicoracoids  meet  in 
the  mid- ventral  line.  Passing  forward  from  these,  also  in  the  middle 
line,  is  a  short  bony  rod,  the  omosternum,  and  in  front  of  this  is  a 
thin  circular  plate  of  cartilage,  the  episternum.  Behind  the 
epicoracoids  is  a  bony  bar,  the  sternum  or  mesosternum,  and  posterior 
to  this  again  is  a  bilobed  cartilaginous  expansion,  the  Xiphisternum. 
It  has  been  pointed  out  previously  that  in  addition  to  supporting 
the  fore-limbs  and  giving  attachment  to  their  muscles  the  ventral 
part  of  the  pectoral  girdle  forms  a  protection  for  the  heart.  Although 
single  in  the  adult,  the  various  sternal  elements  arise  from  the  paired 
rudiments.  It  is  not  yet  clear  how  far  they  correspond  to  the  breast 
bones  in  man,  for  they  arise  in  connection  with  the  coracoid  bones, 
whereas  in  man  the  sternum  is  developed  in  relation  to  the  ventral 
ends  of  the  ribs. 

The  bones  of  the  hind-limb  are  very  similar  to  those  of  the 
fore-limb  and  homologous  with  them.  The  bone  of  the  thigh  is  the 
femur,  a  long  slightly  curved  bone  with  a  well-marked  rounded  head 
and  the  distal  end  expanded  laterally.  The  leg  contains  the  tibio- 
fibula,  and  the  well-marked  groove  running  along  this  indicates  its 
origin  from  two  separate  bones.  In  the  tarsus  are  two  rows  each 
of  two  bones.  The  proximal  two  are  quite  long  and  united  together 
at  both  ends.  The  pre-axial  bone  is  termed  the  astragalus,  and  the 
slightly  larger  post-axial  one  the  calcaneum.  The  distal  tarsalia 
are  quite  small  and  easily  overlooked.  Unlike  the  hand,  five  com- 
plete digits  are  present  in  the  foot,  and  there  is  also  a  small  additional 
toe.  Each  digit  has  a  well-developed  metatarsal,  and  the  number  of 
phalanges  is  two  on  the  first  and  second  toes,  three  on  the  third  and 
fifth,  and  four  on  the  fourth.  The  extra  toe,  or  calcar,  consists  of  a 
very  short  broad  metatarsal  on  the  pre-axial  side  of  the  hallux  or 
big  toe,  and  sometimes  also  indications  of  a  phalanx.  Supporting 
the  calcar  is  a  small  bony  nodule  that  may  represent  a  displaced 
tarsal. 

The  hind-limb  is  suspended  by  the  pelvic  girdle  In  its  primitive 
condition  this  girdle,  like  the  pectoral,  forms  an  incomplete  hoop 
around  the  posterior  part  of  the  body.  It  has  been  modified  in  the 
frog  in  accordance  with  the  jumping  habits  of  the  animal,  and  the 
ventral  end  has  rotated  through  an  angle  of  nearly  90  degrees,  so 
that  instead  of  passing  to  the  ventral  side  it  is  dorsal  and  almost  in 
a  line  with  the  backbone.  The  conspicuous  hump  on  the  back  of  the 
frog  marks  the  point  of  attachment  of  the  pelvic  girdle  to  the  back- 
bone. The  girdle  is  similar  in  shape  to  the  "  wish-bone  "  or  merry- 
thought of  a  bird,  and  it  exhibits  on  each  side  a  deep  cup-shaped 
hollow,  the  acetabulum,  into  which  the  head  of  the  humerus  fits 


THE   FROG 


33 


The  two  long  bones  articulating  with  the  processes  of  the  sacral 
vertebrae  are  the  ilia.  Each  possesses  a  well- 
marked  crest,  and  unites  with  its  fellow 
posteriorly  in  the  middle  line  to  form  a 
part  of  the  acetabulum.  The  remaining 
part  of  the  wall  of  this  depression  is 
formed  by  two  bones,  in  front  by  the  small 
pubis,  which  never  completely  ossifies,  and 
the  larger  posterior  part  by  the  bony 
ischium.  These  three  bones,  ilium,  ischium, 
and  pubis,  meet  the  corresponding  ones 
of  the  opposite  side  in  the  middle  line  in 
junctions  termed  symphyses. 


Before  leaving  the  skeleton  it  will 
be  well  to  examine  its  structure  in  detail  as 
revealed  by  the  aid  of  the  microscope.    Good 
B 


cf 


_f 


QC. 


FIG.   7. — A,  Left  hind  limb  Rana  ;    B,  Pelvic  girdle  Rana  viewed  from 

right  side. 

a.,  astragalus  ;  ac.,  acetabulum  ;  c.,  calcaneum  ;  ca.,  calcar  ;  d.t.t  distal  tarsalia  ;  /.,  femur; 
i.e.,  iliac  crest ;  il.,  ilium  ;  is.,  ischium  ;  tnt.,  metatarsal ;  p.,  phalanges  ;  pu.,  pubis  ;  t.f.,  tibio- 
fibula  ;  I.~V.,  digits. 

examples  of  cartilage  may  be  obtained  by  taking  thin  slices  of  the 
head  of  a  large  bone,  or  of  the  epi-  or  xiphisternum,  and  examining 
them  in  normal  salt  solution  under  the  microscope.  It  consists  of 
a  large  number  of  slightly  granular  cells  embedded  in  homogeneous 
fairly  clear  substance,  the  matrix.  The  nuclei  of  the  cells  can  be 
readily  made  out  ^by  the  addition  of  a  drop  or  two  of  weak  acetic 
acid  and  of  methyl  green.  This  treatment  also  shows  that  a  thin 
layer  of  the  matrix  immediately  surrounding  the  cell,  termed  the 
capsule,  stains  more  deeply  than  the  rest.  The  cells,  although  some- 
times single,  are  more  often  to  be  found  in  groups  of  two,  three  or 
four,  in  each  case  derived  from  the  division  of  a  single  cell.  From  the 
fact  that  the  matrix  in  these  cases  is  very  clear,  the  substance  is 

D 


34 


AN   INTRODUCTION   TO  ZOOLOGY 


described  as  hyaline  cartilage.  The  supra-scapular  yields  another 
variety  of  cartilage,  namely,  calcified  cartilage,  so  called  because  the 
matrix  is  more  or  less  impregnated  with  calcareous  granules.  These 
may  be  removed  by  immersion  for  some  time  in  weak  hydrochloric 
acid,  and  the  matrix  then  appears  full  of  a  number  of  tiny  spaces  in 
which  the  granules  were  previously  lodged. 

Young  cartilage  is  easily  distinguishable  from  old,  because  it 
possesses  much  less  intercellular  matrix,  and  consequently  a 
relatively  larger  number  of  cells.  All  cartilage  on  boiling  yields  a 
substance  called  chondrin,  which  sets  in  a  jelly  when  cold.  It  is 


•       •*  :  -»  ' 

V         •         »** 

ft  V  > 


FIG  8. — Section  of  cartilage,  showing  cartilage  cells  (c.c.)  embedded  in  the 
transparent  intercellular  matrix  or  ground  substance  (m).  x  390. — 
From  a  photograph  (Dendy). 

covered  by  a  tissue  containing  a  rich  supply  of  blood-vessels,  known 
as  the  perichondrium. 

Bone  differs  markedly  from  cartilage  in  many  ways, 
although  it  agrees  with  it  in  being  built  up  of  cells  surrounded  by  a 
large  amount  of  intercellular  matrix..  The  matrix  consists  of  a 
tough  organic  basis,  that  yields  gelatin  on  boiling,  densely  impreg- 
nated with  inorganic  salts.  The  intimate  way  in  which  organic 
and  inorganic  constituents  are  mixed  may  be  seen  by  taking  two 
bones  and  placing  one  in  weak  Hydrochloric  acid  and  heating  the 
other  in  a  test  tube  until  it  is  thoroughly  charred.  The  former  will 
have  all  the  inorganic  salts  removed,  i.e.  be  decalcified,  and  will  be 
soft  and  flexible,  whereas  the  latter  will  have  no  organic  matter  left, 
and  will  be  hard  and  very  brittle.  Both,  however,  will  remain 


THE  FROG  35 

exact  models  of  the  original  bone.  The  decalcified  one  will  be  about 
one-third  the  weight  it  was  originally,  and  the  burnt  one  about 
two-thirds.  The  inorganic  salts  present  in  the  matrix  are  mainly 
Calcium  phosphate,  but  small  amounts  of  Calcium  carbonate  and 
fluoride  and  Magnesium  phosphate  are  also  present. 

Most  of  the  long  bones  are  hollow,  the  central  cavity  in  life  being 
filled  with  a  soft  red  tissue,  the  marrow.  This  consists  of  a  plentiful 
supply  of  blood-vessels  embedded  in  a  fatty  substance,  in  which 
are  special  cells,  the  marrow  cells,  concerned  with  the  storing  of  fat 
and  the  production  of  red  blood  corpuscles.  The  outer  layer  of  the 


2 


FIG.  9.  —  Transverse  section  of  the  compact  tissue  of  bone.     Magnified 
150  diameters.  —  From  Quain. 

i,  Haversian  canals  ;   2,  lacunae  ;  3,  laminae. 

bone,  i.e.  the  compact  bone,  is  much  denser  and  firmer  than  the 
inner  layer  surrounding  the  marrow  cavity,  which  is  termed  can- 
cellous,  but  both  have  the  same  fundamental  structure.  The 
interior  of  many  of  the  smaller  bones,  although  containing  no  cavity, 
is  cancellous. 

The  matrix  of  bone  is  quite  impermeable  to  nutriment,  and 
consequently  we  find  an  elaborate  arrangement  to  allow  of  its 
distribution.  Bone,  unlike  cartilage,  is  penetrated  by  a  series  of 
blood  vessels  running  in  canals  called  after  their  discoverer,  the 
Haversian  canals.  In  a  transverse  section  these  appear  as  circular 
spaces,  often  black  when  viewed  under  the  microscope,  owing  to 
their  being  filled  with  air  or  dirt.  Around  these  the  bone  is  arranged 
in  a  series  of  thin  concentric  sheets,  the  Haversian  lamellae,  marked 


36  AN  INTRODUCTION  TO  ZOOLOGY 

off  by  a  similar  concentric  arrangement  of  spaces,  lacunae,  in  which 
the  bone  cells  or  corpuscles  are  situated.  A  number  of  branching 
canals,  the  canaliculi,  come  off  from  the  lacunae  and  traverse  the 
lamellae,  thus  connecting  together  the  rings  of  the  lacunae  and  the 
Haversian  canal.  The  lacunae  and  canaliculi  are  also  black  in 
sections,  but  in  life  are  filled  by  cells  which  give  off  fine  branches, 
and  in  this  way  provide  a  series  of  channels  through  which  the  food 
substances  brought  by  the  blood  may  be  dispersed  throughout  the 
bone.  The  whole  arrangement  of  canal,  lacunae  and  canaliculi  is 
known  as  an  Haversian  system. 

In  addition  to  the  Haversian,  there  is  always  a  series  of  lamellae 
on  the  outside  of  the  bone  and  parallel  with  it.  These  are  termed 
the  peripheral  or  circumferential  lamellae.  In  the  long  bones  the 
marrow  cavity  is  similarly  lined  by  a  series  of  perimedullary  lamellae; 
The  small  spaces  between  the  three  various  sets  are  filled  in  by  yet 
other  lamellae,  the  interstitial. 

The  foregoing  description  of  the  appearance  of  bone  applies 
equally  to  both  membrane  and  cartilage  bone,  the  difference  between 
them  being  solely  whether  they  are  preceded  by  cartilage  or  not. 
It  is,  however,  based  on  the  structure  of  the  bones  in  the  higher 
vertebrates,  such  as  the  rabbit  or  man,  and  not  strictly  applicable 
to  the  frog.  In  this  animal  we  find  that  the  Haversian  systems  are 
never  fully  developed,  so  that  in  a  transverse  section  of  a  long  bone 
we  find  only  a  continuous  series  of  concentric  lamellae,  the  outer 
of  which  may  be  termed  peripheral  and  the  inner  perimedullary. 

The  free  surface  of  bone  is  covered  by  a  formative  and  nutritive 
tissue,  the  periosteum,  composed  of  two  layers.  The  outer  is  fibrous 
and  plentifully  supplied  with  blood-vessels,  and  the  inner  composed 
of  a  layer  of  bone-forming  cells  or  osteoblasts,  left  over  from  those 
that  produced  the  bone. 

The  process  of  bone  formation  or  ossification  is  complex, 
and  only  its  main  features  need  be  noted.  In  the  case  of  a  membrane 
bone,  the  place  it  will  take  is  occupied  by  connective  tissue,  whose 
structure  will  be  dealt  with  more  fully  later.  Thin  strands  of  the 
matrix  of  the  future  bone  are  laid  down,  and  the  osteoblasts  or 
bone-formers  arrange  themselves  in  a  more  or  less  regular  layer 
around  them.  Here  they  deposit  layers  of  bone,  with  the  result 
that  bony  spicules  are  produced.  More  strands  of  matrix  are  laid 
down  and  surrounded  by  bone  in  a  similar  way,  and  so  a  network 
of  spicules  is  produced,  at  first  loose  but  becoming  more  and  more 
compact.  During  this  process  certain  osteoblasts  are  included  in 
the  bone,  and  form  the  future  bone  corpuscles.  In  cartilage  bone 
a  model  of  the  bone  is  already  present  and  enclosed  in  peri- 
chondrium.  Ossification  takes  place  in  this  surrounding  tissue  in 


THE   FROG  37 

a  way  similar  to  that  already  described,  and  so  produces  the 
perichondral  bone  outside  the  cartilage.  Calcification  or  the 
deposition  of  lime  salts  has  already  occurred  to  a  certain 
extent  in  the  cartilage.  While  the  perichondral  bone  is  forming, 
the  formative  layer  sends  processes  inwards,  which  first  destroy  the 
cartilage  by  means  of  special  cells,  the  osteoclasts,  and  then  lay 
down  strands  of  matrix,  around  which  bony  spicules  are  formed. 
The  bone  thus  formed  in  the  actual  position  of  the  cartilage 
is  termed  endochondral.  Ossification  continues  until  peri-  and 
endochondral  bone  have  joined  up  and  the  whole  of  the  original 
cartilage  has  been  replaced.  We  are  now  in  a  position  to  under- 
stand a  little  more  clearly  what  is  meant  by  the  term  "  epiphysis." 
In  a  large  number  of  bones,  particularly  the  long  bones,  the  process 
of  ossification  starts  from  three  places,  the  middle  and  each  end,  so 
that  the  bone  really  consists  of  three  pieces,  which  grow  together  by 
an  interlocking  of  the  bony  spicules.  This  interlocking  is  not 
always  complete,  so  that  in  a  dried  skeleton  the  epiphyses  may 
fall  off. 

Connective  Tissue. 

Another  tissue  may  conveniently  be  considered  here, 
although  it  cannot  strictly  be  considered  as  forming  part  of  the 
skeleton.  It  helps  to  bind  the  various  soft  parts  to  one  another 
and  to  the  skeleton,  and  is  therefore  called  connective  tissue.  In 
Rana,  although  it  is  present  in  small  quantities  between  the  fibres 
of  the  muscles,  we  do  not  find  the  simplest  kind  of  connective  tissue 
nearly  so  plentifully  as  in  the  rabbit,  where  it  may  be  obtained 
readily  immediately  under  the  skin.  Under  the  microscope  the 
sub-cutaneous  or  areolar  tissue  appears  as  a  network  of  interlacing 
fibres  and  a  number  of  more  or  less  isolated  cells  contained  in  a 
colourless  matrix.  The  cells,  termed  connective  tissue  corpuscles, 
are  of  at  least  three  different  kinds.  The  lamellar  cells  are  flattened, 
have  a  large  nucleus,  and  give  off  a  number  of  processes,  which  may 
unite  with  the  similar  processes  from  neighbouring  cells.  The 
granular  cells  are  more  regular  in  shape  with  a  well-marked  nucleus, 
and  owe  their  name  to  their  granular  contents.  The  granules  are 
coarse  and  stain  deeply  with  eosin  and  certain  other  acid  dyes,  so 
that  they  are  often  termed  eosinophilous.  The  vacuolated  cells  are 
frothy  in  appearance,  owing  to  the  number  of  minute  vacuoles 
present  in  them. 

In  addition  to  these  cells,  more  or  less  constant  constituents, 
others  may  be  present.  A  certain  number  of  leucocytes  are  almost 
always  to  be  found  in  the  frog  pigment  cells  also. 

Two  sorts  of    fibres  will  be  easily  distinguished  crossing  and 


38  AN   INTRODUCTION  TO  ZOOLOGY 

intercrossing  through  the  matrix  in  all  directions.  White  fibres, 
so  called  from  their  colour,  appear  as  fairly  large  bundles  of  smaller 
parallel  fibres  running  together  and  having  an  undulating  course. 
These  do  not  branch,  and  fibres  from  one  bundle  do  not  join  with 
those  from  another.  On  boiling  they  yield  gelatin,  and  the  addition 
of  a  drop  of  acetic  acid  while  they  are  still  fresh  causes  them  to 
swell  up  and  dissolve.  The  yellow  elastic  fibres  differ  from  the 
former  in  a  number  of  ways.  Their  course  is  nearly  straight,  or  only 
in  very  shallow  curves,  and  they  branch  frequently,  the  branches 
anastomosing  with  neighbouring  fibres.  They  are  yellow  in  colour, 


x 


- 


FIG.  10. — Areolar  connective  tissue  of  the  frog. — From  Borradaile. 
c.,  cells  ;  £./.,  elastic  fibres  ;  «'./.,  white  fibres. 

and  their  cut  ends  curl  up,  indicating  their  elastic  nature.  Acetic 
acid  has  no  effect  on  them,  and  when  boiled  they  give  elastin  and 
not  gelatin. 

Various  modifications  of  this  tissue  occur  to  meet  different 
requirements.  In  tendon  and  ligament  the  white  fibres  run  parallel 
with  one  another  and  nearly  straight,  and  they  form  practically  the 
whole  of  the  structure,  the  other  elements  being  correspondingly 
reduced.  In  the  mammals  a  strong  ligament,  the  ligamentum  nuchee, 
runs  along  the  anterior  part  of  the  backbone  and  is  attached  to  the 
head,  which  it  helps  to  hold  up.  This  consists  of  yellow  elastic 
tissue,  in  which  the  elastic  fibres  form  the  major  part. 


THE  FROG  39 

Besides  the  hyaline  cartilage  already  described,  two  other 
varieties,  white  fibre-cartilage  and  elastic  fibre-cartilage,  are  met 
with  in  'tke  mammals.  As  their  names  imply,  one  or  other  sort  of 
fibre  is  present  in  addition  to  the  hyaline  matrix.  Membrane  bone 
is  laid  down  in  connective  tissue,  and,  as  has  been  pointed  out,  during 
the  formation  of  cartilage  bone  the  cartilage  is  destroyed  and  the 
bone  actually  deposited  in  a  sort  of  connective  tissue.  Hence  it  is 
that  some  writers  regard  both  cartilage  and  bone  as  highly  modified 
forms  of  connective  tissue. 

The  corpuscles  of  connective  tissue  may  take  up  fat,  and 
so  the  tissue  becomes  somewhat  fatty.  This  must  not  be  confused 
with  true  fat  or  adipose  tissue.  In  this  we  find  numbers  of  cells, 
each  containing  a  relatively  enormous  globule  of  fat,  and  the  proto- 
plasm is  reduced  to  a  very  thin  enclosing  layer,  thickened  at  one 
point  where  the  nucleus  is  situated.  These  fat  cells  are  bound 
together  in  groups  or  lobules  by  connective  tissue.  The  fat  is 
formed  gradually  as  a  number  of  minute  globules  within  the  cell, 
which  ultimately  run  together  and  form  the  large  globule. 

We  have  thus  seen  that  the  skeleton  forms  a  framework  of 
firm  supports  of  characteristic  structure,  articulating  one  with 
another  by  means  of  joints.  The  soft  parts  of  the  body  are  connected 
with  the  skeleton,  and  the  various  parts  of  this  with  one  another  by 
means  of  connective  tissue. 

Muscular  System  and  Integument. 

It  has  already  been  seen  that  the  flesh  or  muscle  is  situated 
beneath  the  skin  and  forms  the  mass  of  the  limbs  and  the  body  wall. 
Muscular  tissue  is  the  tissue  that  is  specially  modified  in  order  to 
bring  about  movements  of  all  kinds.  The  property  of  contractility 
is  common  to  all  protoplasm  to  a  certain  limited  extent,  but  in 
muscle  it  is  much  more  highly  developed  than  elsewhere.  Any 
change  that  is  capable  of  bringing  into  activity  the  whole  or  any 
part  of  an  animal,  however  small,  is  spoken  of  as  a  stimulus. 
Stimuli  may  come  from  inside  the  organism,  i.e.  be  internal,  or  from 
its  surroundings,  i.e.  be  external.  The  stimulus  that  causes  muscle 
to  contract  is  usually  nervous,  taking  the  form  of  a  message  from  a 
nerve  cell  in  the  brain  or  elsewhere,  although,  when  a  muscle  is 
removed  from  the  body,  the  same  response  may  be  obtained  by 
mechanical,  chemical  or  electrical  means,  or  by  the  application  of 
heat.  When  muscle  contracts  in  reply  to  a  stimulus  it  shortens  in  the 
direction  of  its  long  axis  and  increases  correspondingly  in  thickness. 
The  energy  necessary  for  such  activity  is  obtained  by  the  oxidation 
of  the  substances  of  the  protoplasm  itself,  i.e.  by  katabolism. 


AN   INTRODUCTION   TO  ZOOLOGY 


The  muscles  in  the  frog  are  fibrous  in  structure  and  of  two 
kinds.  The  first  are  those  forming  the  muscles  of  the  body  wall 
and  limbs,  and  their  fibres,  grouped  in  bundles,  appear  transversely 
striped  when  examined  under  the  microscope.  They  are  conse- 
quently termed  striate  or,  because  they  can  be  made  to  contract 
at  will,  voluntary  muscles.  The  second  kind  is  found  in  the  walls  of 
the  alimentary  canal,  the  bladder  and  blood-vessels,  and  does  not 
appear  striped.  They  are  called  non-striate  muscles,  and,  as  they 
are  not  under  the  control  of  the  will,  involuntary.  In  general  we 
find  that  striate  muscles  are  directly  or  indirectly 
connected  with  the  skeleton,  whereas  the  non-striate 
are  related  to  the  internal  organs,  and  hence  the 
two  are  sometimes  distinguished  as  skeletal  and 
visceral  respectively. 

Involuntary  muscles  consists  of  a  number  of 
long  spindle-shaped  cells  about  I  mm.  in  length, 
with  pointed  ends,  that  may  in  rare  instances  be 
bifurcated.  The  cell  body  is  enclosed  by  a  definite 
membrane,  and  within  it  is  situated  a  characteristic 
elongated  nucleus.  The  cell  as  a  whole  is  highly 
refractive,  but  distinctly  granular  in  the  neighbour- 
hood of  the  nucleus,  and  may  appear  very  faintly 
striated  in  the  longitudinal  direction,  owing  to  the 
presence  in  it  of  a  number  of  extremely  fine  fibrils. 
The  cells  are  situated  in  layers  closely  bound  to- 
gether, so  that  their  individual  outlines  cannot  be 
made  out  save  in  preparations  from  material  that 
has  been  allowed  to  stand  in  certain  solutions  *  and 
then  teased,  i.e.  torn  up  with  fine  needles.  They  lie 
parallel  with  one  another,  and  are  bound  together 
by  a  small  amount  of  intercellular  substance.  Such 
muscles  do  not  appear  to  be  capable  of  quick  move- 
ment in  unison,  but  are  adapted  for  slower  ones, 
such  as  are  required  in  the  blood-vessels  and  bladder,  or  the 
rhythmic  contractions,  peristalsis,  that  pass  along  the  alimentary 
canal  during  digestion. 

'  Many  of  the  skeletal  muscles,  such  as  the  biceps,  the  large  muscle 
that  draws  up  the  fore-arm,  and  the  gastrocnemius,  that  forms  the 
calf  of  the  leg,  have  a  very  characteristic  shape.  They  are  spindle- 
shaped,  tapering  off  at  each  end  and  swelling  out  in  the  middle  to 
form  the  belly.  The  ends  are  continued  on  as  the  connective 
tissue  ligaments,  and  these  in  turn  are  joined  to  the  skeleton.  When 

*  Such  a  process  is  termed  maceration.  The  fluid  may  be  very  weak  spirit 
or  a  weak  solution  of  Potassium  bichromate. 


FIG.  ii. — Two 
non  -  striate 
muscle  cells 
from  wall  of 
i  n testine 
showing 
nuclei  and 
longitudinal 
striations. 


THE   FROG  41 

the  muscle  contracts  one  of  the  bones  remains  more  or  less  still  and 
the  other  is  drawn  towards  it.  The  tendon  attached  to  the  relatively 
fixed  point  is  termed  the  origin,  and  that  attached  to  the  movable 
bone  the  insertion.  Usually  a  muscle  has  but  one  origin  and  one 
insertion,  but  it  may  have  two,  as,  for  example,  in  the  biceps,  which 
receives  its  name  from  the  fact  that  it  has  a  double  origin.  The 
tendons  are  not  always  united  to  bone.  The  distal  tendon  of  the 
gastrocnemius  is  called  the  tenclo  Achillis,  and  in  the  frog  it  passes 
over  the  place  occupied  by  the  heel  in  ourselves,  and  is  inserted  into  a 
sheet  of  connective  tissue  spread  out  over  the  sole  of  the  foot.  A 
connective  tissue  sheet  serving  for  the  attachment  of  muscle  or 
tendon  is  termed  an  aponeurosis,  and  the  particular  one  just  described, 
from  its  position,  is  known  as  the  aponeurosis  plantaris. 

The  entire  muscle  is  enclosed  in  a  connective  tissue  sheath,  the 

n. 


FIG.  12. — Striped  muscle  fibres  (m.)  from  the  tail  of  a  larval  axolotl, 
showing  their  nuclei  («.).     X  560. — From  a  photograph  (Dendy). 


perimysium,  and  within  this  are  to  be  found  a  number  of  bundles  of 
fibres  or  fasciculi,  each  enclosed  by  a  sheath,  the  endomysium, 
continuous  with  the  external  one.  The  individual  fibres  composing 
the  fasciculi  have  quite  a  fair  diameter,  as  much  as  i  mm.,  and 
may  in  some  cases  reach  a  length  of  120  mm.  (nearly  five  inches). 
They  are  circular  or  polygonal  in  cross-section,  and  as  a  rule  un- 
branched,  although  branched  fibres  are  present  in  the  tongue 
muscles.  Each  fibre  is  enclosed  by  a  very  thin  sheath,  the  sar co- 
lemma,  immediately  beneath  which  are  numerous  nuclei  irregularly 
distributed.  The  interior  is  filled  by  a  semi-fluid  substance,  the 
sarcoplasm,  consisting  of  a  large  number  of  small  fibrils  parallel 
with  one  another,  and  a  certain  amount  of  other  material.  These 
fine  fibrils  are  the  fibrillse  or  sarcostyles,  and  give  the  whole  fibre 


AN   INTRODUCTION   TO  ZOOLOGY 


a  faint  longitudinal  striation.  A  sarcostyle  appears  striated  from 
the  presence  in  it  of  two  substances,  one  of  which  is  clearer  and 
singly  refracting,  isotropic,  and  hence  appears  light,  and  the  other  is 
dense,  doubly  refractive,  i.e.  anistropic,  and  so  appears  dark.  The 
whole  fibre  appears  striped,  because  the  dark  and  light  bands  of  the 
various  fibrillae  coincide.  When  examined  under  a  still  higher  power 
of  the  microscope  the  light  band  is  seen  to  be  divided  into  two  by  a 
very  thin  dark  line,  usually  taken  to  be  partly  membranous,  and 

so  called  Krause's  membrane. 
Between  one  membrane  of 
Krause  and  the  next,  therefore, 
is  included  half  a  light  band 
on  each  side  of  a  complete  dark 
band,  and  this  constitutes  a 
unit  of  striate  muscle,  a  sar- 
comere,  and  a  row  of  such  end 
to  end  forms  a  sarcostyle.  The 
dark  band  is  similarly  divided 
into  two  parts  by  a  narrow 
white  line,  Henson's  line.  The 
fibrillae  may  or  may  not  be  uni- 
formly distributed  throughout 
the  fibre,  and  often  in  a  cross- 
section  we  find  them  grouped 


FIG.  13. — Striate  muscle.  A,  Muscle 
fibre  partly  teased  ;  B,  part  of  single 
fibrilla  or  sarcostyle  in  resting  con- 
dition, highly  magnified  ;  C,  part  of 
single  fibrilla  in  state  of  contraction, 


highly  magnified. 


a.,  ainisotropic  element  ;  H.,  Heuson's  line 
»'.,  isotropic  element ;  K.t  Krause's  membrane 
s.,  sarcostyle  ;  sm.,  sarcomere. 


together  in  polygonal  areas,  the 
areas  of  cohesion  or  muscle 
columns.  Striate  muscle  is  not 
composed  of  simple  uninucleate 

cells,  but,  as  has  been  pointed  out,  beneath  the  sarcolemma  are 
numbers  of  nuclei.  It  is  found  that  in  development  all  the  structures 
in  one  fibre  are  formed  from  and  by  a  long  multinucleate  piece  of 
protoplasm,  which  arises  by  the  union  of  many  cells.  Such  a  cell 
fusion  is  termed  a  syncytium. 

This  striped  muscle  is  well  adapted  for  strong  quick  movements, 
such  as  are  required  in  locomotion,  and  the  various  constituent  fibres 
contract  in  unison,  a  wave  of  contraction  passing  along  them  from 
the  origin  and  affecting  the  sarcomeres  successively.  The  muscle  is 
under  the  control  of  the  will,  and  the  nerve  supplying  it  branches  on 
entering  its  substance.  The  small  terminal  branches  are  joined  to 
the  muscle  fibres  by  characteristic  structures  known  as  end  plates, 
and  so  all  the  fibres  can  be  called  into  activity  simultaneously. 
As  a  result  of  a  change  in  the  disposition  of  its  constituent  sub- 
stances each  sarcomere  shortens  in  length  and  increases  in  diameter. 
It  appears  as  if  the  material  of  the  light  bands  passes  into  that  of  the 


THE  FROG  43 

dark,  so  causing  them  to  expand  laterally.  These  changes  are  also 
accompanied  by  chemical  and  electrical  ones,  and  by  the  production 
of  heat. 

Yet  another  kind  of  muscular  tissue  is  met  with  in  vertebrate 
animals,  but  it  is  confined  to  the  heart,  and  hence  termed  cardiac. 
It  is  involuntary  and  yet  striped,  and  so  constitutes  a  variety  of  its 
own.  It  is  composed  of  flat  short  cells  placed  end  to  end,  with 
short  branches  connecting  them  with  neighbouring  cells.  No 
sarcolemma  is  present,  and  each  cell  possesses  a  single  fairly  large 
nucleus  situated  near  its  centre.  It  exhibits  both  longitudinal  and 
transverse  stripes,  though  the  latter  are  not  so  well  marked  as  in 
voluntary  muscle. 

Integument. 

The  integument  in  the  frog  is  very  simple,  and  consists 
only  of  the  skin,  in  which  no  hair,  nails,  or  any  exoskeletal 
structures  are  developed.  The  skin  is  moderately  thick  and  tough, 
and  fits  loosely,  being  only  attached  to  the  underlying  muscles  here 
and  there.  A  vertical  section  through  the  skin  shows  it  to  be  com- 
posed of  two  very  clearly  defined  layers,  an  outer  or  epidermis,  and 
an  inner  or  dermis.  The  outer  forms  an  epithelium,  that  is  to  say,  a 
sheet  of  cells  covering  a  free  surface,  and  is  compound  or  composed 
of  a  number  of  layers  of  cells.  The  cells  of  the  various  layers  differ 
in  form,  according  to  their  position.  The  cells  at  the  inside  are 
columnar  in  shape  and  granular,  forming  a  layer  known  as  the 
Malpighian  layer.  Above  this  the  cells  are  polygonal,  becoming 
more  and  more  flat,  until  the  outer  layers  are  thin  scale-like  cells. 
In  addition  to  the  flattening  the  protoplasm  of  the  cells  undergoes 
modification,  and  is  progressively  replaced  by  a  substance,  keratin, 
allied  to  horn,  so  that  the  outermost  cells  are  practically  nothing 
but  thin  dead  horny  scales.  The  outer  layer  is  cast  off  periodically, 
a  process  known  as  sloughing,  and  the  cells  are  replaced  by  new  ones 
produced  from  the  Malpighian  layer,  which  is  the  actively  growing 
part  of  the  epidermis.  An  epithelium  thus  composed  of  layers  of 
cells  gradually  changing  in  character  is  spoken  of  as  stratified,  and 
although  found  in  the  epidermis  of  the  frog  is  still  more  marked 
in  ourselves,  where  the  horny  layer  on  the  palms  of  the  hands  and 
soles  of  the  feet  is  extraordinarily  thick.  Belonging  to  the  epidermis 
are  a  number  of  hollow  flask-shaped  structures,  the  cutaneous  glands, 
which  dip  down  into  the  dermis.  Each  is  composed  of  two  parts, 
a  spherical  hollow,  the  alveolus,  in  the  dermis  lined  with  large  cells, 
and  a  narrow  tube,  the  duct,  running  through  the  epidermis  and 
opening  to  the  exterior.  The  cells  lining  the  alveolus  are,  of  course,  * 
epithelial,  but  as  they  are  constantly  engaged  in  forming  in  their 


44 


AN   INTRODUCTION   TO  ZOOLOGY 


interior  droplets  of  a  substance  which  is  afterwards  passed  outside, 
that  is,  they  are  constantly  secreting,  they  constitute  a  glandular 
epithelium.  Since  they  are  only  in  a  single  layer  they  also  form  a 
simple  epithelium.  The  secretion,  a  slimy  fluid,  is  stored  upon  the 
alveolus  and  passed  to  the  surface  through  the  duct,  and  by  this 
means  the  skin  of  the  frog  is  kept  moist.  The  glandular  cells  are 


h.i. 


d.l 


FIG.  14. — A  section  of  the  skin  of  a  frog,  taken  vertically  to  the  surface, 
highly  magnified. — From  Borradaile. 

b.v.,  small  blood-vessels  ;  cap.,  capillaries  ;  d.l.,  dense  layer  of  connective  tissue,  consisting 
of  fibres  which  lie  parallel  to  the  surface  ;  der.,  dcrmis  or  corium  ;  ef>.,  epidermis  ;  gl'.,  gl".,  gl'"., 
glands  of  three  kinds  ;  gl'.  and  gl".  secrete  a  slimy  mucus  and  pass  it  to  the  surface  of  the  skin 
by  ducts  which  are  not  shown  in  the  section  ;  gl"'.  secretes  a  more  watery  secretion  which 
probably  contains  a  substance  of  unpleasant  taste  ;  all  three  kinds  are  simple  glands  of  the 
saccular  type  ;  h.l.,  horny  layer  of  the  epidermis  ;  M.I.,  lowest  row  of  the  Malpighian  layer  of 
the  epidermis  ;  pig.,  pigment  cells  ;  v.f.,  strands  of  vertical  fibres  in  the  connective  tissue. 


continuous  with  those  of  the  duct,  but  the  latter  are  flattened  and 
squamous  instead  of  being  almost  cubical  like  the  former.  The  whole 
is  probably  to  be  regarded  as  a  specialised  part  of  the  epidermis 
that  has  sunk  below  the  general  level,  in  order  to  carry  out  its  function 
more  efficiently.  Gland  cells  are  found  in  many  parts  of  an  animal, 
.  and  frequently  close  together,  so  as  to  form  a  glandular  epithelium. 
When  this  is  brought  together  into  a  definite  organ  we  speak  of  it 


THE   FROG  45 

as  a  gland.  From  their  characteristic  form  these  structures  in  the 
skin  of  the  frog  are  known  as  flask-glands. 

The  dermis  is  much  thicker  than  the  epidermis,  and  is  composed 
of  connective  tissue.  Most  of  the  fibres  in  it  run  parallel  with  the 
surface  of  the  skin,  but  a  certain  number  of  strands  of  them  run  at 
right  angles  to  the  remainder.  Unlike  the  epidermis  the  dermis 
has  a  supply  of  blood-vessels.  They  are  much  more  plentiful  than 
in  other  animals,  for  in  the  frog  the  skin,  in  addition  to  its  other 
functions,  plays  a  large  part  in  the  breathing.  Immediately  beneath 
the  Malpighian  layer  are  a  number  of  cells  deeply  impregnated 
with  granules  of  pigment,  and  another  layer  of  more  sparsely 
scattered  pigment  cells  is  situated  deeper  down.  It  is  to  the  presence 
of  these  pigment  cells  that  the  skin  owes  its  colour,  and  to  their  power 
of  altering  their  shape,  the  possibility  of  altering  the  colour  within 
limits.  The  dermis  is  also  supplied  with  nerves  which  end  just 
beneath  the  epidermis,  or  partly  in  it,  in  sense  corpuscles,  as  they  are 
termed.  These  are  especially  abundant  in  regions  of  great  sensi- 
bility, such  as  the  tongue. 

The  principal  function  of  the  skin  is  protection,  and  we  find  that 
it  presents  a  tough  surface  to  the  outer  world,  and  so  guards  the 
underlying  parts  from  injury.  The  blood-vessels  do  not  come  right 
to  the  surface,  so  that  a  superficial  wound  does  not  result  in  a  loss 
of  blood.  The  skin  is  also  protective  in  another  sense,  for  its  colour 
pattern  and  the  power  of  altering  it  enables  the  animal  to  harmonise 
with  its  surroundings,  and  so  avoid  detection  by  its  enemies.  The 
second  function  of  the  skin  in  the  frog  is  respiration,  and  so  we  find 
a  large  supply  of  blood-vessels  in  it,  and  these  are  not  shut  off  from 
the  air  by  such  a  thick  layer  of  horny  cells  as  we  find  in  many 
animals.  Then  again  this  process  is  aided  by  the  secretion  of  the 
cutaneous  glands,  which  keeps  the  skin  always  moist,  and  so  in  a 
condition  in  which  the  exchange  of  gases  can  most  readily  take  place. 
It  is  not  improbable  that  this  secretion  may  also  help  the  animal  to 
rid  itself  of  certain  obnoxious  substances. 


CHAPTER   III 
THE   FROG— RAN  A    TEMPO  R  ARIA— (continued) 

Alimentary  System — Respiratory  System — Circulatory  System — Urogenital 

System. 

Alimentary  or  Digestive  System. 

The  alimentary  canal  commences  at  the  mouth,  which  leads 
directly  into  the  buccal  cavity.  The  structures  in  this,  and  the  way 
in  which  it  narrows  down  at  the  back  to  form  an  indistinct  pharynx 
leading  into  the  oesophagus,  have  already  been  described.  The 
oesophagus  or  gullet  is  a  wide  short  tube  situated  in  the  pleuro- 
peritoneal  cavity,  and  attached  to  the  dorsal  wall  by  a  mesentery. 
It  passes  on  without  a  sharp  line  of  demarcation  into  the  stomach,  a 
wide  slightly  curved  sac  lying  to  the  left  of  the  middle  line.  The 
walls  of  the  stomach  are  very  well  supplied  with  muscles,  by  means  of 
which  a  churning  movement  is  maintained  as  long  as  it  contains 
food.  At  the  lower  end  of  the  stomach  a  band  of  circular  muscles,  a 
sphincter,  is  developed,  by  means  of  which  the  food  can  be  retained. 
The  position  of  this  muscle  is  marked  externally  by  a  slight  con- 
striction, and  the  slightly  swollen  part  of  the  stomach  in  its  imme- 
diate neighbourhood  is  known  as  the  pylorus.  From  this  point  the 
first  part  of  the  intestine,  the  duodenum,  runs  forward  nearly  parallel 
with  the  stomach,  and  united  to  it  by  a  fold  of  peritoneum,  the 
gastro-duodenal  omentum.  At  the  posterior  end  of  the  liver,  to 
which  it  is  bound  by  the  duodeno-hepatic  omentum,  it  turns  sharply 
backwards,  and  is  known  as  the  small  intestine  or  ileum.  This 
pursues  a  complicated  course,  and  is  thrown  into  a  number  of  con- 
volutions all  held  together  by  mesentery,  and  finally  it  returns  to 
the  level  of  the  hinder  end  of  the  stomach,  where  it  swells  out  to  form 
the  large  intestine  or  rectum.  This  runs  straight  backwards  for 
just  over  an  inch,  and  opens  by  the  cloaca  to  the  exterior.  At  the 
point  where  the  large  intestine  joins  the  small  it  gives  off  a  small 
dorsal  projection,  which  may  perhaps  be  homologous  with  a  large 
sac-like  structure,  called  the  ccecum  in  the  higher  animals.  The 
cloaca  is  practically  the  end  of  the  intestine,  but  as  it  has  connected 
with  it  the  excretory  and  reproductive  organs  it  is  more  conveniently 

46 


THE  FROG  47 

treated  in  dealing  with  these.  The  whole  of  the  alimentary  canal 
is  lined  by  a  thin  cellular  membrane,  whose  actual  structure  varies  in 
different  parts,  but  as  it  is  always  kept  moist  by  secretion  from 
some  of  its  constituent  cells,  it  is  spoken  of  as  the  mucous  membrane 
or  mucosa. 

The  canal,  its  various  parts,  and  the  glands  connected  with 
it,  are  kept  in  position  by  the  same  peritoneum  that  lines  the  coelomic 
cavity,  and  this  is  reflected  round  them,  so  as  to  form  thin  sheets  of 
supporting  tissue  which  bind  them  all  together.  The  sheet  that  is 
attached  to  the  original  dorsal  side  of  the  canal,  and  runs  from  it  to 
the  body  wall  immediately  beneath  the  vertebral  column,  is  known 
as  a  mesentery,  while  the  various  side  folds  tying  different  laterally 
situated  parts  and  organs  to  this  main  sheet  are  distinguished  as 
omenta.  Thus  the  one  enclosing  the  liver  and  joining  it  to  the 
stomach  is  known  as  the  gastro-hepatic  omentum. 

Intimately  connected  with  the  alimentary  canal  are  two 
important  glands,  the  liver  and  the  pancreas.  The  liver  is  a  large 
brownish  mass  situated  just  behind  the  heart,  and  occupying  a  large 
part  of  the  anterior  end  of  the  body  cavity.  It  is  composed  of  two 
parts,  one  on  each  side  of  the  middle  line,  the  right  and  left  lobes, 
joined  by  a  small  connecting  piece.  The  larger  left  lobe  is  partially 
subdivided  into  two  smaller  lobes.  Between  the  two  main  lobes  is 
situated  a  dark  green  spheroidal  sac  with  thin  walls,  the  gall-bladder, 
in  which  the  gall  or  bile  secreted  by  the  li ver  is  stored  until  required 
for  use.  Three  small  ducts,  the  hepatic  ducts,  issue  from  the  liver 
substance  and  unite  toform  a  common  or  cystic  duct,  that  opens  into 
the  bladder  and  serves  for  the  conveyance  of  the  bile  fiom  the  liver. 
Hence  it  is  taken  to  the  duodenum  by  a  single  tube,  the  bile  duct  or 
ductus  choledocus,  which  is  formed  by  the  union  of  three  small  tubes 
coming  from  the  cystic  duct.  This  duct  passes  through  the  sub- 
stance of  the  pancreas  and  some  way  along  is  joined  by  two  or  three 
small  ducts  coming  from  the  liver.  Its  point  of  entry  into  the 
duodenum  may  easily  be  seen  if  the  duodenum  be  slit  up  and  washed 
out,  then  when  the  gall  bladder  is  pressed  a  drop  of  dark  green  liquid 
will  make  its  appearance  on  the  wall  of  the  intestine  not  far  from  the 
pylorus. 

The  pancreas  is  a  pale  yellow,  slightly  lobed,  elongated 
gland  lying  between  the  duodenum  and  the  stomach,  slightly  towards 
their  dorsal  side.  It  is  traversed  from  end  to  end  by  the  bile  duct, 
into  which  its  own  numerous  small  ducts  open,  so  that  it  acts  as  a 
common  channel  for  bile  and  pancreatic  fluid,  and  opens  into 
the  duodenum  shortly  after  leaving  the  posterior  end  of  the 
pancreas. 

Both  the  liver  and  pancreas,  as  we  have  seen,  are  connected 


48 


AN   INTRODUCTION   TO  ZOOLOGY 


directly  with  the  alimentary  canal,  but  more  than  this,  they  arise 
in  the  developmental  stages  of  the  animal  as  actual  outgrowths  of 
the  canal,  and  so  may  be  regarded  as  parts  of  it  separated  off  and 
modified  for  special  functions. 

The  front  part  of  the  buccal  cavity  is  lined  by  a  layer  of 
very  thin,  flat,  approximately  polygonal  cells,  i.e.  a  squamous 
epithelium.  In  reality  it  is  a  stratified  epithelium  of  many  layers 
with  squamous  cells  on  the  outside.  Each  cell  is  a  very  thin  scale 
composed  of  coarsely  granular  protoplasm  with  a 
distinct  granular  nucleus.  The  various  cells  are 
held  together  by  a  certain  amount  of  intercellular 
connective  substance.*  A  number  of  glands  open 
on  the  roof  of  the  mouth  just  behind  the  pre- 
maxillae,  and  they  secrete  a  mucous-like  substance, 
whose  principal  function  is  in  connection  with  the 
capture  of  food.  As  the  tongue  is  slung  out  of  the 
mouth  it  wipes  this  region,  and  so  becomes  covered 
with  sticky  secretion  to  which  the  prey  adheres. 
No  glands  comparable  with  those  producing  the 
saliva  in  the  higher  animals  are  present  in  the 
frog,  and  the  digestion  does  not  commence  until 
the  food  has  reached  the  stomach.  The  teeth,  too, 


the  mouth. 
Magnified  260 
diameters. — 
From  Quain. 


FIG.  15. — Over- 
lapping squam- 
ous cells  from 
the  inside  of  are  not  used  for  mastication,  but  only  for  holding 

the  prey  during  the  process  of  swallowing. 

Further  back,  in  the  region  of  the  pharynx,  the 
mouth  is  lined  by  highly  specialised  cells.  Each 
cell  is  roughly  cone-shaped,  with  its  inner  pointed 
end  resting  upon  a  basement  membrane,  and  possesses  a  large 
distinct  nucleus  within  its  finely  granular  protoplasm.  The  side 
of  it  turned  towards  the  mouth  is  covered  with  a  coating  of  very 
fine  hair-like  processes,  termed  cilia,  capable  of  executing  quick 
whip-like  movements.  The  cilia  are  minute  projections  of  highly- 
modified  protoplasm,  and  lash  sharply  in  one  direction  and  then 
return  more  slowly  to  their  initial  position.  Their  structure  and 
mode  of  action  may  readily  be  studied  by  examining  a  scraping  from 
the  pharynx  of  a  freshly  killed  frog.j  At  the  edge  of  the  cell,  just 
beneath  the  bases  of  the  cilia,  are  a  number  of  small  refractive 
granules,  and  in  suitably  prepared  material  it  appears  as  if  each 
granule  bears  a  cilium,  and  is  also  continued  as  a  thin  fibril  into  the 

*  More  typical  squamous  cells  may  be  obtained  by  gently  scraping  the 
inside  of  the  human  cheek  with  the  handle  of  a  scalpel  and  their  structure 
examined  by  mounting  the  scraping  in  a  drop  of  salt  solution  on  a  slide. 

f  Very  good  examples  of  such  cells  may  also  be  obtained  by  teasing  up 
portions  of  the  gill  of  the  mussel  in  salt  solution. 


THE   FROG 


49 


FIG, 


1 6. — Ciliated  cells, 
Rana. 


pharynx  of 


c,,  cilia  ;  c,b.t  cell  body  ;   ».,  nucleus. 


body  of  the  cell.  The  whole  pharyngeal  region  is  covered  by  this 
ciliated  epithelium,  and  its  constituent  cells  do  not  behave  as 
isolated  units,  but  act  together.  Bands  of  contraction  pass  over  it, 
producing  the  same  wave-like  effect  that  is  seen  when  the  wind 
blows  over  a  cornfield,  only,  of  course,  on  a  very  minute  scale.  The 
purpose  of  this  action  may 
readily  be  ascertained  by  plac-  11(1111/411  c 
ing  a  small  piece  of  cork  on 
the  pharynx  of  a  frog  immedi- 
ately after  killing  ;  it  will  be 
slowly  carried  along  towards 
the  oesophagus.  It  will  be 
seen,  from  the  fact  that  the 
cilia  maintain  their  lashing  in 
the  individual  cells  when  sepa- 
rated from  their  fellows,  that 

their  motion  is  due  to  the  activity  of  the  protoplasm  of  the  cell, 
and  not  to  stimuli  received  from  the  nerves. 

The  transition  from  pharynx  to  oesophagus  is  marked  by  a 
change  in  the  character  of  the  mucous  membrane,  and  the  single- 
layered  ciliated  epithelium  gives  place  to  a  many-layered  compound 
one.  This  compound  epithelium  is  in  the  main  similar  to  that  of  the 
skin,  its  deeper  cells  are  polyhedral  and  the  superficial  ones  flat  and 
non-ciliated.  The  membrane  is  thrown  into  slight  folds  running  in 
the  longitudinal  direction,  it  is  surrounded  by  connective  tissue, 
and  outside  this  again  is  a  well-developed  sheath  of  muscles,  by 
means  of  which  the  action  of  swallowing  is  brought  about.  The 
whole  is  covered  by  the  visceral  part  of  the  pleuro-peritoneal 
membrane. 

The  change  to  the  stomach,  not  noticeable  externally,  is  again 
accompanied  by  an  alterntion  in  the  mucous  membrane.  The  slight 
folds  become  very  marked  and  form  high  ridges  (rugee)  passing  along 
the  organ.  The  epithelium  itself  takes  on  a  glandular  character, 
and  instead  of  being  a  more  or  less  level  covering  to  the  folds,  it 
forms  a  large  number  of  simple  or  branched  tubes,  the  gastric  glands, 
which  sink  down  into  the  connective  tissue.  These  do  not  possess 
the  flask  form  of  the  cutaneous  glands,  but  are  test-tube  shaped,  and 
constitute  examples  of  simple  tubular  or  compound  tubular  glands, 
according  as  to  whether  they  consist  of  one  tube  or  several  opening 
by  a  common  duct.  The  cells  of  the  single  layer  lining  the  duct  are 
columnar  in  shape,  with  more  or  less  clear  cytoplasm  and  a  distinct 
nucleus  lying  towards  their  base.  In  the  deeper  part,  the  fundus 
of  the  gland,  the  cells  are  larger  and  somewhat  cubical.  They 
possess  a  basally  situated  nucleus  and  very  granular  protoplasm, 


AN   INTRODUCTION  TO  ZOOLOGY 


the  granules  being  termed  the  zymogen  granules.  In  the  rabbit  the 
difference  between  the  two  types  of  cell  is  much  more  strongly 
marked,  and  there  are  present  in  addition  certain  large  sub-spherical 
or  ovoid  cells.  These  cells  are  situated  below  the  others  upon  the 
basement  membrane  bounding  them,  and,  as  they  are  regarded  as 
being  specially  concerned  with  the  production  of  Hydrochloric  acid, 
they  are  termed  the  Oxyntic  cells.  Acid  is  also  produced  in  the 

gastric  glands  of  Rana,  but  the  cells 
producing  it  are  not  easily  distin- 
guishable from  the  remainder. 

The  function  of  the  glands  is  to 
secrete  a  fluid,  the  gastric  juice, 
that  plays  a  very  important  part 
in  the  digestion  of  the  food.  In 
addition  to  the  Hydrochloric  acid 
already  mentioned,  it  contains  certain 
other  substances  that  are  very  active 
chemically.  They  are  termed  fer- 
ments, or  preferably  enzymes,  a 
name  given  to  substances  produced 
by  living  matter  that  are  capable  of 
producing  chemical  changes  in  other 
substances  with  which  they  are  in 
contact  while  they  remain  unaltered 
in  themselves.  The  enzymes  appear 
to  be  derived  from  the  zymogen 
granules  which  undergo  a  certain 
change  when  discharged  from  their 
cells,  and  their  action  on  the  food  will 
be  considered  later  in  dealing  with 
digestion. 

The  spaces  between  the  gland 
tubules  are  filled  with  connective 
tissue,  which  also  extends  beyond 
them  and  forms  a  fairly  thick  coat, 

the  sub-mucosa,  plentifully  supplied  with  blood-vessels.  This 
connective  tissue  is  interrupted  by  a  very  thin  double  circle 
of  muscles,  the  muscularis  mucosae,  which  follow  the  outline  of 
the  mucous  membrane.  The  inner  part  of  the  band  is  formed  by 
circular  muscles,  i.e.  muscles  whose  fibres  run  around  the  stomach, 
and  the  outer  part  by  fibres  at  right  angles  to  them,  the  longi- 
tudinal muscles.  Outside  the  sub-mucosa  is  the  large  muscular 
coat  of  the  stomach,  also  consisting  of  two  parts,  an  inner  thick 
layer  of  circular  muscles  and  an  outer  much  thinner  layer  of 


FIG.  17. — Peptic  gland  from  the 
mucous  membrane  of  the 
stomach.  Highly  magnified. 
— From  Gray. 


THE  FROG  51 

longitudinal  muscles.  The  whole,  like  the  oesophagus,  is  enclosed 
in  a  portion  of  the  pleuro-peritoneal  membrane,  in  this  case  it  is 
the  peritoneum  itself. 

The  end  of  the  stomach  is  marked  by  the  pylorus,  at  which  point 
the  long  folds  of  the  mucous  membrane  disappear,  and  the  outer 
band  of  circular  muscles  becomes  greatly  thickened.  This  ring  of 
muscles,  a  sphincter,  enables  the  stomach  to  be  cut  off  almost,  if 
not  entirely,  from  the  intestine.  The  first  part  of  the  intestine  is  the 
duodenum,  into  which  the  bile  and  pancreatic  fluid  are  passed.  No 
tabular  glands  are  present  in  its  mucous  membrane,  which  is  not 
thrown  into  folds,  but  is  raised  into  a  number  of  small  conical 
projections,  papillae,  irregularly  distributed.  In  the  next  part  of  the 
intestine,  the  ileum,  the  longitudinal  folds  again  make  their  appear- 
ance and  are  closely  set  together.  The  rectum  is  devoid  of  papillae 
or  folds,  and  covered  with  a  plain  epithelium.  The  mucous  mem- 
brane of  the  entire  intestine  consists  of  a  columnar  epithelium,  whose 
cells  have  distinct,  basally  situated  nuclei,  and  rest  upon  a  basement 
membrane.  A  large  number  of  glandular  cells  are  to  be  found 
among  the  apparently  unspecialised  epithelial  cells.  These  cells, 
called  goblet  cells,  from  the  fact  that  they  often  contain  a  large  drop 
of  secretion  at  the  outer  end,  and  hence  present  a  somewhat  fanciful 
resemblance  to  a  globet,  do  not  appear  to  be  aggregated  into 
definite  glandular  areas  as  in  the  frog,  as  some  of  them  are  in  the 
rabbit.  The  remaining  layers  of  the  intestinal  wall  are  similar  to 
those  in  the  stomach,  save  that  the  muscularis  mucosae  is  so  thin 
that  it  is  often  overlooked,  and  the  sub-mucosa  is  very  plentifully 
supplied  with  blood-vessels.  The  whole  is  encased  in  peritoneum. 
During  the  time  that  the  food  is  undergoing  digestion  a  slow  move- 
ment of  the  intestinal  walls  is  kept  up.  It  takes  the  form  of  a  series 
of  waves  of  contraction,  which  slowly  pass  from  pylorus  to  rectum,  so 
assisting  the  digestive  juices  to  mix  with  the  food  and  at  the  same 
time  gradually  passing  it  onwards.  As  has  been  previously  noted, 
this  movement  is  termed  peristalsis,  and  is  brought  about  by  the 
involuntary  muscles  of  the  muscular  coats. 

The  pancreas  furnishes  an  example  of  a  complex  gland,  and 
is  more  intricate  than  the  branched  glands  of  the  stomach.  A  section 
shows  that  it  is  composed  of  lobules,  each  consisting  of  a  group  of 
large  granular  cubical  cells  around  a  small  central  cavity  or  lumen. 
The  cavities  or  alveoli  of  neighbouring  lobules  open  into  a  small 
tube  or  duct,  and  these  ducts  join  with  one  another  and  ultimately 
open  into  the  bile  duct.  The  ducts  are  readily  detected  in  a  section, 
owing  to  their  comparatively  large  lumen  and  the  flat  cells  forming 
their  walls.  It  will  be  seen  then  that  such  a  gland  bears  some 
resemblance  to  a  bunch  of  grapes  squeezed  together,  the  ducts 


AN   INTRODUCTION  TO  ZOOLOGY 


c° 


FIG.  28. — Small  portion  of  pancreas. 

a.,  alveolus  ;  c.c.,  central  cells  of  alveolus  ;  c.t.,  con- 
nective tissue  ;  d.,  duct  wall  (cubical  epithelium)  ;  I.d., 
lumen  of  duct ;  *.g.,  zymogen  granules. 


represent  the  stalks  and  the  alveoli  the  grapes,  and  for  this  reason  it 
is  termed  a  racemose  gland.     The  glandular  cells  contain  numerous 

zymogen  granules,  which, 
when  discharged,  give  rise 
to  the  enzymes  of  the 
pancreatic  juice  that  are 
utilised  in  digestion. 

The  liver,  the 
largest  gland  in  the  body, 
is  also  a  complex  gland. 
It  arises  as  a  compound 
tubular  gland,  but  during 
its  growth  the  various  tubes 
unite  with  one  another  into 
a  close  reticulation,  so  that 
in  the  adult  its  structure  is 
difficult  to  make  out.  In 
section  it  is  seen  to  consist 
of  large  numbers  of  poly- 
hedral cells  with  distinct 
nuclei  embedded  in  granu- 
lar cytoplasm.  It  has  also 

a  very  plentiful  supply  of  blood-vessels.  Here  and  there  ducts  of 
different  sizes  will  be  seen,  and  masses  of  pigment  are  irregularly 
distributed  in  it.  The  functions  of  the  liver  are  three  in  number. 
In  the  first  place  it  secretes  a  fluid,  the  bile,  that  takes  part  in  the 
digestive  processes.  This  bile  is  collected  by  numerous  small 
ducts,  and  thence  taken  to  the  cystic  ducts.  The  second  function 
is  the  storage  in  its  cells  of  a  substance,  glyeogen  or  animal  starch. 
The  various  sugars  obtained  from  the  food  are  brought  to  the  liver 
by  the  blood,  and  there  transformed  into  glyeogen,  which  is  in- 
soluble. When  sugar  is  again  required  by  the  blood  the  glyeogen 
is  re-converted  into  soluble  sugar  and  given  up  to  the  blood.  By 
this  means  the  proportion  of  sugar  in  the  blood  is  kept  fairly 
constant,  and  the  liver  acts  as  a  storehouse  in  which  the  excess 
may  be  kept  until  required.  Sugar  and  substances  of  a  similar 
chemical  nature,  the  carbohydrates,  play  an  important  role  in 
supplying  the  energy  necessary  for  the  bodily  activities.  The 
third  function  of  the  liver  is  connected  with  the  removal  of  waste 
matter  from  the  system.  Certain  of  the  waste  products  are 
brought  to  the  liver  by  the  blood,  and  there  transformed  into  a 
substance  called  urea.  This  is  secreted  back  again  into  the  blood, 
whence  it  is  eliminated  by  the  kidneys. 


THE  FROG  53 

Digestion. 

We  must  now  turn  our  attention  briefly  to  the  actual  process 
of  digestion  itself,  and  in  doing  so  we  shall  consider  this  process 
in  general,  and  not  in  the  frog  in  particular.  The  chief  point 
of  difference  between  the  process  in  the  frog  and  the  rabbit  is 
that  the  former  does  not  possess  any  salivary  glands,  and  hence 
the  digestion  does  not  actually  commence  in  the  mouth.  In 
correlation  with  this  we  find  that  the  frog  does  not  masticate  its 
food.  Whether  the  animals  considered  be  carnivorous,  i.e.  flesh- 
eating,  or  herbivorous,  i.e.  plant-eating,  the  reactions  are  funda- 
mentally the  same,  for  although  the  actual  food  substances  differ 
slightly  in  the  two  cases  they  are  closely  allied  chemically.  The  food 
taken  in  consists  of  matter  that  is,  or  has  been,  living,  and  the 
greater  part  of  it  falls  into  a  small  number  of  groups  of  substances, 
sometimes  spoken  of  as  the  proximate  principles.  One  most  im- 
portant constituent  of  all  food  is  water,  in  addition  to  which  we 
find: 

1.  Mineral  salts,  such  as  common  salt,  etc. 

2.  Proteins,  i.e.  compounds  formed  by  living  matter  and  contain- 
ing Nitrogen.     Ultimate  analysis  shows  them  to  contain  Carbon, 
Hydrogen,  Oxygen,  Nitrogen  and  Sulphur  in  various  proportions. 

3.  Fats,  i.e.  compounds  of  fatty  acids  with  Glycerine.* 

4.  Carbohydrates,   i.e.   compounds  of  Carbon,   Hydrogen  and 
Oxygen,  in  which  the  last  two  are  in  the  same  proportion  as  in 
water,  f 

The  first  group  contains  inorganic  substances,  and  the  last  three 
organic,  that  is  to  say,  substances  that  in  nature  only  occur  in  con- 
nection with  living  matter.  Nearly  all  of  them  are  in  solution,  or 
readily  soluble.  Matter  in  solution  falls  into  two  classes,  according 
as  to  whether  it  can  or  cannot  pass  through  an  organic  membrane. 
If  a  vessel  be  divided  into  two  compartments  by  means  of  an  organic 
membrane  (e.g.  parchment) ,  and  a  solution  of  common  salt  be  poured 
into  one  side  and  ordinary  water  into  the  other,  it  will  be  found 
after  a  time  that  salt  is  present  in  both  compartments.  It  will 
continue  to  pass  through  the  membrane  until  the  solution  on  both 
sides  is  of  equal  strength.  Such  a  passage  is  termed  osmosis  or 
dialysis,  and  a  compound  capable  of  performing  it  is  distinguished 
as  a  crystalloid,  as  such  substances  are  usually  easily  obtainable  in  a 
crystalline  condition.  The  other  class  of  substances  would  be  quite 

*  The  fatty  acids  are  a  series  of  acids  derived  from  the  oxidation  of  mono- 
tomic  alcohols. 

f  This  definition  is  convenient  rather  than  accurate,  for  while  including  the 
Carbohydrates  it  also  includes  a  few  compounds  like  acetic  and  laetic^acids 
that  are  not  carbohydrates. 


54  AN   INTRODUCTION   TO  ZOOLOGY 

unable  to  pass  through  the  membrane  in  this  manner,  and  are  termed 
colloids. 

We  have  already  seen  that  the  alimentary  canal  is  a  complicated 
tube  running  from  mouth  to  anus,  but  nowhere  opening  into  the 
body  itself.  The  food  then,  in  order  to  be  utilised  by  the  animal, 
must  find  its  way  into  the  tissues  through  the  walls  of  the  canal,  but 
these  walls  consist  of  living  cells,  and  hence  form  an  organic  mem- 
brane. Some  constituents  of  food  are  insoluble,  and  of  the  re- 
mainder only  the  mineral  salts  and  certain  of  the  carbohydrates  are 
crystalloids,  while  the  remainder  of  the  carbohydrates,  the  fats,  and 
the  proteins  are  colloids.  The  problem  confronting  an  animal  when 
its  food  is  secured  therefore  is,  how  can  the  insoluble  substances 
be  made  soluble,  and  how  can  the  colloids  in  solution  be  converted 
into  crystalloids  so  as  to  pass  through  the  gut  wall  ?  In  order  to 
effect  the  necessary  changes,  we  find  in  the  higher  animals  a  complex 
system  of  glands  has  been  evolved,  and  the  food  is  subjected  to 
the  action  of  a  number  of  different  substances  produced  by  them. 
The  various  alterations  undergone  by  the  food  up  till  the  time  it 
passes  into  the  wall  of  the  canal  are  all  included  in  the  term  digestion. 

It  is  beyond  our  scope  to  enter  into  the  details  of  the  chemical 
changes  brought  about  by  the  enzymes  in  the  digestive  juices,  but 
a  consideration  of  a  few  of  the  more  important  will  serve  to  illustrate 
their  general  mode  of  action.  The  principal  ones  act  by  Hydrolysis, 
i.e.  they  cause  the  molecules  of  the  substance  to  decompose  or 
undergo  cleavage  by  a  reaction  with  water.*  They  may  be  separated 
into  protein-splitting  enzymes,  starch-splitting  enzymes,  fat- 
splitting  enzymes,  sugar-splitting  enzymes,  and  so  on,  according  to 
the  material  upon  which  they  act. 

Digestion  commences  in  the  mouth  by  the  action  of  the  ptyalin 
of  the  saliva  attacking  and  splitting  up  the  starchy  food.  It  is 
continued  in  the  stomach  where,  in  addition  to  encountering  the 
hydrochloric  acid,  it  is  acted  upon  by  the  two  ferments,  pepsin  and 
rennin,  which  are  present  in  the  gastric  juice.  The  former  acts 
upon  proteins,  breaking  them  down  to  simpler  substances,  peptones, 
and  the  latter  coagulates  milk.  The  food  now,  after  mastication 
and  partial  digestion  in  the  stomach,  assumes  a  batter-like  con- 
sistency and  gives  an  acid  reaction  to  litmus.  In  this  condition  it 
is  termed  chyme.  After  it  has  been  thoroughly  acted  upon  by  the 
gastric  juice  it  is  passed  on  to  the  duodenum,  and  here  comes  under 
the  influence  of  the  bile  and  pancreatic  juice. 

*  Thus,  for  example,  one  carbohydrate,  maltose,  is  transformed  into  another, 
dextrose,  by  the  action  of  the  enzyme  maltase — 

C12H22On+H20=C6H1206+CbH1206. 

(maltose)  (dextrose)      (dextrose) 


THE  FROG  55 

Bile  is  in  part  an  excretion,  and  conveys  out  of  the  body 
certain  waste  materials  derived  from  haemoglobin.  Separately 
it  has  practically  no  digestive  action,  though  in  the  body  it  greatly 
augments  the  action  of  the  pancreatic  juice,  more  particularly  in 
regard  to  its  action  in  fat-splitting,  and  its  salts  help  in  emulsification. 

Pancreatic  juice  is  a  far  more  active  digestive  agent,  and, 
in  addition  to  being  alkaline,  contains  three  principal  ferments. 
Trypsin  is  an  enzyme  splitting  up  the  peptones  to  amino-acids,  and 
this  has  much  the  same  action  as  pepsin  in  breaking  down  proteins, 
save  that  it  acts  in  an  alkaline  solution.*  Amylase  is  a  starch- 
splitting  enzyme  that  is  more  active  than  the  pytalin  of  the  saliva. 
Lipase  acts  upon  certain  fats,  breaking  them  down  into  glycerol  and 
fatty  acids,  which,  in  their  turn,  unite  with  the  alkali  present  in  the 
pancreatic  juice  to  form  soap.  The  soap  acts  upon  the  remaining 
fats  in  a  mechanical  way,  aided  also  by  the  bile  salts,  and  forms  an 
emulsion,  i.e.  a  white  milky-looking  fluid,  formed  by  minute  fat 
globules  being  suspended  in  a  liquid.  The  result  of  these  various 
actions  is  that  the  food,  having  of  coarse  an  alkaline  reaction,  is  of 
the  consistency  of  milk,  and  is  now  termed  chyle. 

During  its  passage  through  the  remaining  part  of  the 
intestine  ereptase  continues  splitting  up  the  amino-acids,  the  various 
digestive  processes  are  completed,  and  the  food  is  taken  up  or 
absorbed  by  the  walls.  The  colloids,  now  transformed  into  crystal- 
loids, pass  through  the  mucous  membrane  by  osmosis  into  the  under- 
lying blood-vessels,  and  are  thus  distributed  to  various  parts  to  be 
utilised  in  the  general  anabolism.  The  fats,  however  finely  emulsi- 
fied, remain  colloidal,  and  it  appears  as  if  the  cells  of  the  intestinal 
wall  actually  take  the  small  globules  in  and  pass  them  out  at  their 
inner  end  into  special  vessels,  the  lacteals.  These  vessels,  with 
similar  ones  in  all  parts  of  the  body,  form  an  auxiliary  part  of  the 
circulatory  system  known  as  the  lymphatic  system,  and  are  specially 
numerous  in  the  intestinal  walls. 

A  certain  amount  of  insoluble  matter  is  always  taken  in 
with  the  food,  and  this,  together  with  the  undigested  residue, 
accumulates  in  the  rectum  as  more  or  less  solid  pellets,  the  faeces, 
which  are  discharged  as  excreta  from  time  to  time.  It  is  noticeable 
when  comparing  an  herbivorous  am'mal,  such  as  the  rabbit,  with  a 
carnivore,  like  the  frog,  that  the  intestine  is  a  great  deal  longer 


*  The  action  of  trypsin  is  dependent  on  the  presence  of  another  substance, 
enterokinase,  which  is  produced  by  the  mucous  membrane  of  the  duodenum 
when  it  is  acted  upon  by  an  acid .  The  very  pancreatic  fluid  itself  is  not  secreted 
until  a  substance,  secretin,  is  made  by  the  mucosa  of  the  duodenum  and  con- 
veyed to  the  pancreas  by  the  blood.  Indeed,  the  digestive  actions  in  this  part 
of  the  gut  are  too  complex  to  be  adequately  dealt  with  here. 


56  AN   INTRODUCTION  TO  ZOOLOGY 

relatively  to  the  size  of  the  animal,  and  the  amount  of  excreta  is 
also  greater  in  the  former. 

We  have  seen  then  that  the  alimentary  canal  is  a  long  tube, 
whose  various  regions  are  modified  for  different  purposes.  With  it 
are  connected  glands,  which  arise  in  embryonic  life  as  outgrowths 
from  its  walls,  each  of  which  produces  a  specific  substance  or  sub- 
stances, termed  enzymes,  playing  definite  parts  in  the  splitting  up  of 
certain  colloidal  compounds.  The  whole  system  acts  together  for 
the  digestion  and  absorption  of  the  food,  in  order  that  it  can  be 
utilised  by  the  body. 

Respiratory  System. 

Attention  has  already  been  directed  to  the  slit-like  opening 
at  the  back  of  the  floor  of  the  mouth,  known  as  the  glottis.  It  is 
situated  upon  a  slight  median  elevation  lying  between  the  posterior 
cornua  of  the  hyoid  plate,  and  it  leads  into  a  space,  the  laryngo- 
tracheal  chamber,  or,  more  briefly,  the  larynx,  whose  walls  are 
supported  by  cartilages.  The  cartilaginous  structures  are  five  in 
number,  a  complex  ring-like  cartilage,  the  cricoid,  runs  around  its 
median  walls,  while  the  roof  of  it  is  supported  by  a  pair  of  semi- 
lunar  cartilages,  the  arytenoids,  and  at  the  middle  of  the  inner  edge 
of  each  of  these  is  a  small  cartilage,  the  pre-arytenoid.  It  is  these 
two  pairs  of  cartilages  that  project  slightly  into  the  buccal  cavity, 
and  between  them  lies  the  glottis.  Two  flat  bands  of  connective 
tissue,  the  vocal  cords,  related  to  the  cartilages,  stretch  across  the 
laryngo-trachial  chamber,  leaving  between  them  a  somewhat  long 
and  narrow  opening,  the  rima  glottidis.  They  can  be  approximated 
by  means  of  a  special  set  of  muscles,  and  so  enable  the  frog  to  croak 
by  expelling  the  air  from  the  lungs  sharply  through  the  reduced 
opening.  The  volume  of  sound  thus  produced  is  increased  in  the 
male  by  the  presence  of  two  bags,  the  vocal  sacs,  in  the  floor  of  the 
mouth,  which  can  be  inflated  with  air  and  so  act  as  resonators.  They 
are  particularly  well  developed  in  R.  esculenta.  The  laryngo- 
tracheal  cartilages  are  provided  with  a  series  of  muscles,  by  means  of 
which  the  glottis  can  be  closed,  while  food  is  being  swallowed,  and 
opened,  and  the  chamber  distended  during  the  taking  in  of  air,  and 
so  on. 

The  laryngo-tracheal  chamber  leads  directly  into  the  lungs,  a 
pair  of  dark-coloured,  thin- walled  sacs  lying  far  forward  in  the  body 
cavity  on  its  dorsal  side  near  the  heart.  The  lungs,  like  the  digestive 
glands,  arise  during  early  life  from  the  wall  of  the  alimentary  canal.' 
At  first  they  are  represented  by  a  single  outgrowth  from  the  floor  of 
the  pharynx,  but  as  this  grows  backwards  it  splits  into  two.  Their 
original  connection  with  the  pharynxes  retained  as  the  glottis. 


THE  FROG  57 

Each  lung  is  an  oval  sac  with  a  pointed  posterior  end,  with  extremely 
elastic  walls,  and  usually  is  found  in  a  collapsed  condition.  It  may, 
however,  be  inflated  easily  by  means  of  a  blowpipe  inserted  into  the 
larynx.  The  interior  is  divided  up  by  a  network  of  partitions  into  a 
number  of  chambers  incomplete  at  their  inner  ends.  These  in  their 
turn  are  sub-divided  into  a  larger  number  of  smaller  cells,  the 
alveoli,  by  a  multitude  of  smaller  partitions. 

The  act  of  breathing  is  somewhat  complex,  and  differs  from 
that  in  man.  The  frog  possesses  no  ribs  whereby  it  can  increase 
the  size  of  the  chest  cavity  and  so  cause  the  air  to  rush  in  ;  on  the 
contrary,  the  lungs  are  highly  elastic  and  tend  to  expel  the  air,  which, 
therefore,  has  to  be  forced  into  them.  The  forcing  is  carried  out  by 
the  floor  of  the  mouth  in  conjunction  with  certain  valves  in  the 
nostrils.  Internal  and  external  nares  are  connected  by  a  continuous 
passage,  allowing  of  easy  ingress  and  egress  of  air  when  not  closed 
by  the  valves.  Three  stages  can  be  recognised  in  the  process  : 
firstly,  aspiration,  during  which  air  is  drawn  in  through  the  nostrils 
by  means  of  lowering  the  floor  of  the  mouth,  the  glottis  being  kept 
closed  ;  secondly,  expiration,  when  the  contraction  of  the  trunk 
muscles  and  the  elasticity  of  the  lungs  expel  some  of  the  air  from  the 
lungs  through  the  glottis,  now  open,  into  the  buccal  cavity  ;  lastly, 
immediately  following  upon  the  foregoing,  inspiration,  in  which  the 
mixed  air  is  pumped  into  the  lungs  by  raising  the  floor  of  the  mouth 
and  at  the  same  time  keeping  the  nares  closed.  It  is  obvious  from 
this  that  the  air  breathed  into  the  lungs  is  not  fresh,  but  a  mixture 
of  pure  air  taken  in  through  the  nostrils  with  impure  air  that  has 
already  been  in  the  lungs.  If  a  living  frog  is  observed  it  will  be 
noticed  that  the  floor  of  the  mouth  is  constantly  being  moved  up  and 
down,  these  movements  being  concerned  with  respiration. 

The  lungs  are  extremely  well  supplied  with  blood-vessels  whose 
smallest  branches  ramify  in  a  close  network  in  the  walls  of  the 
alveoli.  They  are  covered  only  by  the  alveolar  epithelium,  which  is 
composed  of  a  single  layer  of  flattened  cells,  and  so  the  blood  in 
them  is  only  separated  from  the  air  by  their  own  very  thin  walls 
and  the  alveolar  epithelium.  Thus  the  oxygen  in  the  air  is  enabled 
to  diffuse  into  the  blood,  where  it  forms  an  unstable  compound 
with  the  colouring  matter  of  the  blood,  and  in  this  condition  is 
carried  to  the  tissues,  where  it  is  utilised  to  release  energy  by  the 
oxidation  of  certain  substances.  In  this  way  the  carbonic  acid  gas, 
one  of  the  main  products  of  the  oxidation  process,  that  has  been 
collected  up  from  the  various  parts  and  dissolved  in  the  fluid  portion 
of  the  blood  is  able  to  pass  from  it  into  the  air  in  the  lungs,  whence  it 
is  expelled. 

This  taking  in  of  oxygen  from  the  air  and  giving  off  of  carbonic 


58  AN   INTRODUCTION  TO  ZOOLOGY 

acid  gas  from  the  blood  constitutes  respiration  and  the  exchange  of 
the  two  gases  is  termed  the  respiratory  exchange. 

Some  authorities  maintain  that  the  mere  osmotic  interchange  of 
gases  just  outlined  is  not  sufficient  to  account  for  the  total  amount  of 
gases  taken  up  and  given  off,  and  suggest  that  the  alveolar  epithelium 
also  plays  a  part  in  it  by  actually  absorbing  the  gas  from  the  one 
side  and  passing  it  through  to  the  other. 

The  lungs  are  the  principal  centre  of  respiration,  and  in  the 
higher  animals  the  only  places  where  it  occurs.  In  the  frog, 
however,  this  respiration,  i.e.  pulmonary,  is  supplemented  by  two 
other  kinds.  The  small  blood-vessels  in  the  skin  are  much  larger 
than  in  other  animals  and  come  near  to  the  surface,  which  is  always 
moist,  and  so  allow  of  a  cutaneous  respiration.  Again,  the  vessels 
in  the  wall  of  the  pharynx  and  buccal  cavity  give  off  curious  sac-like 
diverticula,  which  lie  between  the  epithelial  cells  lining  those  regions. 
This  brings  the  blood  into  contact  with  the  air,  and  a  buccal  or 
pharyngeal  respiration  occurs.  These  two  last  forms  of  respiration 
are  peculiar  to  the  Amphibia,  the  class  to  which  frogs  belong,  among 
the  Vertebrates,  and  are  not  equally  important  in  all  members  of 
that  class. 

Respiration  is  the  exchange  of  carbonic  acid  gas  in  the 
blood  for  oxygen  in  the  air,  and  is  carried  out  mainly  by  osmosis  in 
the  very  vascular  lungs.  The  exchange  is  aided  by  a  force-pump 
action  of  the  floor  of  the  mouth,  and  in  Rana  is  supplemented  by 
similar  exchanges  in  the  skin  and  lining  of  the  mouth  and 
pharynx. 

Circulatory  System. 

The  circulatory  or  vascular  system  of  a  vertebrate  consists 
of  a  series  of  tubes  by  means  of  which  the  blood  is  carried  round  the 
body.  The  central  point  in  this  system  is  the  heart,  an  elaborate 
structure  which  pumps  the  blood  into  the  blood-vessels,  and  by 
means  of  series  of  valves  causes  it  to  keep  circulating  in  the  same 
direction.  Its  position  in  the  pericardium  and  the  main  vessels 
connected  with  it  have  already  been  noted.  The  large  trunks  con- 
veying blood  away  from  the  heart  are  termed  the  arteries,  and  these 
break  up  into  smaller  and  smaller  branches  as  they  get  farther  from 
the  heart.  The  smallest  of  these  are  termed  arterioles,  and  they 
penetrate  into  all  the  tissues  of  the  body.  In  the  tissues  they  break 
up  into  an  interlacing  network  of  very  minute  vessels  with  extremely 
thin  walls,  the  capillaries,  which  unite  again  to  form  somewhat 
larger  vessels,  the  venules.  These  in  their  turn  unite  to  form  larger 
and  larger  trunks  carrying  the  blood  back  to  the  heart,  these  are  the 
veins. 


THE  FROG 


59 


The  heart,  then,  may  be  regarded  as  the  centre  of  the  vascu- 
lar system  which,  as  we  have  seen,  is  a  closed  series  of  tubes.  It 
consists  of  five  chambers,  a  sinus  venosus,  two  auricles,  a  ventricle 
and  a  conus  arteriosus.  As  has  already  been  noticed,  it  is  situated 
far  forwards  in  the  body  in  front  of  the  main  mass  of  the  liver,  and 
ventral  to  the  lungs.  Around  it  is  a  more  or  less  closely  fitting  bag, 
the  pericardium,  composed  of  a  white,  glistening,  semi-transparent 
membrane  that  is  reflected  back  and  covers  the  heart  very 


ven 


FIG.  19. — A,  The  frog's  heart  dissected  from  the  ventral  surface.  B,  an 
enlarged  semi-diagrammatic  view  of  the  truncus  arteriosus  ;  the  ventral 
wall  has  been  cut  through  somewhat  to  the  observer's  right  of  the  middle 
line,  and  the  walls  have  been  turned  back. — From  Bourne. 

aw.,  auriculo-ventricular  valve,  with  its  cordse  tendineae  ;  c.ao.,  cavum  aorticum  of  the 
truncus  arteriosus  ;  car.,  carotid  artery  ;  c.pm.,  cavum  pulmonale  of  the  truncus  arteriosus  ; 
l.au.,  left  auricle  ;  p.ca.,  opening  of  the  pulmo-cutaneous  arches  into  the  cavum  pulmonale  ; 
pmc.,  pulmo-cutaneous  artery ;  pv.,  opening  of  the  pulmonary  vein  into  the  left  auricle  ;  r.au., 
right  auricle  ;  r.sa.,  opening  of  the  right  systemic  artery  ;  in  B  a  rod  is  passed  up  the  opening  and 
projects  from  the  cut  end  of  the  middle  or  systemic  channel  of  the  right  aortic  arch  ;  s.m.,  septum 
medium  of  the  upper  part  of  the  truncus  ;  s.p.,  septum  principale  ;  s/>.t>.,  spiral  valve,  in  B  the 
reference  line  points  to  the  surface  of  attachment  to  the  ventral  wall  of  the  truncus  which  has  been 
cut  through  ;  sv.,  opening  of  the  sinus  venosus  into  the  right  auricle  ;  sys.,  systemic  artery  ;  v.1, 
proximal  row  of  semi-lunar  valves  guarding  the  passage  from  the  ventricle  into  the  truncus  ; 
v.z,  distal  row  of  semi-lunar  valves  in  the  truncus  ;  ten.,  ventricle. 

intimately.     Thus  the  heart  comes  to  lie  in  a  specially  separated 
part  of  the  body  cavity. 

The  sinus  venosus  is  a  dark-coloured  thin-walled  sac  lying  on 
the  dorsal  side  of  the  heart  ;  it  is  triangular  in  shape,  with  the  apex 
pointing  backwards  and  into  its  three  corners  open  the  three  main 
veins  of  the  body,  the  Venae  cavae  or  Caval  veins.  It  opens  into  the 
right  auricle  by  a  fairly  large  transverse  slit  whose  edges  are  guarded 
by  valves,  the  sinu-auricular  valves,  which  allow  the  blood  to  pass 
from  sinus  to  auricle,  but  not  in  the  reverse  direction. 


60  AN   INTRODUCTION  TO  ZOOLOGY 

The  two  auricles  together  form  a  large  dark  hemispherical  sac, 
with  thin  walls,  often  termed  the  atrium,  lying  immediately  in  front 
of  the  ventricle  from  which  it  is  separated  by  a  deep  furrow,  the 
coronary  sulcus.  This  sac  is  completely  divided  by  a  vertical 
partition,  the  inter-auricular  septum,  into  two  chambers,  a  large  right 
and  a  smaller  left  auricle.  The  right  communicates  with  the  sinus, 
as  already  noted,  and  into  the  left  opens  the  common  pulmonary 
vein  formed  by  the  union  of  the  two  pulmonary  veins,  one  from  each 
lung.  The  two  auricles  open  into  the  ventricle  by  a  single  aperture, 
the  auriculo-ventricular  aperture,  which  is  partly  divided  by  the 
posterior  free  edge  of  the  septum.  The  opening  is  guarded  by  two 
auriculo-ventricular  valves,  one  dorsal  and  one  ventral,  which  stop 
the  blood  flowing  from  ventricle  to  auricle.  These  are  two  mem- 
branous flaps  whose  free  edges  are  tied  to  the  wall  of  the  ventricle 
by  a  number  of  tough  tendinous  cords,  the  chordae  tendinse,  and  are 
so  enabled  to  withstand  the  pressure  of  the  blood  during  the  con- 
traction of  the  ventricle. 

The  ventricle  is  a  moderately  elongated  pinkish  structure  bluntly 
pointed  at  its  posterior  end.  A  horizontal  cut  shows  it  to  have  a 
comparatively  small  cavity  surrounded  by  very  thick  muscular 
walls.  The  walls  appear  spongy,  owing  to  the  presence  of  a  number 
of  interlacing  projecting  muscular  ridges  between  which  the  ventri- 
cular cavity  dips  deeply.  This  sponginess  of  the  ventricular  wall 
plays  a  part  in  the  separation  of  the  two  different  sorts  of  blood 
brought  to  the  heart. 

The  Conus  arteriosus  is  a  fairly  stout  tube,  with  walls  composed 
of  cardiac  muscle,  which  arises  from  the  anterior  ventral  corner  of 
the  ventricle  on  the  right  and  runs  forward  obliquely  towards  the 
left  on  the  ventral  surface  of  the  atrium.  Its  exit  from  the  ventricle 
is  guarded  by  three  semi-lunar  valves,  i.e.  valves  shaped  something 
like  small  watch  pockets,  whose  free  edges  can  meet  in  the  lumen 
and  stop  the  blood  returning  to  the  ventricle.  Within  it  is  a  spiral 
fold  of  membrane,  and  its  anterior  end,  marked  by  another  set  of 
three  semi-lunar  valves  ;  it  is  continued  into  a  small  terminal  portion, 
the  truncus  arteriosus,  forming  a  common  chamber  from  which  two 
branches  are  given  off.  Each  of  these  is  divided  internally  into 
three  ;  the  carotid  in  front,  the  systemic  in  the  middle,  and  the 
pulmo-cutaneous  behind,  which  soon  manifest  themselves  externally 
as  three  separate  arteries.  These  three  arteries  on  each  side 
constitute  the  aortic  arches.  By  some  authors  the  conus  is  termed 
the  pylangium,  and  the  truncus  the  synangium,  quite  superfluous 
terms.* 

*  Considerable  confusion  exists  in  the  way  these  terms  are  used  in  text- 
books. The  term  conus  arteriosus  (bulbus  cordis  or  pylangium)  should  be 


THE  FROG  61 

The  spiral  fold  or  valve  within  the  conus  is  a  flap  of  membrane 
running  obliquely  and  spirally  forwards,  practically  dividing  its 
lumen  into  two  passages.  It  begins  on  the  ventral  side  near  the 
right  semi- lunar  valve  of  the  posterior  series,  and,  for  the  most  part, 
attached  on  the  dorsal  side  with  its  ventral  edge  free,  it  ends  on  the 
dorsal  side  of  the  conus  near  the  large  right  semi-lunar  valve. 
Owing  to  the  way  in  which  it  is  attached,  the  cavity  of  the  conus 
is  divided  into  a  right  channel,  the  cavum  aorticum,  which  leads 
off  directly  from  the  ventricle  and  a  left  channel,  the  cavum  pulmo- 
cutaneum,  not  directly  continuous  with  the  ventricle. 

The  truncus  is  divided  by  a  horizontal  septum  into  dorsal  and 
ventral  compartments.  The  dorsal  chamber  at  the  one  end  com- 
municates with  the  cavum  pulmo-cutaneum  by  an  aperture  guarded 
by  one  semi-lunar  valve,  and  at  the  other  passes  over  on  each  side 
into  the  pulmo-cutaneous  artery.  The  ventral  chamber  is  con- 
tinuous with  the  cavum  aorticum,  and  is  further  subdivided  by  a 
median  vertical  partition,  the  septum  medium,  into  a  right  vacity 
which  leads  into  the  right  systemic  arch,  "and  also  the  two  carotid 
arches  and  a  left  chamber  which  leads  into  the  left  systemic  arch. 

Arterial  System. 

The  arteries  of  the  body,  forming  the  arterial  system,  all 
arise  from  the  three  arches  coming  off  from  the  truncus  arteriosus, 
namely,  the  carotid,  the  systemic  and  the  pulmo-cutaneous.  While 
still  united  in  a  common  trunk  they  pass  outwards  round  the 
oesophagus  for  a  short  distance  before  dividing  into  separate  vessels 
The  Carotid  Arch,  soon  after  becoming  separate,  gives  off  a 
branch,  the  external  carotid  (often  termed  the  lingual),  which  supplies 
the  tongue,  lower  jaw  and  hyoid  apparatus,  and  then  swells  out  to 
form,  an  enlargement,  the  so-called  carotid  "  gland,"  in  which  the 
carotid  breaks  up  into  a  considerable  number  of  small  vessels  which 
reunite  to  form  the  internal  carotid  artery.  This  "  gland  "  is  the 
remains  of  a  vascular  connection  present  in  the  tadpole,  and 
it  is  suggested  by  some  authorities  that  it  plays  a  part  in  the 
regulation  of  the  blood  pressure  in  the  carotid  arch.  The  internal 
carotid  runs  outwards  and  upwards  to  the  base  of  the  skull,  where 

applied  in  the  frog  to  that  part  of  the  heart  marked  off  at  each  end  by  the 
semi-lunar  valves.  Its  walls  are  composed  of  cardiac  muscle,  it  is  rhythmically 
contractile,  and  it  is  homologous  with  the  similarly  named  portion  of  the 
dogfish  heart.  For  some  reason  or  other  it  has  been  erroneously  termed  in 
some  books,  truncus  arteriosus.  The  truncus  arteriosus  (pylangium),  however, 
is  merely  the  terminal  chamber,  and  represents  in  a  very  abbreviated  form  the 
Ventral  aorta  (truncus  arteriosus  or  truncus  aortae)  of  the  dogfish.  The  posses- 
sion of  a  short  truncus  arteriosus  makes  the  heart  of  Rana  intermediate  between 
that  of  Scy Ilium,  where  it  is  long,  and  that  of  the  rabbit,  where  it  is  absent 
altogether. 


62 


AN   INTRODUCTION  TO  ZOOLOGY 


C.G 


I.C. 


L.B 


it  divides  into  a  palatine  branch,  supplying  the  palate,  part  of  the 
oesophagus  and  the  orbit,  and  a  cerebral  branch  which  passes  through 
a  foramen  in  the  base  of  the  skull  and  spreads  out  over  the  brain. 

The  Systemic  Arch  runs  on  laterally  around  the  oesophagus  to 
unite  partially  with  its  fellow  of  the  opposite  side,  and  so  give  rise 

to  the  dorsal  aorta,  the  main 
arterial  trunk  of  the  posterior 
part  of  the  body.  During  its 
passage  around  the  oesophagus 
the  systemic  arch  on  each  side 
gives  off  a  number  of  branches. 
The  laryngeal  and  oesophageal  are 
two  fairly  small  twigs,  supplying 
the  larynx  and  the  oesophagus 
respectively.  They  are  followed 
by  the  occipito-vertebral  artery,  a 
short  trunk  that  divides  into  the 
occipital,  running  forwards  to  the 
back  and  sides  of  the  head,  and 
the  vertebral,  which  runs  back 
parallel  with  the  vertebral  column 
and  gives  off  branches  to  the 
muscles  of  the  back  and  to  the 
spinal  cord.  The  sub-clavian  is 
the  largest  of  the  arteries  coming 
off  from  the  arch.  It  runs  straight 
outwards,  supplying  the  shoulder 
girdle  and  the  fore  limb. 

The  two  arches  run  together  on 
the  dorsal  side  of  the  body  cavity 
just  below  the  vertebral  column 
about  the  level  of  the  sixth  verte- 
bra, and  the  left  communicates 
with  the  right  by  a  small  opening, 
but  it  is  in  the  main  continuous 
with  thecoeliaco-mesenteric  artery, 
the  first  great  branch  coming  off 
from  the  dorsal  aorta.  The  cceliaco- 
mesenteric  quickly  divides  into  two  the  cceliac  and  the  mesenteric. 
From  the  cceliac  spring  the  gastric  artery  supplying  the  stomach  and 
pancreas,  and  the  hepatic  supplying  the  liver.  The  mesenteric,  after 
giving  off  the  splenic  to  the  spleen,  splits  into  two  main  branches, 
going  to  the  remaining  parts  of  the  alimentary  canal  exclusive  of  the 
end  of  the  rectum. 


FIG.  20, — Diagram  of  arterial 
system.     Rana. 

C.,  coeliac  ;  C.A.,  conus  arteriosus  ;  C.G., 
carotid  gland  ;  C.M.,  coeliaco-mesenteric  ; 
Cu.,  cutaneous  ;  D.O.,  dorsal  aorta  ;  E.,  epi- 
gastric ;  E.G.,  external  carotid  ;  F.,  femoral ; 
G.,  gastric  ;  Ge.,  genital ;  H.,  Hepatic  ;  I., 
iliac  ;  I.C.,  internal  carotid  ;  L.,  laryngeal ; 
L.B.,  ligamentum  (ductus)  botalli;  L.S.,  left 
systemic  arch  ;  M.,  mesenteric  and  intes- 
tinal ;  O.,  occipital ;  Oe.,  oesophageal ;  O.V., 
occipito-vertebral ;  P.,  pulmonary  ;  P.M., 
posterior  mesenteric  ;  R.S.,  right  systemic  ; 
R.V.,  recto-vesicular  ;  S.,  sciatic  ;  S.C.,  sub- 
clavian  ;  Sp.,  spermatic  ;  T.A.,  truncus  arte- 
riosus ;  V.,  vertebral. 


THE  FROG  63 

The  dorsal  aorta  then  passes  backwards  to  lie  between  the 
kidneys,  to  which  it  gives  off  a  branch  on  each  side  that  in  its  turn 
sends  a  genital  artery  (spermatic  in  the  male  and  ovarian  in  the 
female),  running  to  the  gonad  and  its  accompanying  fat  body. 
Immediately  following  this  are  from  three  to  five  renal  arteries 
passing  off  to  the  kidneys.  In  this  part  of  its  course  also  a  few  short 
arteries,  the  lumbars,  arising  from  the  dorsal  aorta,  supply  the 
muscles  of  the  back.  Behind  the  kidneys  the  dorsal  aorta  gives 
off  a  small  posterior  mesenteric  and  then  divides  into  two  large 
trunks,  the  iliacs,  one  running  towards  each  leg.  After  travers- 
ing a  short  distance  the  iliac  gives  rise  to  a  branch  that  almost 
immediately  divides  into  an  epigastric,  supplying  the  posterior 
portion  of  the  ventral  body  wall,  and  a  recto- vesicular,  supplying  the 
end  of  the  rectum  and  the  bladder.  Soon  after  this  the  iliac  splits 
into  a  femoral,  feeding  the  muscles  and  skin  of  the  upper  part  of  the 
thigh,  and  a  sciatic,  that  is  distributed  to  the  remainder  of  the  hind 
limb. 

The  external  carotid  artery  just  beyond  the  carotid  "  gland  "  is 
tied  to  the  systemic  arch  by  a  thin  strand  of  tissue.  In  the  tadpole 
this  is  an  open  tube,  and  in  consequence  it  is  termed  the  Ductus 
caroticus,  a  similar  remnant,  only  between  the  systemic  and  pul- 
monary arches,  is  found  in  mammals,  and  there  called  the  Ductus 
Botalli,  after  its  discoverer,  Botallus. 

The  Pulmo-cutaneous  Arch,  the  hindermost  of  the  three,  also 
passes  round  to  the  side  of  the  oesophagus  where  the  pulmonary 
artery  arises  and  runs  along  on  the  outer  side  of  the  .lung.  The 
great  cutaneous  artery,  as  the  trunk  is  now  called,  is  a  large  vessel 
which  after  passing  close  to  the  ear  is  reflected  backwards  and 
ramifies  over  the  under  surface  of  the  skin. 

From  the  main  arteries,  briefly  mentioned  above,  smaller  arteries 
and  ultimately  arterioles  arise,  and  so  constitute  a  network  of  vessels 
running  throughout  all  the  various  parts  of  the  body,  where,  as 
has  been  noted  already,  they  break  up  into  minute  capillaries,  and 
so  allow  the  blood  to  be  distributed  everywhere. 

Venous  System. 

As  we  have  already  seen,  the  blood  is  collected  up  from 
all  over  the  body  by  a  series  of  veins  which  unite  to  form  larger 
and  larger  trunks  and,  except  the  blood  from  the  lungs;  it  is  all 
returned  to  the  sinus  venosus  b)'  the  three  caval  veins. 

The  pulmonary  veins  are  vessels  running  up  the  dorso-lateral 
borders  of  the  lungs  and  uniting  to  form  a  single  vein  which  enters 
the  left  auricle  immediately  in  front  of  the  anterior  edge  of  the 
sinus  venosus. 


64 


AN   INTRODUCTION  TO  ZOOLOGY 


M. 


I.J. 


S.C. 


M.C. 


In  the  same  way  that  the  arteries  fall  into  three  groups,  according 
to  the  arches  from  which  they  spring,  so  the  veins  can  be  considered 
in  groups  according  to  the  trunks  they  ultimately  go  to  form.  The 
veins  may  be  dealt  with  in  the  following  groups  ;  those  contributing 

to  the  formation  of  the 
pre-caval  veins,  the 
renal  portal  and  ante- 
rior abdominal  veins 
and  the  hepatic  portal 
s.s  __  ^iX  ^^Zd  i }  /~  vein. 

Blood  from  the 
tongue  and  hyoid  is 
collected  by  the  lingual 
vein  that  runs  parallel 
with  the  external  ca- 
rotid artery.  The 
mandibular  vein  drains 
the  region  around  the 
lower  jaw  and  unites 
with  the  lingual  in  the 
neighbourhood  of  the 
carotid  "  gland "  to 
form  the  external  jugu- 
lar, a  short  trunk  run- 
ning towards  the  heart. 
Various  tributaries 
from  the  dorsal  side  of 
the  head,  the  eye  and 
the  brain  unite  near 
the  posterior  border  of 
the  orbit  to  form  an 
internal  jugular  vein 
which  passes  backwards 
for  a  short  distance. 
It  is  joined  by  the  sub- 
scapular  vein  coming 
from  the  shoulder  and 
part  of  the  muscles  of 

the  back,  and  the  common  vessel  so  formed,  termed  the  innominate, 
also  goes  directly  inwards  towards  the  heart. 

A  large  vein,  the  brachial,  collects  the  blood  from  the  fore  limb, 
shortly  after  leaving  which  it  is  confluent  with  the  musculo-cutaneous 
vein  near  the  shoulder  joint.  The  musculo-cutaneous  is  a  large 
vein  formed  by  the  union  of  a  number  of  tributaries  coming  mainly 


FIG.  21. — Diagram  of  venous  system  excluding 
pulmonary  veins.     Rana. 

A.A.,  anterior  abdominal ;  B.,  brachial ;  C.,  cardiac  ; 
D.L.,  dorso-lumbar  ;  E.J.,  external  jugular  ;  F.,  femoral ; 
G.,  genital ;  H.,  hepatic  ;  H.P.,  hepatic  portal  main  trunk 
and  branch  to  left  lobe  of  liver ;  H.P'.,  branch  of  hepatic 
portal  to  right  lobe  of  liver  ;  I.,  innominate  ;  I.J.,  internal 
jugular  ;  L.,  lingual ;  L.Pr.,  left  precaval ;  M.,  mandibular  ; 
M.C.,  musculo-cutaneous  ;  P.,  pelvic  ;  P.C.,  post  caval ; 
P.C'.,  inter-renal  portion  of  post  caval ;  R.,  renal ;  R.C., 
ramus  communicans  iliacus ;  R.P.,  renal  portal;  R.Pr., 
right  precaval ;  S.,  sciatic  ;  S.C.,  sub-clavian  ;  S.S.,  sub- 
scapular  ;  S.V.,  sinus  venosus. 


THE   FROG  65 

from  the  skin,  but  also,  as  its  name  implies,  from  the  muscles  of  the 
side  of  the  head  and  body.  The  common  vein  is  known  as  the  sub- 
clavian,  as  it  is  situated  just  inside  the  clavicle  and  it  goes  inwards 
and  slightly  forwards  in  the  direction  of  the  heart. 

The  external  jugular,  the  innominate  and  the  sub-clavian  veins 
all  unite,  often  at  one  point,  but  sometimes  the  first  two  join  a  very 
short  distance  before  flowing  into  the  third,  and  give  rise  to  the 
pre-caval  vein,  or  the  vena  cava  anterior.  The  pre-caval  vein  on 
each  side  enters  the  sinus  venosus  at  its  anterior  corner. 

From  each  kidney  arise  four  or  five  renal  veins  (venae 
renales  revehentes),  and  the  two  sets  unite  in  the  middle  line  to  form 
the  inter-renal  portion  of  the  post-caval  vein,  or  vena  cava  posterior. 
Into  the  anterior  of  these  renals  flows  a  genital  vein  (spermatic  in 
the  male  and  ovarian  in  the  female),  coming  from  the  gonad  and  fat 
body.  The  post-caval  vein  then  runs  forward,  parallel  with  and 
immediately  ventral  to  the  dorsal  aorta  to  the  liver.  It  goes  on 
partly  embedded  in  the  substance  of  this  gland  to  which  it  gives 
no  branches,  but  from  which  it  receives  the  wide  short  hepatic 
vein  on  each  side,  and  then  enters  the  posterior  corner  of  the 
sinus  venosus. 

It  will  be  seen  from  the  foregoing  account  that  a  number  of 
parts  of  the  body  have  not  been  included  in  those  drained  by  the 
vessels  entering  the  sinus  venosus.  The  blood  from  these  portions, 
instead  of  being  conveyed  straight  to  the  heart,  passes  first  by  special 
veins  to  another  organ,  the  kidney  or  liver,  in  which  these  veins 
break  up  into  capillaries,  so  that  both  these  glands  have  a  double 
blood  supply,  venous  and  arterial.  These  capillaries  are  greatly 
distended,  for  which  reason  they  are  sometimes  called  sinusoids, 
and  form  comparatively  large  vessels  with  extremely  thin  walls, 
so  that  the  blood  they  contain  is  brought  into  close  proximity  to 
the  active  secretory  cells  of  these  two  organs.  Such  an  arrangement, 
whereby  the  blood  is  collected  up  by  a  vein  which  then  takes  it  to 
the  capillaries  of  one  of  the  organs  of  the  body,  is  termed  a  portal 
system,  so  that  in  the  frog  we  have  renal  portal  and  hepatic  portal 
systems.* 

The  blood  conveyed  by  these  systems  to  the  organs  is,  of  course, 
taken  from  them  by  the  renal  and  hepatic  veins  respectively. 

The  blood  from  the  hind  limb  is  collected  by  two  veins,  the  femoral 
and  the  sciatic.  The  femoral,  returning  blood  from  the  major  part 
of  the  leg,  is  situated  on  the  anterior  dorsal  side  of  the  thigh  and  runs 

*  The  portal  arrangement  is  not  to  be  confused  with  that  met  with,  for 
example,  in  the  carotid  "  gland,"  where  a  vessel,  usually  an  artery,  breaks  up 
into  a  network  of  small  branches  or  even  capillaries,  but  then  reunites  and 
continues  as  a  single  trunk.  This  is  often  designated  a  rete  mirabile. 


66  AN   INTRODUCTION  TO  ZOOLOGY 

dorsally  to  the  acetabulum  into  the  coelom.  Here  it  divides  into  two 
branches,  the  external  iliac  vein  going  forward  approximately 
parallel  with  the  ilium,  and  the  pelvic  vein  running  ventro-mesially 
across  the  hinder  part  of  the  pelvis.  The  sciatic,  draining  the  muscles 
and  skin  of  the  back  of  the  thigh,  runs  up  to  join  the  external  iliac 
a  short  distance  behind  the  kidney.  The  trunk  formed  by  this 
union  is  the  renal  portal  vein,  and  it  passes  to  the  lateral  edge  of  the 
kidney,  along  which  it  runs,  gradually  decreasing  in  calibre.  Near 
the  middle  of  its  course  along  the  kidney  the  dorso-lumbar  vein 
opens  into  it,  bringing  blood  from  the  back  and  lumbar  region,  and 
from  it  pass  off  a  number  of  branches  (venae  renales  advehentes) 
into  the  kidney  substance. 

The  two  pelvics  unite  in  the  middle  line  on  the  inner  surface 
of  the  body  wall  to  form  the  single  anterior  abdominal  vein,  which 
in  its  forward  course  underlies  the  linea  alba.  Shortly  after  its 
origin  it  receives  a  well-marked  tributary  from  the  bladder,  the 
vesicular  vein,  and  a  number  of  smaller  parietal  veins  enter  it  on  its 
way  forward.  At  the  level  of  the  posterior  end  of  the  heart  it 
passes  up  to  the  liver,  receiving  a  small  vessel,  the  cardiac  vein,  as 
it  does  so.  It  enters  the  hepatic  portal  vein  as  this  vessel  divides 
into  two,  and  from  just  before  its  point  of  union  it  sends  a  small 
branch  to  the  left  lobe  of  the  liver. 

The  hepatic  portal  vein  is  formed  by  a  number  of  factors  from 
the  various  parts  of  the  alimentary  canal,  chief  among  which 
are  the  intestinal,  composed  of  branches  from  the  intestine  and 
duodenum,  the  splenic  from  the  spleen  and  the  gastric  from  the 
stomach.  It  divides  into  two,  one  going  to  each  lobe  of  the  liver,  and 
it  receives  the  anterior  abdominal  just  where  it  divides. 

Lymphatic  System. 

While  traversing  the  capillaries  a  certain  amount  of  liquid 
passes  off  from  the  blood  so  that  the  tissues  are  bathed  in  this  fluid, 
called  the  lymph.  In  addition  to  the  arteries,  veins  and  capillaries 
already  described,  there  is  present  in  all  parts  of  the  body  a  network 
of  small  vessels  for  the  conveyance  of  the  lymph,  the  lymphatics, 
which  have  extremely  thin  walls.  They  collapse  very  readily,  and 
as  they  only  contain  the  colourless  lymph  they  are  generally  over- 
looked in  dissection.  Connected  with  the  lymphatics  are  large 
spaces  also  containing  lymph.  It  will  be  remembered  that  the  skin 
of  the  frog  is  only  loosely  attached  by  means  of  septa  to  the  under- 
lying muscles  of  the  body.  The  spaces  separated  from  one  another 
by  these  partitions  form  the.  great  series  pf  sub-cutaneous  lymph 
sacs.  Beneath  the  peritoneum  and  the  muscles  of  the  body  wall 
are  also  lymph  spaces,  which  communicate  with  the  ccelom  by 


THE    FROG  67 

minute  pores  or  stomata  perforating  the  peritoneum.  The  spaces 
are  well  developed  in  the  posterior  dorsal  region,  where  they  are 
termed  the  abdominal  or  sub-vertebral  lymph  sacs,  indeed  so  large 
are  they  that  the  kidneys  are  situate  in  them.  On  the  surface  of 
the  kidney,  are  minute  ciliated  funnels  leading  into  small  veins,  and 
so  putting  the  lymph  sacs  in  direct  communication  with  the  blood 
stream.  The  excess  of  lymph  is  also  returned  to  the  blood  by 
means  of  two  pairs  of  small  pulsating  vesicles  connected  with  the 
lymphatics.  The  anterior  pair  are  situated  immediately  ventral 
to  the  enlarged  transverse  process  of  the  third  vertebra  and  open 
into  the  sub-scapular  vein.  The  posterior  pair  lie  one  on  each  side 
of  the  urostyle  and  open  by  a  short  duct  into  the  ramus  com- 
municans  iliacus,  a  small  vein  joining  the.  femoral  and  sciatic 
veins. 

The  lymphatics  plentifully  distributed  in  the  wall  of  the  intestine 
are  especially  concerned  with  the  collection  of  the  fat  obtained  from 
the  food,  are  always  full  a  short  time  after  a  meal.  The  fat  is  in  the 
form  of  a  very  fine  emulsion,  and  so  the  contents  of  the  vessels  have 
a  milky  appearance,  hence  the  lymphatics  in  these  parts  are  often 
termed  lacteals. 

Thus  the  lymphatic  vessels  and  sacs  constitute  a  part  of  the 
circulatory  system,  and  one  that  is  auxiliary  to  the  blood-vascular 
system. 

Arteries  and  veins  are  not  only  to  be  distinguished  from  one 
another  by  the  fact  that  the  former  convey  blood  to,  and  the  latter 
from,  the  heart,  but  they  also  differ  in  structure. 

An  examination  of  a  transverse  section  of  an  artery  shows  that 
its  wall  consists  of  three  layers  or  coats.  The  outer  coat  or  tunica 
adventitia,  consists  of  a  layer  of  areolar  tissue  through  which  pass  a 
number  of  elastic  fibres,  and  it  is  for  this  reason  sometimes  referred 
to  as  the  outer  elastic  coat.  The  middle  coat,  the  tunica  media, 
is  very  thick  and  composed  largely  of  unstriped  muscles,  most  of 
which  run  in  a  circular  direction,  but  some  are  longitudinal.  It  also 
contains  a  few  elastic  fibres.  The  innermost  coat,  the  tunica 
interna  or  fenestrated  membrane  of  Henle,  is  not  so  thick  as  either  of 
the  others,  but  is  itself  composed  of  three  separate  layers.  The 
outer  is  an  elastic  layer,  the  middle  a  layer  of  fine  connective 
tissue,  the  sub-endothelial  layer,  and  the  innermost  is  an  epi- 
thelial layer,  the  endothelium,  one  cell  deep  forming  the  actual 
lining  of  the  vessel.  All  these  structures  together  give  a  tough 
elastic  wall  to  the  artery,  which  will  remain  open  when  cut  and 
empty  of  blood. 

A  transverse  section  of  a  vein  shows  its  wall  to  consist  of  the 
same  parts  as  that  of  an  artery,  the  only  difference  being  the  relative 


68 


AN   INTRODUCTION   TO  ZOOLOGY 


thickness  of  the  coats.  The  tunica  adventitia  is  thicker,  but  con- 
tains a  smaller  number  of  elastic  fibres.  The  tunica  media  and 
tunica  interna  are  very  much  thinner,  especially  the  former,  and 
neither  has  so  much  elastic  tissue.  The  result  is  that  the  wall  of 
the  vein  is  thinner  and  inelastic,  so  that  when  empty  and  cut  it 

collapses.  As  an  artery  and  a  vein 
often  run  side  by  side,  embedded  in 
the  same  connective  tissue,  a  section 
of  this  will  show  at  a  glance  the 
structure  of  each  and  the  difference 
between  them.  In  the  capillaries 
the  walls  are  considerably  reduced, 
and  consist  simply  of  the  endothe- 
lium,  thus  bringing  the  blood  into 
more  intimate  relation  with  the 
tissues. 

We  have  already  seen  that 
the  blood  consists  of  a  fluid  portion, 
the  plasma,  in  which  are  suspended 
countless  numbers  of  minute  nucle- 
ated cells,  the  corpuscles.  The 
plasma  is  rich  in  proteid  matter,  and 
one  of  the  compounds  present  in  it  is 
termed  fibrinogen.  This  substance, 
when  exposed  to  air,  as  for  example 
when  a  blood-vessel  is  cut,  forms  an 
interlacing  network  of  threads  of 
another  and  solid  body,  fibrin.  The 
fibrin  mass  entangles  all  the  corpus- 
cles, and  as  it  shrinks  squeezes  out 
a  clear  pale  yellow- coloured  fluid, 

the  serum.  This  is  the  well-known  phenomenon  of  clotting,  and  the 
serum  represents  the  plasma,  from  which  the  fibrinogen  has  been 
deposited  as  solid  fibrin  threads.  The  corpuscles  are  of  two  kinds, 
colourless,  the  leucocytes,  and  red  corpuscles  or  erythrocytes.  Several 
varieties  of  the  former  are  recognised,  according  to  the  number  of 
nuclei  they  contain.  They  possess  the  power  of  being  able  to  creep 
through  the  walls  of  a  capillary  without  leaving  a  wound  in  it. 
This  they  frequently  do,  and  are  in  consequence  to  be  found  widely 
distributed  through  the  body  and  also  in  the  lymph.  They  appear 
to  play  the  part  of  scavengers,  and  help  in  the  removal  of  waste 
matters  from  the  tissues.  In  addition  to  this,  they  have  the  power 
under  certain  conditions  of  ridding  the  body  of  bacteria,  and  hence 
are  of  importance  in  helping  to  repel  bacterial  invasion — a  process 


FIG.  22. —  Transverse  section 
through  a  small  artery  and 
vein. — From  Gray. 

A,  artery ;  V,  vein  ;  e.,  epithelial 
lining  ;  m.,  middle  muscular  and  elastic 
coat,  thick  in  the  artery,  much  thinner 
in  the  vein  ;  a.,  outer  coat  of  areolar 
tissue  (magnified  350  diameters). 


THE  FROG  69 

termed  phagocytosis,  and  the  particular  cells  engaged  in  it  are  often 
called  phagocytes.  The  red  corpuscles  owe  their  colour  to  a  proteid- 
like  substance,  containing  a  certain  amount  of  iron,  haemoglobin. 
This  pigment  plays  an  important  part  in  respiration.  In  the  capil- 
laries of  the  lungs  it  enters  into  loose  combination  with  the  oxygen 
of  the  air  to  form  an  unstable  compound,  oxyhaemoglobin,  of  a  bright 
scarlet  colour.  This  blood  is  conveyed  to  the  heart  by  the  pulmonary 
veins,  and  thence  by  the  arteries  all  over  the  body.  Hence  the 
arteries  (except  the  pulmonary)  contain  bright  red  oxygenated  blood, 
and  such  blood  is  sometimes  termed  arterial.  In  the  capillaries  of 
the  tissues  the  oxygen  is  yielded  up  and  haemoglobin  of  a  darker 
bluish-red  colour  is  again  produced.  This  is  collected  by  the  veins,  so 
that  these  vessels  (again  with  the  exception  of  the  pulmonary  veins) 
contain  a  non-oxygenated  darker  blood,  often  called  venous  blood. 
The  carbon  dioxide  produced  in  the  organs  and  tissues  is  on  the 
other  hand  not  carried  in  a  combined  form  ;  it  simply  passes  into 
solution  in  the  blood  and  lymph,  and  is  passed  out  again  in  the 
capillaries  of  the  lungs.  The  blood  then  plays  an  extremely  impor- 
tant  part  in  respiration,  being  the  agent,  by  means  of  which  the 
oxygen  is  distributed  to  and  the  carbon  dioxide  collected  from  all 
parts.  This  is  but  one  of  the  functions  of  the  blood,  and  in  a  like  way 
it  acts  as  a  collector  of  food  in  the  intestine,  and  then  conveys  it  all 
over  the  body.  It  will  also  be  remembered  that  one  of  the  activities 
of  the  liver  is  to  transform  the  nitrogenous  waste  matter  brought 
to  it  by  the  blood  into  urea,  which  it  returns  to  that  fluid,  so  that  it 
may  be  taken  to  the  kidneys,  where  it  is  eliminated.  In  the  warm- 
blooded animals,  like  the  rabbit,  the  blood  is  also  concerned  with  the 
equalisation  of  temperature.  For  example,  when  we  are  exerting 
ourselves  the  blood  comes  to  the  surface  of  the  body,  where  it  is 
slightly  cooled  down.  The  distributing  power  of  the  blood  is  also 
manifested  in  the  transportation  of  certain  active  substances,  the 
hormones  or  internal  secretions,  which  will  be  dealt  with  more 
fully  later. 

Before  leaving  the  blood,  another  very  interesting  form  of 
activity,  intimately  connected  with  it;  calls  for  attention,  and  that  is 
the  phenomenon  termed  immunity.  It  is  not  possible  here  to  do 
more  than  mention  one  or  two  of  its  most  obvious  points,  but  it  is 
a  subject  of  great  importance  in  practical  medicine  and  interesting 
to  the  zoologist,  since  it  is  sometimes  the  factor  determining  whether 
or  not  an  animal  can  live  on  a  certain  area.  The  meaning  of  the 
term  is  readily  made  clear  by  considering  what  happens  when  an 
epidemic  of  an  infectious  disease  breaks  out.  Many  persons  contract 
the  complaint  severely,  perhaps  even  fatally,  others  less  severely, 
and  lastly,  certain  persons  do  not  catch  it,  however  much  they  may 


70  AN   INTRODUCTION  TO  ZOOLOGY 

be  brought  into  contact  with  infected  people.  These  last  are  said 
to  be  immune  to  the  disease,  and  those  only  slightly  affected  to  be 
partially  immune.  In  a  similar  way,  one  kind  of  animal  is  quite 
immune  to  diseases  dangerous  to  others,  for  example,  birds  are  not 
susceptible  to  the  particular  malaria  common  to  man.  These  are 
examples  of  what  may  be  termed  natural  immunity  as  opposed  to 
another  variety,  namely,  acquired  immunity.  After  an  attack  of 
some  diseases,  like  whooping-cough  and  chicken-pox,  usually  over- 
come in  childhood,  the  person  is  not  liable  to  a  second  attack,  because 
immunity  has  been  acquired  as  a  result  of  the  first  attack.  Immunity 
may  also  be  acquired  in  an  indirect  way,  by  means  of  vaccination, 
as  was  first  shown  by  Jenner.  Thus  in  the  case  of  small-pox,  it  has 
been  found  that  if  a  calf  is  inoculated  with  the  disease,  there  is 
produced  in  it  a  strain  of  germs  whose  virility  has  been  very  consider- 
ably lowered.  These  when  re-inoculated  into  a  human  being  are 
not  strong  enough  to  produce  more  than  a  temporary  disturbance, 
but  yet  are  strong  enough  to  produce  a  measure  of  immunity.  Most 
of  the  ill  effects  of  these  complaints  are  due  to  poisonous  substances, 
toxins,  produced  by  the  organism  causing  the  disease.  When  im- 
munity has  been  acquired,  it  is  found  that  the  blood  contains  sub- 
stances capable  of  combining  with  the  toxins  and  rendering  them 
inert  and  harmless,  and  these  are  termed  the  antibodies  or  antitoxins. 
The  available  evidence  seems  to  show  that  these  bodies  are  produced 
in  the  tissues  and  not  in  the  blood,  which  only  serves  as  a  distributing 
agent .  They  continue  to  be  produced  long  after  the  original  stimulus 
has  disappeared  ;  in  some  cases  it  may  be  for  years.  This  immunity 
by  the  production  of  antitoxins  is  not  limited  to  the  effects  of  disease 
germs,  for  it  applies  also  in  the  case  of  most  snake  poisons  and  some 
plant  poisons. 

Yet  another  kind  of  immunity  is  to  be  noted,  and  this  is  a  most 
important  one  in  the  case  of  men  living  under  war  conditions.  In 
this  variety  no  antitoxin  appears  to  be  produced,  but  by  the  injection 
of  so  many  million  bodies  of  dead  bacteria  the  tissues  and  fluids  of 
the  body  produce  a  bacteriolytic  substance  or  substances,  whereby 
they  acquire  the  power  of  being  able  to  at  once  attack  and  digest 
the  living  bacteria,  hence  the  disease  can  make  no  headway  and  the 
invading  germs  are  destroyed. 

The  power  of  a  man  to  withstand  diseases  is  then  dependent 
to  a  very  large  extent  on  this  ability  to  produce  antitoxins  and 
bacteriolytic  substances,  and  also  on  the  readiness  with  which  the 
lymphocytes  will  exhibit  phagocytosis.  The  ability  varies  in  the 
same  individual  from  time  to  time,  but  may  be  tested  by  certain 
reactions,  and  what  is  termed  an  opsonic  index,  or  an  approximate 
measure  of  this  capacity  can  be  obtained. 


THE  FROG  71 

Uro-genital  System, 

Under  the  present  heading  two  systems,  the  excretory  and 
reproductive,  are  dealt  with  together,  not  only  for  convenience,  but 
also  because  they  are  closely  related  structurally  and  development  ally. 
The  component  parts  of  this  joint  system  naturally  differ  in  the  two 
sexes,  which  will  therefore  be  considered  separately.  The  adult 
female  has  the  two  sets  of  organs  separate,  and  so  presents  a  some- 
what simpler  condition  than  the  male. 

The  kidneys  are  two  elongated,  flat,  oval  structures  of  a 
dark  red  colour,  lying  in  the  sub-vertebral  lymph  sinus  close  to  the 
vertebral  column  at  the  posterior  end  of  the  body.  They  are 
actually  outside  the  ccelom,  as  they  are  beneath  the  peritoneum. 
Their  inner  edges  are  indented  by  a  few  notches  and  their  outer 
edges  intact.  On  the  ventral  surface  of  each  is  a  narrow  irregular 
strip  of  an  orange-coloured  tissue,  the  supra-renal  body.  From  the 
outer  side  of  the  kidney  arises  a  whitish  tube,  the  ureter,  which 
becomes  free  a  short  distance  from  its  hinder  end  and  runs  straight 
backwards  to  open  into  the  end  of  the  alimentary  canal.  The  two 
ureters  open  on  small  papillae  situated  close  together  on  the  dorsal 
side  of  the  terminal  portion  of  the  rectum,  the  cloaca.  On  the 
ventral  wall  of  the  cloaca,  immediately  opposite  these  papillae,  is  the 
single  opening  of  the  urinary  bladder.  The  bladder  is  a  large  bilobed, 
thin-walled  sac,  lying  in  the  posterior  ventral  part  of  the  body  cavity. 
These  various  organs  together  constitute  the  excretory  system. 

The  ovaries  are  two  irregular  masses,  one  on  each  side 
immediately  ventral  to  the  kidneys.  They  are  attached  to  the 
dorsal  wall  of  the  ccelom  by  two  folds  of  the  peritoneum,  known  as 
the  mesovaria.  The  presence  in  the  ovaries  of  a  large  number  of 
spherical  eggs,  like  small  shot,  each  half  black  and  half  white  (during 
a  large  part  of  the  year)  renders  them  very  conspicuous.  Each 
egg  is  enclosed  in  a  tightly  fitting  sac  composed  of  a  simple  layer  of 
cells,  and  termed  the  follicle.  To  the  front  end  of  each  ovary  is 
attached  a  number  of  finger-shaped  lobes  of  yellow  or  orange-coloured 
fat,  these  are  the  so-called  fat  bodies  or  corpora  adiposa.  A  long 
coiled  tube,  the  oviduct,  is  situated  laterally  to  the  kidneys  on  each 
side  of  the  dorsal  wall  of  the  body  cavity,  from  which  it  is  suspended 
by  a  fold  of  the  peritoneum,  the  mesometrium.  The  front  end  of 
the  oviduct  forms  a  wide  funnel-  shaped  opening,  the  oviducal  funnel, 
lying  far  forwards  dorsally  to  the  liver.  Immediately  behind  this 
the  duct  is  a  slender  tube,  which,  however,  soon  becomes  larger,  owing 
to  the  fact  that  its  walls  become  very  glandular,  and  pursues  a  very 
convoluted  course  to  the  level  of  the  hinder  end  of  the  kidney.  Here 
it  widens  out  to  form  a  large  thin- walled  sac,  the  ovisac,  which  opens 


72  AN   INTRODUCTION  TO  ZOOLOGY 

into  the  cloaca.  These  oviducal  openings  appear  as  slit-like  aper- 
tures in  the  dorsal  wall  of  the  cloaca,  just  in  front  of  the  openings  of 
the  ureters.  Thus  the  cloaca  of  the  female  frog  possesses  five  pores 


leading  into  it  :  a  pair  of  genital  openings,  a  pair  of  urinary  openings, 
and  the  opening  of  the  bladder.  When  ripe  the  eggs  are  discharged 
from  the  ovaries  into  the  ccelom,  and  find  their  way  forward  to  the 


THE  FROG  73 

oviducal  funnels,  into  which  they  are  taken.  They  then  pass  along 
the  oviduct,  where  they  are  coated  with  albuminous  matter,  to  be 
stored  in  the  ovisac  until  they  are  finally  laid.  All  these  parts  of 
the  female  reproductive  system  are  subject  to  variations  at  different 
times  of  the  yeai.  During  the  summer  they  are  smallest,  but  the 
fat  bodies  enlarge.  In  winter,  and  particularly  in  the  early  months  of 
the  year,  the  ovaries  enlarge  very  markedly,  while  the  fat  bodies 
begin  to  dwindle  in  size,  yielding  up  their  food  store  to  the  ovaries 
to  enable  them  to  produce  an  enormous  number  of  eggs.  Shortly 
after  the  glandular  portions  of  the  oviducts  begin  to  increase  in  size, 
their  glands  becoming  active  and  preparing  the  albuminous  matter 
with  which  the  egg  is  surrounded  as  it  passes  down  the  ducts.  Finally, 
in  early  spring  the  ova  are  discharged  and  stored  in  the  ovisac,  so 
that  the  ovary  diminishes  markedly,  and  the  ovisacs  become  greatly 
distended  with  the  waiting  eggs.  After  the  eggs  are  laid  the  various 
organs  all  become  smaller  again. 

The  primary  male  organs  are  the  testes,  a  pair  of  yellowish 
white  oval  bodies,  lying  immediately  ventral  to  the  kidneys,  and 
suspended  in  folds  of  the  peritoneum  often  deeply  pigmented,  known 
as  the  mesorchia.  To  their  front  ends  are  attached  corpora  adiposa, 
similar  in  all  respects  to  those  of  the  female.  Each  testis  is  connected 
to  the  corresponding  kidney  by  ten  or  a  dozen  fine  white  tubes,  the 
vasa  efferentia,  running  through  the  mesorchium.  The  spermatozoa 
produced  in  the  testes  are  not  discharged  into  the  body  cavity,  as 
are  the  ova  from  the  ovary,  but  are  conveyed  to  the  kidney  by  the 
vasa  efferentia.  The  kidneys  resemble  those  of  the  female,  and  have 
ureters  arising  from  their  lateral  edges.  The  ureter  serves  also  to 
convey  the  spermatozoa  to  the  cloaca,  and  so  functions  as  a  sperm 
duct  or  vas  deferens.  Between  kidney  and  cloaca  the  vas  deferens 
gives  off  laterally  a  sac-like  dilatation,  the  vesicula  seminalis,  in  which 
the  sperms  are  stored  until  required.  Thus  there  are  only  three 
pores  leading  into  the  cloaca  of  the  male :  a  pair  of  uro-genital  openings 
situated  on  papillae  on  its  dorsal  wall,  and  the  single  opening  of  the 
urinary  bladder  on  the  ventral  wall  opposite.  The  various  parts  of, 
the  male  system,  i.e.  fat  bodies,  testes,  and  vesiculae  seminales,  also 
vary  in  size  at  different  times  of  the  year,  but  not  to  such  a  marked 
degree  as  the  female  organs. 

A  section  of  the  ovary  shows  it  to  consist  of  a  hollow  sac 
with  folded  walls,  from  which  a  number  of  partitions  pass  inwards. 
From  the  walls  and  partitions  the  eggs  project  towards  the  interior. 
The  ovum  or  female  reproductive  cell  itself  is  a  spherical  cell,  varying 
in  size  according  to  its  stage  of  development,  with  a  large  vesicular 
nucleus  situated  eccentrically  and  containing  several  distinct  nucleoli. 
The  cytoplasm  of  the  egg  is  extremely  granular,  owing  to  the  presence 


74  AN   INTRODUCTION  TO  ZOOLOGY 

in  it  of  a  large  number  of  spherical  yolk  granules,  small  masses  serving 
as  food  reserves  for  the  subsequent  development  of  the  ovum.  The 
ovum  is  surrounded  by  a  very  thin  homogeneous  membrane,  the 
vitelline  membrane,  and  held  to  the  wall  of  the  ovary  by  the  epithe- 
lium and  connective  tissue  of  the  latter,  which  is  reflected  round  it. 
It  thus  comes  to  lie  in  a  sac  composed  of  epithelium  covered  by  a  thin 
layer  of  connective  tissue,  and  termed  the  follicle.  In  the  course  of 
development  the  egg  grows  larger,  and  one  half  become  impregnated 
with  a  dense  black  pigment. 

The  testes  are  compact  bodies,  and  section  shows  them  to 
consist  of  a  number  of  seminiferous  tubules,  whose  walls  are  several 
cells  thick.  The  outermost  layer  contains  ordinary  cells,  showing 
little  specialisation  ;  as  they  pass  inwards,  however,  they  become 
more  and  more  differentiated,  as  the  result  of  a  process  known  as 
maturation,  that  will  be  considered  in  greater  detail  later.  The  inner- 
most cells  are  very  highly  modified,  consisting  of  a  rod-shaped 
head  containing  the  nucleus,  a  short  continuation  of  this,  the  middle 
piece,  and  a  long  thread-like  tail,  capable  of  moving  rapidly  and 
forming  an  organ  by  means  of  which  the  whole  cell  is  able  to 
move  about.  These  are  the  spermatozoa,  or  male  reproductive  cells, 
actively  motile  cells  with  the  power  of  existing  and  swimming 
independently  for  a  short  time  under  suitable  conditions. 

In  the  breeding  season  both  male  and  female  frogs'  resort  to 
ponds  and  pools,  and  when  the  eggs  are  ready  to  be  laid  they  associate 
in  pairs.  The  male  takes  up  a  position  on  the  back  of  the  female 
and  clasps  her  tightly  by  means  of  the  pads  on  the  first  finger,  which 
have  become  enlarged  and  rough.  As  the  eggs  are  laid  the  male 
pours  over  them  the  milt  or  spermatic  fluid,  in  which  the  sperma- 
tozoa are  contained.  The  sperm  swims  about  actively  until  it 
reaches  an  egg  into  which  it  penetrates,  and  with  which  it  fuses. 
Only  one  sperm  can  enter  and  fertilise  an  ovum,  for  immediately 
after  this  has  taken  place  the  vitelline  membrane  undergoes  a  slight 
alteration  that  prevents  the  entrance  of  other  spermatozoa.  Far 
emore  sperms  are  discharged  than  eggs,  and  consequently  under 
normal  conditions  practically  all  the  eggs  are  fertilised.  They  are 
now  ready  to  develop  into  small  free-swimming  animals,  tadpoles, 
which  after  a  period  of  growth  turn  into  frogs. 

The  kidney  is  composed  of  an  enormous  number  of  small 
tubes,  the  uriniferous  tubules,  bound  together  by  a  small  amount  of 
connective  tissue  richly  supplied  with  blood  vessels.  Each  tubule 
commences  as  a  Malpighian  body,  situated  near  the  ventral  side  of 
the  kidney.  This  body  consists  of  a  hollow  cup  with  double  thin 
walls,  Bowman's  capsule,  the  interior  of  which  is  filled  up  by  a 
glomerulus.  The  glomerulus  is  a  rete  mirable,  formed  by  a  branch 


THE  FROG  75 

of  the  renal  artery,  which  enters  the  capsule,  breaks  up  into  a  mass 
of  capillaries,  and  unties  again  to  form  a  single  vessel  still  to  be 
regarded  as  an  artery.  From  the  Malpighian  body  the  tubule  runs 
fairly  straight  dorsally,  and  becomes  much  coiled  near  the  dorsal 
surface.  It  then  runs  ventrally,  again  becomes  coiled,  and  finally 
passes  dorsally  once  more  to  open  into  a  collecting  tubule,  extending 
nearly  transversely  beneath  the  dorsal  surface  of  the  kidney.  Thus 
in  a  transverse  section  the  middle  portion  consists  of  tubules  cut 
more  or  less  longitudinally,  and  on  the  dorsal  and  ventral  sides  of 
this  is  a  strip  where  the  coiled  parts  of  the  tubes  are  cut  in  all  sorts  of 
directions.  The  ventral  border  is  easily  recognised  by  the  presence 
of  the  conspicuous  Malpighian  bodies,  and  also  because  the  nephro- 
stomes  are  situated  on  this  side.  The  nephrostomes  are  ciliated 
funnels,  that  originate  in  connection  with  the  urinary  tubules,  and 
are  to  be  found  as  such  in  the  young  tadpole.  During  the  course 
of  development  they  lose  their  connections  and  acquire  new  ones 
with  branches  of  the  renal  veins,  hence  serving  as  a  means  of  com- 
munication between  the  lymph  in  the  sub-vertebral  sinuses  and  the 
circulatory  system.  The  urinary  tubules  are  lined  by  a  ciliated 
epithelium,  whose  character  varies  slightly  in  different  parts.  The 
collecting  tubule  opens  into  a  longitudinal  canal  on  each  side  ;  that 
on  the  inner  edge  of  the  kidney  is  called  Bidder's  canal,  and  that  on 
the  outer  side  is  of  course  the  ureter.  The  vasa  efferentia  open  into 
Bidder's  canal,  and  the  sperms  are  conveyed  thence  by  the  collecting 
tubules  to  the  ureter,  so  that  they  do  not  enter  the  uriniferous  tubules. 
As  already  noticed,  blood  is  brought  to  the  kidneys  by  the  renal 
portal  vein,  and  this  vessel  breaks  up  into  dilated  capillaries,  sinusoids, 
whose  walls  are  closely  apposed  to  the  uriniferous  tubules.  The 
artery  coming  from  Bowman's  capsule,  also  bringing  blood  to  the 
kidney,  breaks  up  into  capillaries  that  open  into  the  sinusoids. 
Blood  is  carried  away  from  these  vessels  by  factors  of  the  renal 
veins.  It  appears  probable  that  the  function  of  the  capsule  is  to 
remove  excess  of  water  from  the  blood,  while  the  nitrogenous  waste, 
in  the  form  of  urea,  is  taken  from  it  by  the  tubules. 

The  urinary  bladder  is  lined  by  a  peculiar  type  of  epithelium, 
known  as  transitional  epithelium.  The  cells  are  only  two  or  three 
deep,  fit  together  irregularly,  and  are  not  arranged  in  definite  layers 
one  above  the  other,  as  in  stratified  epithelium.  Outside  the  epithe- 
lium is  a  muscular  layer,  composed  of  numerous  strands  of  non- 
striate  muscles  distributed  in  an  irregular  manner,  so  as  to  form  a 
network  in  a  thin  layer  of  connective  tissue.  The  whole  is  enclosed 
in  a  close  fitting  extension  of  the  peritoneum. 


CHAPTER   IV 
THE   FROG— RAN  A    TEM  FOR  ARIA— (continued) 

Nervous  System  and  Sense  Organs — -Ductless  Glands- — Life  History — Animals 
and  Plants — Classification. 

Nervous  System  and  Sense  Organs. 

The  nervous  system  and  organs  of  the  senses  are  so  inti- 
mately related  that  they  may  be  regarded  as  forming  one  system, 
whose  main  function  is  the  appreciation  of  messages  from  the  out- 
side world  and  meeting  them  in  the  proper  way.  For  convenience 
of  description,  we  may  subdivide  them  into  the  Central  Nervous 
System,  the  Peripheral  Nervous  System,  and  the  Sense  Organs. 

The  Central  Nervous  System  is  composed  of  two  parts,  the  brain, 
lodged  within  the  skull,  and  the  spinal  cord,  which  is  contained  in 
the  neural  canal,  formed  by  the  neural  arches  of  the  vertebrae. 
The  peripheral  nervous  system  comprises  three  groups  of  structures  : 
nerves  given  off  from  and  going  to  the  brain,  i.e.  the  Cranial  Nerves, 
similar  nerves,  the  Spinal  Nerves,  related  to  the  spinal  cord,  and  the 
Sympathetic  or  Involuntary  Nervous  System,  a  double  chain  of  small 
nerve  centres  lying  in  the  ccelom  close  to  the  vertebral  column. 
The  sense  organs  are  the  olfactory  organ,  or  organ  of  smell,  the  eye, 
the  ear,  and  the  organs  of  taste  and  touch. 

The  tissue  composing  the  central  nervous  system  is  soft,  but  is 
of  vital  importance  to  the  animal,  and  in  consequence  we  find  that 
it  is  not  only  enclosed  within  the  bony  axial  skeleton,  but  also,  inside 
that  again,  it  is  protected  by  two  membranes,  the  meninges.  The 
outermost  of  the  membranes,  the  dura  mater,  is  tough  and  pigmented, 
and  applied  fairly  closely  to  the  inner  side  of  the  bones  of  the  skull 
and  vertebral  column.  The  inner  membrane,  the  pia  mater,  corre- 
sponds to  the  pia  mater  together  with  the  arachnoid  tissue,  which 
forms  an  intermediate  layer  in  the  higher  vertebrates.  It  is  richly 
supplied  with  blood-vessels,  and  attached  to  the  surface  of  the  central 
nervous  system,  the  outline  of  which  it  follows  closely,  dipping  down 
into  all  the  folds.  Where  it  covers  the  parts  of  the  brain  known  as 
the  optic  lobes  it  is  deeply  pigmented. 

76 


THE  FROG 


77 


The  brain  is  an  elongated  mass  of  nervous  tissue  filling  up 
the  whole  cavity  of  the  cranium.  It  is  composed  of  two  distinct 
kinds  of  tissue,  as  will  be  seen  if  it  is  cut  in  section  :  a  layer  of  grey 
matter  around  the  outside  and  white  matter  within.  Looked  at 
from  the  dorsal  aspect,  it  will  be  seen  that  the  anterior  half,  the  fore- 
brain  or  prosencephalon,  is  mainly  composed  of  two  elongated 
ovoidal  masses,  the  cerebral  hemispheres  or  telencephalon.  These 
are  separated  from  one  another  in  the  middle  line  by  a  deep  groove, 
the  sagittal  fissure.  In  front  the  hemisphere  is  continued  forward 
into  the  olfactory  lobe  or  rhinencephalon,  and  these  unite  in  the 


FIG.  24. — A,  the  brain  of  the  frog :    dorsal  surface.      X  4.     B,  the  brain  of 
the  frog  :    ventral  surface,      x  4. — From  Marshall  and  Gamble. 

C.,  cerebellum  ;  C.H.,  cerebral  hemisphere  ;  .C.P.,  choroid  plexus  of  third  ventricle  ;  F.,  fourth 
ventricle  ;  IN.,  tuber  cinereum ;  M.,  medualla  oblongata ;  O.,  olfactory  lobe ;  O.C.,  optic  chiasma ; 
O.L.,  optic  lobe  ;  P.,  stalk  of  pineal  body  ;  P. B.,  pituitary  body  ;  T.,  thalamencephalon. 

I.,  olfactory  nerve  ;  II.,  optic  nerve  ;  III.,  third  or  motor  pculi  nerve  ;  IV.,  fourth  nerve  ;V.,  fifth 
or  trigeminal  nerve ;  VI.,  sixth  nerve ;  VII.  and  VIII.,  combined  root  of  facial  and  auditory  nerves ; 
IX.  and  X.,  combined  root  of  glossopharyngeal  and  pneumogastric  nerves. 

middle  line,  so  forming  the  anterior  limit  of  the  fissure.  The 
posterior  limit  is  formed  by  the  lamina  terminalis,  the  front  wall  of 
the  median  part  of  the  thalamencephalon,  as  the  next  part  of  the 
brain  is  called.  It  is  a  comparatively  small  median  portion,  situated 
between  the  divergent  hinder  ends  of  the  hemispheres  and  behind 
them.  The  top  of  it  is  fairly  conspicuous  in  the  freshly  killed  animal, 
appealing  as  a  reddish  area,  the  anterior  choroid  plexus,  formed  by  a 
large  increase  in  the  size  and  number  of  blood-vessels  of  the  pia 
mater  in  this  region.  From  the  posterior  end  of  the  thalamence- 
phalon a  thin  stalk  runs  forward  over  the  surface  of  the  brain,  to 


78  AN   INTRODUCTION  TO  ZOOLOGY 

terminate  in  a  small  knob-like  enlargement  just  beneath  the  fronto- 
parietal  bones.  This  is  the  pineal  body  or  epiphysis  cerebri,  the 
vestigial  remnant  of  what  was  once  apparently  a  pair  of  eye-like 
structures  ;  it  is  generally  removed  in  dissecting  out  the  brain,  but 
its  stalk  can  usually  be  made  out  readily. 

The  succeeding  part  of  the  brain  is  the  mid-brain  or  mesence- 
phalon.  Dorsally  it  takes  the  form  of  two  conspicuous  ovoid  lobes, 
the  optic  lobes  or  corpora  bigemina,  whose  long  axis  is  inclined  out- 
wards at  an  angle  of  about  45  degrees  to  the  median  line,  and  over 
which,  as  already  noticed,  the  pia  mater  is  deeply  pigmented.  The 
hind-brain  or  rhombencephalon  is  divided  into  two  portions,  that  in 
front,  immediately  behind  the  optic  lobes,  is  the  cerebellum  or 
metencephalon,  appearing  as  a  small  transversely  running  fold  of 
tissue.  The  remaining  part  of  the  hind-brain  is  a  good  deal  larger, 
almost  as  long  as  the  cerebral  hemispheres,  and  variously  known  as 
the  bulb,  medulla  oblongata  or  myelencephalon.  This  again  is 
conspicuous  in  the  freshly  killed  animal,  owing  to  the  presence  of  a 
blood  network,  the  posterior  choroid  plexus,  which  in  the  form  of  a 
long  isosceles  triangle  covers  a  large  part  of  its  dorsal  surface.  The 
hinder  part  of  the  medulla  is  continuous  with  the  spinal  cord. 

Turning  now  to  the  ventral  surface  of  the  brain,  we  find  in  front 
the  olfactory  lobes,  followed  by  the  cerebral  hemispheres  separated 
by  a  marked  groove.  The  thalamencephalon  is  a  small  but  an 
important  part  of  the  brain,  and  under  it  the  nerves  going  to  the 
eyes,  the  optic  nerves,  form  a  very  characteristic  X-shaped  structure 
known  as  the  optic  chiasma.  Immediately  behind  this  is  a  bi-lobed 
swelling  with  a  median  groove,  the  tuber  cinereum  or  infundibulum. 
It  lies  in  the  mid-ventral  line  and  has  attached  to  it  the  pituitary 
body  or  hypophysis  cerebri.  This  is  composed  of  a  flat  median 
cushion,  immediately  behind  the  tuber  cinereum,  which  gives  off 
two  small  lateral  tongue-like  processes  running  forward. 

The  ventral  part  of  the  mid-brain  is  formed  by  two  large  columns 
of  nervous  matter,  the  crura  cerebri,  that  connect  up  the  hemispheres 
with  the  medulla  which  forms  the  remaining  part  of  the  brain  and 
has  running  down  the  middle  of  it  the  ventral  fissure. 

The  whole  of  the  central  nervous  system  is  hollow,  and  in  the  brain 
the  central  cavity  swells  out  to  form  a  series  of  spaces  known  as 
ventricles.  The  first  are  the  ventricles  in  the  cerebral  hemispheres, 
an  outgrowth  from  each  of  which  extends  forward  into  the  olfactory 
lobes.  The  cavity  of  the  thalamencephalon  is  known  as  the  third 
ventricle.  Its  roof  is  formed  mainly  by  the  anterior  choroid  plexus, 
and  in  its  nervous  portion  run  two  transverse  bands  of  fibres,  the 
superior  and  posterior  cerebral  commissures,  joining  the  cortex  of 
one  hemisphere  to  that  of  the  other.  From  its  floor  a  pocket  projects 


THE  FROG 


79 


downwards  and  backwards,  it  is  termed  the  infundibulum  and  forms, 
when  viewed  from  the  outside,  the  projection  on  the  ventral  surface 
of  the  brain  known  as  the  tuber  cinereum.  The  front  wall  of  the 
third  ventricle  is  formed  by  the  lamina  terminalis,  in  which  runs 
the  anterior  cerebral  commissure,  and  the  lateral  ventricles  of  the 
hemispheres  here  open  by  two  apertures,  known  as  the  foramina  of 
Munro.  The  fourth  ventricle  is  situated  in  the  medulla,  and  is  a 
large  cavity  whose  roof  is  formed  by  the  posterior  choroid  plexus. 
It  is  joined  to  the  preceding  cavity  by  a  passage  running  through  the 
mid-brain.  In  the  frog  it  is  fairly  large  and  continuous  with  two 
spaces  in  the  optic  lobes,  the  optic  ventricles,  but  in  the  higher  ani- 
mals it  is  only  a  small  hole,  consequently  it  is  termed  the  aqueduct 


AC. 


AM 


FM. 


PC. 


C.H. 


01- 


C.C. 


H. 


FIG.  25. — Median  longitudinal  section  of  brain,  Rana,  adapted  from  Gaupp. 

A.C.,  anterior  choroid  plexus  ;  A. Co.,  anterior  commissure  ;  A.M.,  anterior  medullary  velum  ; 
C.,  cerebellum;  C.C.,  crura  cerebri ;  C.H.,  median  surface  of  cerebral  hemisphere;  B.C.,  epi- 
physis  cerebii  (pineal  body)  ;  F.M.,  foramen  of  Munro  ;  H.,  hypophysis  cerebri  (pituitary  body)  ; 
I.,  infundibulum  ;  L.T.,  lamina  terminalis  ;  M.,  medulla  oblongata  ;  O.C.,  optic  chiasma  ; 
O.L.,  optic  lobe  ;  OL,  olfactory  lobe  ;  O.N.,  optic  nerve;  O.V.,  optic  ventricle  ;  P.C.,  posterior 
choroid  plexus  ;  P.Co.,  posterior  commissure  ;  P.R.,  pre-optic  recess  ;  S.Co.,  superior  commis- 
sure ;  III.,  third  ventricle  ;  IV.,  fourth  ventricle. 

of  Sylvius  or  the  Iter  (a  contraction  of  Iter  a  tertio  ad  quartum 
ventriculum) . 

The  spinal  cord  is  a  continuation  of  the  brain  and,  like  it,  is 
hollow,  the  small  cavity  running  through  it  being  called  the  central 
canal  or  canalis  centralis.  The  cord  itself  is  a  dorso-ventrally 
flattened,  thick-walled  tube  situated  in  the  neural  canal  of  the 
vertebral  column.  It  is  not  of  uniform  diameter  throughout  its 
course,  but  swells  out  in  the  region  of  the  second  vertebra  to  form 
the  brachial  enlargement,  and  again  in  the  region  of  the  sixth  or 
seventh  vertebra  to  form  the  lumbar  enlargement.  After  this  it 
quite  sharply  narrows  off  to  form  a  fine  thread,  the  filum  terminate, 
which  is  continued  on  into  the  urostyle.  It  is  composed  of  the  same 
two  substances  as  the  brain,  i.e.  white  and  grey  matter,  but  their 


80  AN   INTRODUCTION  TO  ZOOLOGY 

relative  positions  are  reversed,  for  here  it  is  the  white  matter  that  is 
outside  and  the  grey  matter  inside.  The  transverse  section  of  the 
cord  presents  a  very  characteristic  appearance.  It  is  approximately 
trapezoidal  with  well-rounded  corners.  On  the  dorsal  side  is  a  very 
shallow  groove,  the  dorsal  fissure,  and  on  the  ventral  side  is  a  well- 
marked  fairly  deep  furrow,  the  ventral  fissure.  The  grey  matter  is 
arranged  within  the  white  in  such  a  way  that  its  outline  is  somewhat 
similar,  only  exaggerated,  and  it  is  possible  to  recognise  two  dorsally 
situated  blunt  horns  or  dorsal  cornua,  and  two  ventral  ones,  the 
ventral  cornua.  In  these,  particularly  in  the  latter,  will  be  seen  the 
bodies  of  large  nerve  cells.  Near  the  middle  of  the  section  will  be 
seen  the  small  canalis  centralis  lined  by  a  characteristic  neural 
epithelium.  In  addition  to  the  actual  nerve  cells,  there  are  present 
in  the  spinal  cord  certain  supporting  elements  known  as  the 
neuroglia  cells. 

Peripheral  Nervous  System. 

The  frog  possesses  ten  *  pairs  of  cranial  nerves  coming 
off  from  the  brain,  and  ten  pairs  of  spinal  nerves  coming  from 
the  spinal  cord.  Each  nerve  is  made  up  of  a  large  number 
of  fibres  bound  together,  and  breaks  up  into  smaller  and  smaller 
bundles  as  it  passes  away  to  the  tissues.  A  nerve  fibre  is  only 
capable  of  conveying  a  nervous  message  or  impulse  in  one 
direction,  consequently  one  set  of  fibres  takes  messages  to  the 
central  nervous  system,  and  these  are  termed  afferent  or  sensory 
nerves,  while  the  other  set,  taking  messages  away  from  the  brain, 
are  termed  efferent  or  are  spoken  of  as  motor  nerves.  It  sometimes 
happens  that  all  the  many  fibres  composing  one  nerve  are  of  the  same 
sort,  and  we  then  use  the  terms  "  afferent  "  and  "  efferent "  to  describe 
the  nerve  as  a  whole,  while  if  it  contains  both  kinds  of  fibres  it  is 
called  a  mixed  nerve. 

The  first  pair,  the  olfactory  nerves,  come  from  the  front  end  of 
the  olfactory  lobes  and  are  sensory,  being  distributed  to  the  lining  of 
the  nasal  cavities,  i.e.  the  olfactory  organs.  The  second  or  optic 
nerves,  the  nerves  of  sight,  are  large  sensory  nerves  arising  from  the 
side  of  the  brain  beneath  the  optic  lobes.  They  partly  cross  over 
to  the  other  side  on  the  ventral  side  of  the  brain  forming  the  optic 
chiasma,  and  they  run  through  the  orbit  into  the  eyeball,  where  they 
spread  out  over  its  inner  surface.  Surrounding  them  is  a  muscle, 
the  retractor  bulbi,  which  can  pull  the  eyeball  back  into  its  orbit. 

*  In  addition  to  these  ten  cranial  nerves  there  are  also  two  others,  the 
nervus  terminalis  and  nervus  septalis,  but  they  are  very  small  and  difficult  to 
find.  They  will  be  dealt  with  more  fully  later  in  considering  the  Dogfish  and 
Rabbit,  and  it  is  sufficient  to  simply  notice  their  presence  at  this  place. 


THE  FROG 


81 


The  third  nerve,  a  small  motor  nerve  arising  from  the  ventral  surface 
of  the  brain  between  the  crura  cerebri,  is  distributed  to  four  of  the 
muscles  concerned  with  moving  the  eyeball,  namely,  the  superior 
and  inferior  recti,  the  internal  rectus  and  the  inferior  oblique.  It  is 
termed  the  motor  oculi.  The  fourth,  the  pathetic,  is  also  a  very 
small  motor  nerve  which,  arising  from  the  dorsal  surface  of  the  brain 
between  the  optic  lobes  and  cerebellum,  runs  to  the  superior  oblique, 
another  of  the  eye  muscles.  The  sixth,  or  abducens,  similarly  is 


¥.0 


M 


EN 


FIG.  26. — Diagram  of  lateral  view  of  distribution  of  cranial  nerves, 
Rana,  adapted  from  Howes. 

A.,  atrium  ;  B.,  brachial  nerve  ;  C.A.,  conus  arteriosus  ;  E.,  Eustachian  tube  ;  E.N.,  external 
naris  ;  L.,  lung;  N.,  nasal  bone;  Oe.,  oesophagus  ;  S.,  squamosal  bone;  S.V.,  sinus  venosus ; 
T.,  transverse  process  of  znd  vertebra  ;  V.,  ventricle  ;  V.C.,  vena  cava  posterior ;  II.,  optic 
nerve  ;  V.,  main  trunk  of  trigeminal ;  V.Mn.,  mandibular  branch  of  trigeminal ;  V.Mx.,  maxillary 
branch  of  trigeminal;  V.O.,  ophthalmic  branch  of  trigeminal;  VII.,  main  trunk  of  facial; 
VII.,  hyomandibular  branch  of  facial ;  VI I. P.,  palatine  branch  of  facial ;  IX.,  glossopharyngeal ; 
IX'.,  dorsal  ramus  of  glossopharyngeal ;  X'.,  dorsal  ramus  of  vagus  ;  X.c.,  cardiac  branch  of 
vagus;  X.G.,  gastric  branch  of  vagus;  X.L.,  laryngeal  branch  of  vagus;  X. P.,  pulmonary 
branch  of  vagus  :  I.,  hypoglossal ;  Id.,  dorsal  ramns  of  hypoglossal ;  2,  3,  and  4,  second,  third 
and  fourth  spinal  nerves. 

a  very  small  motor  nerve,  but  it  takes  its  origin  from  the  ventral 
surface  of  the  medulla  just  behind  the  pituitary  body.  It  runs  to 
the  last  of  the  muscles  concerned  with  the  movement  of  the  eye, 
namely,  the  external  rectus. 

The  largest  of  the  cranial  nerves  is  the  fifth,  or  trigeminal.  It 
arises  from  the  side  of  the  medulla  by  two  roots  which  unite  and 
enlarge  to  form  a  swelling,  the  pro-otic  ganglion,  where  it  comes  into 
contact  with  the  sixth  and  seventh  nerves,  and  then  leaving  this 
ganglion  it  passes  through  the  skull  just  in  front  of  the  auditory 


82  AN   INTRODUCTION  TO  ZOOLOGY 

capsule.  Outside  the  skull  it  divides  into  two  branches,  the  oph- 
thalmic and  the  maxillo-mandibular.  The  ophthalmic  branch, 
which  is  mainly  a  sensory  nerve,  runs  forward  through  the  orbit 
along  its  inner  side,  passing  above  all  the  eye  muscles  save  the  superior 
rectus.  At  the  anterior  end  of  the  orbit  it  divides  into  two  main 
branches,  one  to  the  mucous  membrane  of  the  olfactory  organ,  and 
the  other  to  the  skin  round  the  snout.  The  maxillo-mandibular 
branch  is  a  short  mixed  nerve  that  passes  outwards  just  in  front  of 
the  auditory  capsule  and  soon  divides  into  two  branches.  Its  maxil- 
lary branch  runs  forward  in  the  lower  part  of  the  orbit  below  the 
eyeball  and  supplies  the  upper  jaw,  upper  lip,  the  lower  eyelid  with 
its  depressor  muscle  and  the  adjacent  parts.  The  mandibular 
branch  goes  outwards,  downwards  and  backwards  beneath  the  quad- 
rato-jugal  bone  to  the  articulation  of  the  mandible.  It  passes  round 
this  and  forward  along  the  outer  side  of  the  lower  jaw,  to  which  it 
gives  branches  supplying  the  lower  lip  and  skin  muscles  of  the  floor 
of  the  mouth.  The  fifth  nerve  as  a  whole,  then,  is  a  mixed  nerve 
dividing  into  three  main  branches,  hence  its  name,  the  trigeminal. 

The  seventh,  or  facial  nerve,  is  also  a  mixed  nerve,  arising  from  the 
side  of  the  front  end  of  the  medulla,  with  a  distribution  somewhat 
resembling  the  fifth.  Like  this,  it  runs  to  the  pro-otic  ganglion  and 
leaves  the  skull  by  the  same  foramen.  Outside,  it  at  once  divides 
into  two  branches,  the  palatine  and  the  hyomandibular.  The 
palatine  runs  forward  close  above  the  mucous  membrane  of  the  mouth 
in  the  lower  inner  part  of  the  orbit.  At  the  front  end  of  the  orbit 
it  is  joined  by  a  cross  piece,  an  anastomosis,  to  a  twig  from  the 
maxillary  branch  of  the  fifth  nerve.  It  is  distributed  to  the  mucous 
membrane  of  the  front  end  of  the  roof  of  the  mouth.  The  hyomandi- 
bular branch  proceeds  outwards  around  the  auditory  capsule,  past 
the  inner  end  of  the  columella  auris,  just  beyond  which  it  receives 
an  anastomosis  linking  it  with  the  ninth  nerve.  Thence  it  runs 
downwards  in  the  posterior  wall  of  the  Eustachian  tube,  giving  twigs 
to  the  tympanum,  to  the  articulation  of  the  mandible  to  which  it 
sends  twigs.  Here  it  divides  into  two,  one  branch,  the  internal 
mandibular,  runs  in  the  floor  of  the  mouth  near  the  lower  jaw  supply- 
ing the  muscles  and  skin.  The  other,  the  hyoidean,  is  larger  and 
more  posterior  and  passes  forwards  in  the  floor  of  the  mouth  by  the 
side  of  the  anterior  cornu  of  the  hyoid  cartilage,  supplying  the  skin 
and  muscles  on  its  way. 

The  auditory  nerve,  which  is  the  eighth  of  the  series,  is  a  short 
stout  nerve,  purely  sensory,  coming  from  the  side  of  the  medulla 
just  behind  the  seventh.  It  passes  straight  out  into  the  auditory 
capsule  through  a  special  foramen  and  is  distributed  to  the  epithe- 
lium lining  the  various  parts  of  the  internal  ear. 


THE  FROG  83 

The  ninth  nerve,  known  as  the  glosso-pharyngeal,  is  a  mixed  nerve, 
but  mainly  sensory,  arising  from  the  medulla  behind  the  eighth  by 
roots  in  common  with  the  tenth  nerve.  They  leave  the  skull  together 
by  the  same  foramen,  the  jugular  foramen,  situated  on  the  outer  side 
of  the  exoccipital  condyle,  immediately  before  leaving  which  they 
form  a  large  jugular  ganglion.  On  quitting  this  the  ninth  sends  a 
small  branch  forwards,  which,  as  already  noted,  anastomoses  with  the 
hyomandibular  branch  of  the  seventh  nerve.  The  remaining  branch 
passes  round  the  pharynx  to  the  floor  of  the  mouth  and  pursues  a 
characteristic  wavy  course  below  and  inside  the  anterior  cornu  of 
the  hyoid  cartilage.  It  supplies  the  mucous  membrane  of  the  floor 
of  the  mouth  and  the  tongue,  in  which  it  ends. 

The  tenth  is  a  large  mixed  nerve,  called  the  vagus  or  pneumo- 
gastric,  and  it  arises  and  leaves  the  skull  in  the  way  indicated  above. 
It  is  unlike  all  the  preceding  cranial  nerves  in  that  it  is  distributed 
to  structures  outside  the  head.  After  quitting  the  jugular  foramen 
it  gives  off  a  few  twigs  to  the  muscles  of  the  back,  and  passes  in  the 
wall  of  the  pharynx  backwards  and  downwards,  dividing  into  four 
main  branches.  The  first,  the  recurrent  laryngeal,  loops  round  the 
pulmo-cutaneous  artery  and  runs  inwards  to  the  larynx.  The 
second  or  cardiac  passes  to  the  heart,  the  third  or  pulmonary  to  the 
lungs,  and  the  last  or  gastric  breaks  into  two  smaller  branches  going 
to  the  stomach  and  other  viscera. 

Spinal  Nerves. 

There  are  in  the  adult  ten  pairs  of  spinal  nerves,  for 
•although  others  are  present  in  the  tadpole,  they  disappear  during 
the  course  of  its  development.  Each  nerve  is  mixed  and  arises 
from  the  spinal  cord  by  two  quite  distinct  fairly  equi-sized  roots. 
The  dorsal  root  comes  from  the  dorso-lateral  aspect  of  the  cord 
and  is  related  to  the  dorsal  horn  of  grey  matter  within  it.  A 
short  distance  from  its  origin  it  bears  a  swelling,  the  dorsal  root 
ganglion,  and  immediately  after  unites  with  the  ventral  root.  The 
ventral  root  is  similarly  related  to  the  ventral  horn  of  grey  matter, 
and  leaving  the  ventro-lateral  border  of  the  cord  runs,  without  any 
ganglionic  enlargement,  to  join  the  dorsal  root.  The  common  trunk 
so  formed  is  surrounded  by  a  deposition  of  calcium  carbonate  forming 
a  conspicuous  white  patch,  and  passes  out  of  the  vertebral  column 
through  the  intervertebral  foramen.  It  divides  almost  at  once  into 
two  unequal  branches  ;  a  small  dorsal  branch  or  ramus  which  runs 
dorsally  to  the  muscles,  and  a  larger  ventral  ramus  which  is  the  main 
nerve. 

The  first  spinal  nerve  is  known  as  the  Hypoglossal,  and  it  leaves 
the  vertebral  column  by  the  intervertebral  foramen  between  the  first 


FIG.  27. — The  nervous  system  of  the  edible  frog  (Rana  esculenta),  from  the 
ventral  surface. — From  Ecker. 

F.,  facial  nerve  ;  G.,  ganglion  of  pneumogastric  nerve  ;  He.,  cerebral  hemisphere  ;  Lc.,  optic 
tract ;  Lop,  optic  lobe  ;  M.,  boundary  between  medulla  oblongata  and  spinal  cord  ;  M.  i-io,  the 
spinal  nerves  ;  MS.,  connection  between  fourth  spinal  nerve  and  sympathetic  chain;  N.,  nasal  sac  ; 
Ni.,  sciatic  nerve  ;  No.,  crural  nerve  ;  o.,  eyeball ;  S.,  trunk  of  sympathetic ;  S.  i-io,  the  sym- 
pathetic ganglia  ;  Sp.,  continuation  of  sympathetic  into  head. 

I.,  olfactory  nerve  ;  II.,  optic  nerve  ;  III.,  motor  oculi ;  IV.,  fourth  nerve  ;  V.,  trigeminal  and 
facial  nerves  ;  Va.,  ophthalmic  branch  of  trigeminal ;  Vc.,  maxillary  branch  of  trigeminal ;  Vd., 


ramus  anterior  of  glossopharyngeal ;  Xz.,  ramns  posterior  of  glossopharyngeal ;  X.  3-4,  branches 
of  pneumogastric. 


THE  FROG  85 

and  second  vertebrae.  It  runs  forward  in  the  floor  of  the  buccal 
cavity  ventral  to  the  glosso-pharyngeal  nerve,  and  is  distributed  to 
the  muscles  of  the  tongue  and  floor  of  the  mouth.  The  hypoglossal 
nerve  in  the  mammals  has  become  shifted  forward  and  forms  one  of 
the  cranial  nerves,  of  which  there  are  twelve  in  this  group,  and  not 
ten  as  in  the  frog. 

The  second  is  a  large  nerve  running  straight  outwards.  Branches 
from  the  first  and  third,  a  small  nerve,  also  join  it,  forming  a  complex 
termed  the  Brachial  plexus,  which  supplies  by  a  large  branch,  the 
coraco-clavicularis,  the  muscles  of  the  shoulder  girdle.  The  main 
trunk,  the  Brachial,  goes  on  into  the  arm,  dividing  just  above  the 
elbow  into  radial  and  ulnar  branches. 

The  succeeding  three  pairs  of  nerves,  four,  five  and  six,  are  small 
nerves  supplying  the  skin  and  body  wall  in  the  trunk  region. 

The  seventh,  eighth  and  ninth  nerves  unite  in  a  somewhat  com- 
plex manner  to  form  the  large  Sciatic  plexus  well  outside  the  vertebral 
column.  Inside  the  neural  canal  these  nerves,  together  with  the 
tenth  and  the  filum  terminale  of  the  spinal  cord,  run  downwards  in 
a  brush-like  group  known  as  the  cauda  equina.  From  the  sciatic 
plexus,  the  exact  constitution  of  which  is  subject  to  a  certain  amount 
of  variation,  come  off  three  nerves  :  the  ihohypogastric  supplying 
the  muscles  of  the  abdomen,  the  crural  supplying  the  muscles  of  the 
thigh  and  the  large  sciatic,  which  runs  down  the  thigh  and  divides 
into  tibial  and  peroneal  branches,  supplying  the  muscles  and  skin  of 
the  leg  and  foot.  The  last  and  smallest  of  the  spinal  nerves  is  the 
coccygeal,  which  leaves  the  vertebral  column  by  a  foramen  in  the 
urostyle  and  after  giving  a  branch  to  the  sciatic,  branches  over  the 
bladder,  cloaca  and  surrounding  tissue. 

Minute  Structure  of  the  Nervous  System. 

The  structural  unit  of  the  nervous  tissue  is  the  nerve  cell 
or  ganglion  cell.  A  typical  nerve  cell  is  very  large  compared  with 
most  other  cells  of  the  body,  and  consists  of  a  cell  body  from  which 
various  processes  are  given  off.  The  cell  body  is  composed  of  a 
basis  of  ordinary  protoplasm,  which  is  very  granular  owing  to  the 
presence  in  it  of  a  substance,  the  tigroid  substance,  in  the  form  of 
small  grains,  Nissl's  granules.  These  appear  to  form  a  store  of 
reserve  material  that  is  utilised  in  the  periods  of  activity  of  the  cell, 
for  far  more  of  it  is  present  in  a  cell  that  has  been  resting  for  some 
time  than  in  a  similar  cell  that  has  been  very  active.  Appropriate 
staining  also  shows  in  it  a  number  of  very  fine  strands,  the  neuro- 
fibrillae,  which  interlace  freely  and  are  continued  out  into  the  various 
processes,  linking  them  up  with  one  another  in  all  possible  direc- 
tions. Near  the  middle  of  the  cell  is  a  spherical  nucleus  which  is 


86  AN   INTRODUCTION  TO  ZOOLOGY 

fairly  clear,  staining  but  lightly  and  containing  a  well-marked 
nucleolus.  In  certain  very  large  nerve  cells  a  system  of  tiny  canals 
seems  to  be  present  in  the  cytoplasm,  and  it  is  thought  they  conduce 
to  the  ready  removal  of  waste  products  and  to  the  supply  of 
nutriment. 

One  of  the  processes  leaving  the  cell  differs  in  function  from  the 
rest ;  it  takes  nervous  messages  away  from  the  cell  and  is  termed  the 


FIG.  28. — Diagram  of  a  multipolar  nerve  cell  from  the  spinal 
cord  of  an  ox. 

A.,  axon  ;  D.,  dendron  ;  N.,  nucleus  ;  T.,  tigroid  substance. 

axon  or  neuraxis.  In  many  cases  it  differs  also  in  structure,  for  it 
forms  a  long,  in  some  parts  enormously  long  strand  of  fibrillar  proto- 
plasm of  practically  uniform  diameter.  During  its  course  it  gives 
off  a  certain  number  of  very  delicate  fibrils,  the  collaterals,  and  finally 
finishes  up  in  a  little  tuft  of  twigs  known  as  the  terminal  dendrite. 
This  is  always  the  case  in  cells  whose  fibres  leave  the  spinal  cord  and 
pass  out  into  the  tissues,  and  they  may  be  long  enough  to  run  right 
from  the  cord  to  the  tips  of  the  toes.  If,  on  the  other  hand,  the  cell 
remains  within  the  central  nervous  system,  the  axon  is  generally 
not  nearly  so  long  and  resembles  more  closely  the  other  processes, 
and  its  terminal  dendrite  is  often  more  spread  out. 


THE   FROG 


87 


-N.R. 


-A.C. 


The  other  processes  coming  from  the  cell  are  termed  the  dendrons, 
and  as  a  rule  are  shorter  and  branch  more  freely,  forming  a  tree-like 
structure.  They  are  the  receptive  processes  and  convey  the  stimuli 
to  the  cells.  The  processes  of  various  related  ganglion  cells  come 
very  close  to  one  another  in  most  cases,  and  in 
some  appear  to  come  into  actual  contact,  so 
that  the  nerve  impulses,  as  the  nerve  messages 
are  called,  can  be  transferred  from  the  axon 
of  one  cell  to  the  dendrons  of  other  cells 
adjacent  to  it.  The  whole  structure,  cell  body, 
dendrons  and  axon,  is  termed  a  neuron,  and 
the  nervous  system  is  composed  of  countless 
numbers  of  such  closely  bound  up  together, 
with  other  cells  of  a  supporting  nature,  the 
neuroglia  cells.  Three  main  types  of  neurons 
are  met  with  and  are  distinguished  by  the 
number  of  processes  given  off.  If  but  one 
process  is  present,  the  cell  is  termed  unipolar. 
Such  cells  are  found,  but  not  plentifully,  in 
the  dorsal  root  ganglia,  and  as  the  single 
process  divides  into  two  branches,  an  axon 
and  a  dendron,  they  may  perhaps  be  regarded 
as  a  specialised  form  of  the  second  group. 
The  next  group  are  termed  bipolar,  as  they 
have  but  one  dendron  in  addition  to  the  axon, 
and  these  too  occur  in  the  dorsal  root  ganglia. 
All  other  cells  with  more  than  two  processes 
are  termed  multipolar,  and  are  found  through- 
out the  whole  of  the  ventral  nervous  system 
and  are  particularly  well  marked  in  the  ven- 
tral horn  of  grey  matter  in  the  spinal  cord. 
The  actual  form  of  the  neuron  varies  in  dif- 
ferent parts,  and  it  is  possible  to  tell  within 
certain  limits  from  what  part  of  the  brain  a 
section  is  taken,  from  the  shape  and  arrange- 
ment of  its  ganglion  cells. 

The  axons  and  dendrons,  especially 
if  they  have  to  travel  far  from  the  parent 
cell,  are  covered  with  a  thin  but  moderately 
tough  transparent  membrane  variously  known  as  the  neurilemma, 
the  primitive  sheath  or  sheath  of  Schwann.  The  nuclei  belonging 
to  this  covering  are  scattered  irregularly  on  its  inner  surface  between 
it  and  the  nerve  fibre,  which  is  termed  the  axis  cylinder.  The  whole 
structure  is  of  a  greyish  colour  and  is  known  as  a  grey  fibre  or,  in 


-N. 


-M.S. 


-c.s. 


-A.C. 


FIG.  29. — Portions  of 
two  nerve  fibres. 

A.C.,  axis  cylinder ; 
C.S.,  cleft  of  Schmidt; 
M.S.,  medullary  sheath  ; 
N.,  nucleus ;  N.R.,  node 
of  Ranvier  ;  P.S.,  primi- 
tive sheath. 


88  AN  INTRODUCTION  TO  ZOOLOGY 

contradistinction  to  the  second  group  of  fibres,  as  a  non-medullated 
fibre. 

If  the  fibre  is  one  that  leaves  the  central  nervous  system  it  is 
provided  with  yet  another  protective  and  insulating  coat.  Between 
the  primitive  sheath  and  the  axis  cylinder  is  developed  a  compara- 
tively thick  layer,  the  medullary  sheath,  composed  of  a  substance, 
myelin,  closely  allied  to  fat.  This  substance  is  highly  refractive  in 
the  living  condition,  and  consequently  these  nerves  have  a  character- 
istic double  contour  when  seen  under  the  microscope,  but  when 
dead  become  white  and  opaque,  and  hence  such  nerve  fibres  are  termed 
white  or  medullated  fibres.  The  neurilemma  is  present  as  before, 
and  its  nuclei  immediately  beneath  it.  The  medullary  sheath  is 
not  continuous  throughout  the  length  of  the  fibre,  but  divided  into 
a  series  of  fairly  long  segments.  In  the  short  gaps  between  these 
segments  the  primitive  sheath,  slightly  thickened,  comes  into  contact 
with  the  axis  cylinder.  The  ring-like  constrictions  formed  in  this 
way  are  termed  the  nodes  of  Ranvier,  and  the  portion  from  one  node 
to  the  next  is  an  internodal  segment.  In  the  living  condition  the 
medullary  sheath  is  apparently  homogeneous  and  continuous  in  the 
internodal  segment,  but  when  dead  it  appears  to  break  up  into  a 

series  of  overlapping  portions, 
the  segments  of  Schmidt,  which 
are  separated  from  one  another 
by  clear  narrow  fissures  running 
obliquely  to  the  long  axis  of  the 
axis  cylinder.  In  the  higher 
vertebrates  each  internodal  seg- 
ment has  only  one  elongated  oval 
nucleus  lying  in  the  neurilemma, 

E.         ^?Sl|liil^  but  in  fish  a  number  of  nuclei 

are  scattered  in  each  internode. 

It  has  already  been  mentioned 

FIG.  ^o. — Transverse  section  of  a         .->     ,    -,     ,,    -,      •  n  -,          -. 

nerve,  that  both  brain  and  spinal  cord 

are  composed  of  grey  and  white 

A.,  artery  ;  E.,  epmeunum  ;   F.,  fasciculus  ;  *     _,.  J 

p.,  perineurium ;  v.,  vein.  matter.      Ihe    grey    matter    is 

mainly    composed    of    the    cell 

bodies  with  non-medullated  axons  and  dendrons,  while  the  white 
matter  is  almost  entirely  made  up  of  white  fibres. 

If  a  nerve  such  as  we  find  in  any  part  of  the  body,  e.g.  the 
sciatic  nerve,  be  cut  in  transverse  section  it  will  be  seen  that  it  is 
composed  of  an  enormous  number  of  fibres  grouped  together  and 
surrounded  by  a  tough  connective  tissue  envelope,  the  epineurium. 
Within  this  the  fibres  are  seen  to  be  arranged  in  definite  bundles  of 
various  sizes,  the  fasciculi,  each  of  these  has  its  own  sheath,  the 


THE  FROG  89 

perineurium,  whose  substance  is  continuous  with  the  epineurium. 
Yet  again,  the  individual  fibres  themselves  are  separated  by  a  small 
amount  of  the  same  kind  of  tissue,  here  termed  the  endoneurium. 
The  neuroglia  cells  are  found  all  through  the  central  nervous 
system  and  are  much-branched  cells,  whose  fibrous  processes  connect 
up  with  one  another  in  such  a  manner  as  to  make  a  supporting 
scaffolding  for  the  more  delicate  nerve  cells  and  processes. 

Sympathetic  or  Involuntary  Nervous  System. 

The  sympathetic  system  in  Rana  is  well  developed,  and 
consists  of  a  double  chain  of  ganglia  joined  by  longitudinally 
running  strands  and  closely  connected  with  both  the  cranial 
and  spinal  nerves.  The  ganglia  and  cords  are  usually  covered 
with  a  layer  of  black  pigment,  which  makes  them  easy  to  follow. 
It  takes  origin  on  each  side  in  the  pro-otic  ganglion,  and  runs 
backwards  in  the  cranium  to  leave  it  with  the  ninth  and  tenth 
cranial  nerves  through  the  jugular  foramen.  The  first  ganglion 
is  formed  on  the  postero-ventral  aspect  of  the  hypoglossal  nerve, 
and  is  connected  with  this  nerve  by  a  few  fine  fibres.  Thence  it 
passes  back  close  to  the  systemic  arch,  and  then  along  the  sides  of 
the  dorsal  aorta  forming  eight  or  nine  ganglia,  the  number  is  not 
constant,  while  so  doing.  Each  ganglion  is  connected  with  its 
corresponding  spinal  ganglion  by  means  of  a  nerve  stalk,  the  ramus 
communicans,  and  these  increase  in  length  as  they  pass  backwards. 
The  last  ganglion  is  connected  to  the  coccygeal  nerve  by  several  fine 
rami  communicantes.  The  ganglia  also  give  off  numerous  very 
fine  branches,  which  pass  round  the  dorsal  aorta  in  the  form  of  a 
very  delicate  network  or  plexus.  A  similar  but  more  marked  rami- 
fication, the  cardiac  plexus,  is  formed  on  the  surface  of  the  heart, 
where  it  is  connected  with  two  ganglia  to  be  dealt  with  shortly,  and 
around  the  roots  of  the  great  vessels.  Branches  from  the  third, 
fourth  and  fifth  ganglia  on  each  side  pass  vent  rally  to  two  large 
ganglia,  the  coaliac  ganglia,  situated  one  on  each  side  of  the  cceliaco- 
mesenteric  artery,  and  so  form  the  solar  plexus.  Branches  from  this 
plexus  run  to  form  marked  plexuses  of  fibres  in  the  walls  of  the 
stomach  and  intestine,  and  others  in  the  uro-genital  organs  and  the 
remaining  viscera.  In  fact,  branches  of  this  group  of  nerves  spread 
to  all  parts  of  the  body  in  which  involuntary  muscle  fibres  are 
present,  and  to  the  various  viscera  not  directly  under  the  control 
of  the  will,  and  hence  the  term  the  involuntary  nervous  system  is 
quite  applicable. 

The  first  of  the  two  ganglia  in  the  heart  referred  to  above  is 
Remak's  ganglion  lying  in  the  wall  of  the  sinus  venosus,  and  the 
second  is  Bidder's  ganglion,  situated  in  the  auricular  septum  near  the 


go  AN   INTRODUCTION   TO  ZOOLOGY 

auriculo- ventricular  junction.  Both  ganglia  appear  to  be  connected 
with  twigs  from  the  cardiac  branch  of  the  vagus  nerve,  they  are  also 
related  to  the  cardiac  plexus,  and  all  combine  to  regulate  the  speed 
and  strength  of  the  heart  beat. 

As  has  been  pointed  out  previously,  a  nerve  fibre  can  convey 
an  impulse  in  one  direction  only,  thus  the  messages  pass  along  the 
axon  away  from  the  cell  and  in  the  dendrons  towards  the  cell.  The 
exact  nature  of  nervous  impulse  is  not  known,  but  it  is  fairly  closely 
allied  to  an  electric  current,  although  its  rate  of  propagation  is  rela- 
tively very  slow,  being  only  about  125  metres  per  second.  It  has 
been  found  by  experiment,  e.g.  by  cutting  the  roots  of  the  spinal 
nerves  stimulating  the  cut  ends  and  noting  the  results,  that  the 
dorsal  root  is  composed  almost  entirely  of  afferent  or  sensory  fibres, 
conducting  messages  towards  the  spinal  cord,  while  the  ventral  root, 
on  the  other  hand,  is  efferent,  carrying  its  messages  to  the  periphery. 
Efferent  nerves  are  sometimes  termed  motor  nerves,  but  in  the  strict 
meaning  of  the  term  they  are  only  motor  if  -they  pass  to  muscles 
and  by  stimulating  them  cause  their  contraction.  If  they  go  to  a 
gland  their  action  is  probably  to  cause  an  increase  in  glandular 
activity,  and  so  they  may  be  termed  excitatory,  and  those  going  to 
the  heart  may  cause  it  to  beat  more  rapidly,  i.e.  are  acceleratory  or 
produce  the  reverse  effect  when  they  are  termed  depressor  or 
inhibitory. 

Just  as  the  anatomical  unit  of  the  entire  nervous  system  is 
the  neuron,  so  the  functional  or  physiological  unit  is  the  combination 
of  such  neurons,  termed  a  reflex  arc.  This  in  its  simplest  condition 
may  consist  of  two  neurons.  Take  an  example  of  a  spinal  reflex, 
that  is,  one  occurring  in  the  spinal  cord.  It  consists,  in  the  first 
place,  of  the  sensory  epithelium  in  which  the  nerve  terminates  in  a 
series  of  small  sense  organs.  On  the  stimulation  of  these  a  message 
passes  up  the  dendron  of  the  afferent  nerve  to  a  cell  in  the  dorsal  root 
ganglion,  and  thence  on  by  the  axon  into  the  spinal  cord.  Here  the 
terminal  dendrites  come  into  relation  with  the  dendrons  of  an 
efferent  ganglion  cell  situated  in  the  ventral  horn  of  grey  matter,  and 
so  the  message  is  handed  on.  It  can  then  be  sent  out  by  the  axon 
of  the  afferent  cell  through  the  ventral  root  to  the  corresponding 
muscle.  In  this  way  stimulation  of  the  epithelium  will  bring  about 
the  contraction  of  the  muscle  related  to  it  without  the  intervention 
of  the  brain.  The  whole  structure  is  termed  a  reflex  arc,  and  the 
action  a  reflex  action.  The  arc  is  not  always,  indeed  but  rarely, 
so  simple  as  this,  for  generally  the  terminal  dendrite  of  the  dorsal 
root  fibre  come  into  relation  with  the  dendrons  of  a  ganglion  cell 
in  the  dorsal  horn  of  grey  matter,  and  this  cell  sends  out  an  axon 
whose  dendrite  is  related  to  the  dendrons  of  the  ventral  horn  cell. 


THE   FROG  91 

This  intervening  link  is  termed  the  connector  neuron.  In  many  cases 
even  more  neurons  than  this  are  involved,  and  the  reflex  involves 
quite  a  complex  system  of  neurons  and  muscles.  Examples  of  these 
reflexes  can  readily  be  shown  in  a  frog  whose  brain  has  been 
destroyed.  If  such  a  frog  be  suspended  and  a  toe  pinched,  or  the 
toes  dipped  in  weak  acid,  the  foot  will  be  withdrawn.  A  spot  of 
weak  acid  placed  on  the  abdomen  will  be  kicked  off  by  the  hind  foot, 
and  various  other  similar  actions  can  be  brought  about,  all  of  them, 
as  we  should  say,  purposeful.  It  will  be  seen  that  in  these  cases  it 
is  not  merely  the  muscle  actually  touching  the  skin  that  is  con- 
cerned, but  the  whole  of  the  muscles  of  the  leg  and  foot  related  to 
and  bringing  about  the  movement  of  the  part.  Reflex  action  plays 
a  considerable  part  in  the  ordinary  activity  of  the  higher  animals, 
and  the  life  of  the  lower  animals  appears  to  be  almost  entirely  made 
up  of  a  large  number  of  these  co-ordinated  reflexes  often  of  a  complex 
nature. 

When  the  impulse  enters  the  spinal  cord  by  the  dorsal  root  it  is 
not  only  sent  on  to  the  remaining  part  of  the  reflex  arc,  but  it  is  also 
transmitted  to  the  dendrons  of  the  cells  running  longitudinally  in  the 
spinal  cord.  These  cells,  in  their  turn,  hand  it  on  to  similar  cells  and 
so  form  a  relay  path  by  which  the  message  is  conveyed  to  the  brain, 
where  it  may  produce  various  results  according  to  the  requirements 
of  the  animal.  In  our  own  case,  although  many  of  these  reflexes 
are  very  powerful,  it  is  possible  to  override  them  by  an  effort  of  the 
will.  Thus,  for  example,  if  we  purposely  pick  up  a  hot  coal  we  may 
suppress  the  reflex  to  such  an  extent  that  we  do  not  drop  the  coal, 
but  allow  it  to  burn  our  fingers.  One  important  function  of  the  brain, 
then,  is  to  receive  the  stimuli  brought  in  from  all  parts,  and  if  neces- 
sary to  correlate  and  modify  the  local  reflexes  in  such  a  manner  as 
to  make  for  the  well-being  of  the  animal  as  a  whole,  and  not  merely 
of  its  individual  parts.  The  better  able  an  animal  is  to  order  its 
reflexes  to  meet  the  demands  of  its  environment  in  the  very  widest 
meaning  of  the  word,  the  more  advantages  it  possesses  over  its 
fellows  not  so  well  endowed,  and  the  more  highly  developed  we 
regard  it. 

Sense  Organs. 

The  sense  organs  comprise  the  organs  of  smell,  sight,  touch, 
taste  and  hearing.  Three  of  these,  the  first  two  and  the  last,  are 
compact  and  surrounded  by  supporting  structures  known  as  capsules, 
and  the  other  two  consist  of  numerous  tiny  spots  scattered  over  a 
more  or  less  wide  area. 

The  olfactory  organ,  the  organ  of  smell,  is  localised  in  the  olfactory 
capsule,  a  cartilaginous  structure  situated  at  the  anterior  end  of  the 


92  AN   INTRODUCTION  TO  ZOOLOGY 

skull  in  front  of  the  cranium.  There  are  two  of  these  sacs  side  by 
side,  but  completely  separated  from  one  another  in  the  middle  line 
by  the  nasal  septum.  Each  consists  of  a  main  chamber  communi- 
cating with  the  outside  by  means  of  the  external  nares,  and  with 
the  inside  by  the  internal  nares,  which  open  on  the  roof  of  the 
buccal  cavity  just  in  front  of  the  vomers.  The  floor  of  the  main 
cavity  is  occupied  by  a  well-marked  swelling  and  two  smaller  cham- 
bers, a  lateral  and  a  dorsal,  lead  off  from  it.  The  whole  of  the  inside 
of  the  three  chambers  is  lined  by  a  sensory  epithelium.  Transverse 
section  shows  this  epithelium  to  be  simple  and  columnar,  and  to 
contain  two  varieties  of  cells.  The  most  important  are  the  long 
narrow  sensory  cells  which  are  separated  and  kept  in  position  by 
more  numerous  columnar  cells.  At  the  free  end  of  the  olfactory 
cell  is  situated  a  small  tuft  of  very  delicate  sensory  hairs,  and  its 
inner  end  is  continued  on  as  a  fine  fibre.  These  fine  fibres  unite 
together  in  bundles  to  form  small  nerves,  which  pass  into  the  olfactory 
lobes.  Thus  these  cells  are  to  be  regarded  as  peripherally  situated 
neurons  whose  axons  run  into  the  brain  and  whose  dendrons  are 
represented  functionally  by  the  small  hairs,  inasmuch  as  they 
receive  the  impulse  and  convey  it  to  the  cell.  An  epithelium  built 
up  in  this  way  of  neurons  and  supporting  cells  is  sometimes  termed 
a  nervous  epithelium.  Here  and  there  the  epithelium  dips  down  to 
form  simple  saccular  glands,  whose  business  it  is  to  secrete  the  mucous 
that  keeps  the  whole  interior  moist. 

The  main  function  of  this  organ  is  olfactory,  that  is,  it  is  con- 
cerned with  the  perception  of  smells  of  various  sorts.  Owing  to  the 
fact  that  it  opens  internally  as  well  as  externally,  it  is  also  used  for 
respiration  ;  to  aid  in  the  carrying  out  of  this  latter  function  the 
external  nares  are  provided  with  valves  that  open  and  close,  regula- 
ting the  intake  of  air  in  a  way  that  has  already  been  described. 

The  organ  of  sight  is  the  eye,  situated  in  the  orbit  at  the 
side  of  the  cranium.  It  is  a  more  or  less  spherical  structure  kept  in 
position  by  means  of  its  nerve  and  a  series  of  muscles,  which  also 
enable  it  to  be  turned  in  different  directions  at  the  will  of  the  animal. 
When  removed  from  the  skull,  it  will  be  seen  that  the  eyeball,  as  it  is 
termed,  is  tough  and  more  flattened  on  the  outer  than  on  the  inner 
side  and  possesses  a  sort  of  stalk,  the  optic  nerve.  The  inner  side 
also  is  opaque,  whilst  the  outer,  exposed  between  the  eyelids,  is 
transparent,  allowing  certain  internal  parts,  the  iris  and  the  pupil, 
to  be  seen  through  it.  If  cut  in  half,  it  will  be  found  to  be  a  hollow 
structure  with  a  wall  composed  of  three  layers.  The  external  protect- 
ing and  supporting  layer  is  termed  the  sclerotic,  and  is  composed 
of  a  tough  cartilage  which  in  its  transparent  outer  portion  is  known 
as  the  cornea.  The  sclerotic  encloses  the  whole  eyeball  as  a  sort  of 


THE  FROG  93 

optic  capsule,  save  at  the  point  where  the  optic  nerve  leaves  it,  and 
here  it  is  reflected,  getting  thinner  to  form  a  sheath  for  the  nerve. 
The  exposed  surface  of  the  cornea  is  closely  covered  by  skin,  a  con- 
tinuation of  the  same  skin  that  covers  the  whole  body  and  forms  the 
eyelids,  but  in  this  region  it  becomes  very  delicate  and  transparent, 
and  is  termed  the  conjunctiva.  The  second  layer  of  the  eyeball  is 


FIG.  31. — View  of  the  human  eye,  divided  horizontally  through 
the  middle. — From  Furneaux. 

i,  conjunctiva ;  2,  cornea  ;  3,  sclerotic  ;  4,  sheath  of  the  optic  nerve  ;  5,  choroid  ;  6,  ciliary 
processes  ;  7,  iris  ;  8,  pupil ;  9,  retina  ;  10,  anterior  limit  of  the  retina  ;  n,  crystalline  lens  ; 
12,  suspensory  ligament;  13,  ciliary  muscle;  14,  aqueous  chamber;  15,  vitreous  chamber; 
1 6,  yellow  spot  ;  17,  blind  spot. 

known  as  the  choroid  coat,  and  is  formed  of  densely  pigmented  con- 
nective tissue  containing  blood-vessels.  The  choroid  layer  fits 
closely  on  to  the  sclerotic  over  the  inner  parts  and,  like  it,  is  partly 
reflected  over  the  optic  nerve.  At  the  edge  of  the  cornea  the  choroid 
coat  leaves  the  sclerotic  and  becomes  stretched  across  the  eyeball, 
so  dividing  it  into  two  parts  known  as  the  anterior  and  posterior 
chambers.  The  anterior  or  outer  chamber  is  filled  with  a  watery 


94  AN   INTRODUCTION  TO  ZOOLOGY 

fluid,  the  aqueous  humour,  while  the  posterior  chamber  is  rilled  with 
a  transparent  semi-gelatinous  mass,  the  vitreous  humour.  The 
vitreous  humour  is  bounded  by  a  delicate  sheath,  the  hyaloid  mem- 
brane. The  part  of  the  choroid  stretched  curtainwise  across  the  eye 
is  termed  the  iris,  and  it'  is  perforated  in  the  middle  by  an  oval  aper- 
ture, the  pupil.  The  iris  also  is  deeply  pigmented,  appearing  bright 
yellow  on  the  outside,  and  it  acts  in  the  same  way  as  the  diaphragm 
of  a  camera  lens,  regulating  the  amount  of  light  entering  through  the 
pupil,  which  appears  as  a  black  spot.  In  the  iris  run  two  sets  of 
involuntary  muscle  fibres,  one  set  are  circular  and  the  other  radiating, 
and  so  between  them  they  control  the  size  of  the  pupil.  At  the 
line  where  the  iris  leaves  the  sclerotic  the  choroid  coat  is  thrown 
into  a  series  of  tightly  packed  radiating  folds,  well  supplied  with 
blood-vessels,  the  ciliary  processes.  These  have  unstriped  muscles 
running  from  their  outer  edges  to  the  sclerotic  cartilage,  thus  forming 
a  ring  of  fibres,  the  ciliary  muscle,  as  it  is  termed.  Just  behind  and 
touching  the  iris,  and  so  completely  separating  the  anterior  and  pos- 
terior chambers,  is  a  hard  strongly  biconvex  body,  the  lens.  It  is 
crystalline  and  quite  transparent  in  the  living  animal,  but  becomes 
opaque  after  death  and  is  composed  of  a  series  of  concentric  fibres, 
each  derived  from  a  single  cell.  The  lens  is  held  in  position  by  being 
contained  in  a  thin  membranous  bag,  the  lens  capsule,  which  is 
continued  'out  around  its  circumference  into  a  tough  flange,  the 
suspensory  ligament,  in  its  turn  attached  to  the  ciliary  processes. 

Within  the  choroid  we  have  the  third  or  internal  coat  of  the  eye, 
the  retina.  This  is  a  neuro-epithelium,  and  is  the  part  sensitive  to 
light.  It  lies  close  to  the  choroid  in  the  living  animal,  but  is  readily 
detached,  save  at  the  point  of  exit  of  the  optic  nerve  and  when  the 
eye  has  been  cut  in  half  often  hangs  loosely.  The  thickness  of  the 
retina  is  approximately  the  same  over  the  greater  part  of  the  posterior 
chamber,  but  just  behind  the  ciliary  processes  it  suddenly  becomes 
much  thinner,  being  reduced  to  a  columnar  epithelium,  and  the  ridge 
or  step  marking  the  point  of  this  reduction  forms  a  wavy  line,  the 
ora  serrata,  running  round  the  inside  of  the  eyeball.  The  thin  part 
of  the  retina  is  continued  on  over  the  ciliary  processes,  where  it  is 
termed  the  pars  ciliaris  retinae,  and  beyond  this  again  on  to  the  pos- 
terior surface  of  the  iris,  but  here  it  is  represented  only  by  its  pigment 
layer.  The  innermost  surface  of  the  retina  turns  in  to  run  through 
the  posterior  coats  of  the  eye  as  the  optic  nerve,  and  just  at  the  small 
point  where  it  does  so  the  retina  is  not  sensitive  to  light  ;  thus  a 
"  blind  spot  "  is  produced.  At  the  end  of  the  line  passing  through 
the  centre  of  the  lens,  the  optic  axis,  the  retina  becomes  thinner- 
forming  a  tiny  saucer-shaped  depression,  usually  having  a  yellowish 
tinge,  hence  termed  the  yellow  spot  or  macula  lutea,  which  is  the 


THE  FROG  95 

point  where  the  perception  of  the  retina  is  most  keen.     The  blind 
spot  is  a  little  below  and  behind  the  yellow  spot. 

Examined  under  fairly  high  magnification  a  transverse  section 
of  the  wall  of  the  eyeball  shows  the  sclerotic  to  be  mainly  composed 
of  ordinary  hyaline  cartilage.  The  choroid  is  deeply  pigmented 
tissue  with  a  plentiful  supply  of  blood-vessels.  The  retina  exhibits 
quite  a  complex  structure  and  its  full  details  are  difficult  to  make  out, 
save  in  specially  prepared  and  stained  sections.  When  these  are 
employed  the  full  structure  can  be  seen,  but  even  in  ordinary  sections 
viewed  under  comparatively  low  magnification  the  relative  position 
of  the  parts  can  be  easily  made  out.  Under  the  low  power,  the  retina 
appears  to  consist  of  a  series  of  superimposed  layers.  Next  to  the 
choroid  coat  is  a  pigment  layer,  and  then  follow  in  succession  layers 
termed  the  rod  and  cone,  the  outer  nuclear,  the  outer  molecular, 
the  inner  molecular,  the  cellular  and  the  fibrous  layers.  The  explana- 
tion of  this  appearance  becomes  clear  when  we  examine  the  detailed 
structure. 

The  pigment  layer  is  composed  of  a  number  of  cells,  deeply  im- 
pregnated with  pigment,  that  send  processes  down  between  the  rods. 

The  visual  sensory  cells  are  the  rods  and  cones.  The  rod-cells, 
which  are  more  numerous  than  the  others,  consist  of  long  thread- 
like cells.  Each  cell  swells  out  near  the  inner  end  to  form  an  enlarge- 
ment in  which  the  nucleus  is  situated.  These  nuclei  fall  in  the  outer 
nuclear  layer,  the  external  boundary  of  which  is  marked  by  a  thin 
transparent  homogeneous  membrane,  the  outer  limiting  membrane. 
Outside  this  membrane  each  rod-cell  exhibits  a  spindle-shaped 
enlargement  and  then  becomes  a  narrow  cylindrical  rod,  from  which 
the  cell  is  named,  both  of  these  parts  coming  within  the  designation 
of  rods  and  cones.  The  thread-like  portion  of  the  cell,  passing  in- 
wards from  the  nucleus,  terminates  in  a  small  knob-like  swelling. 
The  whole  structure  represents  a  much  modified  sensory  epithelial 
cell,  the  rod-like  portion  corresponding  to  the  dendron,  and  the  knob- 
like  enlargement  the  dendrite  at  the  end  of  its  axon.  The  cone-cell 
has  a  very  similar  structure,  the  principal  difference  being  that  the 
spindle-shaped  swelling  of  the  dendron  is  larger  and  more  marked,  and 
passes  on  into  a  very  short  pointed  rod,  hence  appearing  cone-like. 

Next  come  a  series  of  connecting  elements  in  the  form  of  more 
special  nerve  cells,  whose  bodies  with  their  nuclei  constitute  the 
inner  nuclear  layer.  The  dendron  of  each  cell  passes  out  and  arbo- 
rises round  the  knobs  of  the  rod  and  cone  cells,  thus  giving  rise  to 
the  outer  *  molecular  layer.  The  axon  with  a  large  dendrite  comes 
in  the  inner  *  molecular  layer,  where  it  comes  into  functional 

*  Aborisations  of  other  cells,  that  have  been  omitted  for  the  sake  of 
simplicity,  are  concerned  in  the  formation  of  these  two  molecular  layers. 


96 


AN   INTRODUCTION  TO  ZOOLOGY 


connection  with  the  dendrons  of  the  last  layer  of  cells,  the  trans- 
mitting cells.  The  bodies  of  these  trans- 
mitting elements,  which  are  relatively 
large,  form  the  layer  of  nerve  cells  and 
their  axons,  the  innermost  layer,  that  of 
the  nerve  fibres.  These  pass  out,  via 
the  blind  spot  and  are  continued  on 
outside  the  eye  as  the  optic  nerve  into 
the  brain.  On  the  internal  surface  of 
the  nerve  fibres  is  another  thin  inner 
limiting  membrane.  Between  the  inner 
and  outer  limiting  membrane  stretch 
certain  non-nervous  supporting  cells, 
known  as  Muller's  fibres,  that  form  a 
sort  of  scaffolding  and  whose  nuclei  fall 
in  the  inner  nuclear  layer. 

The  rods  and  cones  are  the  actual 
receptive  elements  in  the  eye,  and  it  is 
interesting  to  note  that  they  are  on  the 
outside  of  the  retina,  so  that  the  light 
has  to  pass  through  all  the  other  layers 
before  reaching  them,  after  doing  which 
it  is  stopped  by  the  pigment  cells.  From 
the  evidence  afforded  by  diseases  of  the 
eye,  it  seems  probable  that  the  cones 
are  more  particularly  concerned  with  the 
perception  of  colour  and  the  rods  with 
variations  of  light  intensity.  The  whole 
eye  forms  a  kind  of  photographic  camera, 
and  the  amount  of  light  let  in  is  regulated 
by  the  iris,  which  therefore  plays  the 
part  of  the  diaphragm.  By  means  of 
the  lens  an  image  of  what  the  animal 
wishes  to  see  is  focussed  upon  the  retina, 
with  the  central  part  of  the  picture  fall- 
ing upon  the  yellow  spot.  As  the  things 
looked  at  are  at  different  distances  from 
the  eye,  it  is  necessary  that  the  lens 
should  be  capable  of  forming  clear 
images  of  both  near  and  distant  objects. 
The  distance  between  the  lens  and  the 
plate  (i.e.  the  retina)  cannot  be  altered 

as   in  a   camera,  but   in  order   to  produce  the  same   result    the 
actual  shape  of  the  lens  itself  is  altered  ;    it   is  more  flattened 


NF 


It 

FIG.  32. — Vertical'  section 
through  the  posterior  wall 
of  the  eye  of  a  frog  ;  the 
section  passes  through  the 
sclerotic,  the  choroid,  and 
the  entire  thickness  of  the 
retina.  X  300.  —  From 
Marshall  and  Gamble. 

B.C.,  red  blood  corpuscle  ;  C., 
cone  ;  G.,  ganglion  cell ;  I.L.,  inner 
limiting  membrane  ;  I.M.,  inner 
molecular  layer ;  I.N.,  inner  nuclear 
layer  ;  N.F.,  layer  of  nerve  fibres  ; 
O.L.,  outer  limiting  membrane  ; 
O.M.,  outer  molecular  layer  ;  O.N., 
outer  nuclear  layer ;  P.,  pigment 
cell  of  retina ;  P.C.,  pigment  of 
choroid  ;  R.,  rod  ;  R.F.,  radial  or 
Muller's  fibre  ;  S.,  the  cartilaginous 
sclerotic. 


THE   FROG 


97 


o.s. 


R.A. 


for  distant  view  and  more  convex  for  things  close  at  hand.     This 

alteration,   termed  accommodation,   is  brought    about  by   means 

of     the     ciliary     muscles. 

Under    normal    conditions  A 

the  suspensory  ligament  is 

pulled  tight,   and    so   the 

lens   is  compressed  by  its 

capsule.       When    a    near 


object  is  examined  the 
ciliary  muscles  contract,  so  R.F? 
pulling  the  choroid  for- 
ward, thereby  relaxing  the 
ligament  and  allowing  the 
lens  by  its  own  elasticity 
to  assume  a  more  curved 
shape.  When  the  image, 
which  is  inverted,  falls  on 
the  retina  it  is,  as  it  were, 
analysed  by  the  rods  and 
cones  and  then  transmitted 
to  the  brain,  by  the  relays 
already  noted,  and  there  it 
is  interpreted.  With  the 
advance  of  age  in  ourselves, 
there  is  usually  a  gradual 
increase  in  the  rigidity  of 
the  lens,  and  consequently 
it  becomes  harder  to  see 
things  close  at  hand,  so 
that  glasses  have  to  be 
worn  when  reading,  etc. 

In  order  to  point  the 
eye  in  the  right  direction, 
it  is  provided  with  a  series 
of  muscles  which  are  ar- 
ranged in  two  groups.  The 
first  is  a  group  of  four  mus- 
cles, arising  close  together 
at  the  inner  posterior  angle 
of  the  orbit  and  inserted  on 
the  top,  bottom,  inner  and 

outer  sides  of  the  eyeball.  They  are  termed  collectively  the  recti 
muscles,  and  individually  the  Rectus  superior,  Rectus  inferior,  Rectus 
internus  or  anterior  and  Rectus  externus  or  posterior  respectively. 


R.s. 


3. 


R.R 


R.I. 


FIG.    33. — Diagram  of  the  muscles  of  the 
eye  of  a  vertebrate,  Scyllium. 

A,  eyeball  removed,  viewed  from  side  ;  B,  viewed 
from  above;  C.,  cranial  wall;  E.,  eyeball;  I.,  iris; 
O.T., inferior  oblique ;  O.S.,  superior  oblique ;  P.,  pupil ; 
R.A.,  anterior  or  internal  rectus  ;  R.I.,  inferior  rectus  ; 
R.P.,  posterior  or  external  rectus ;  R.S.,  superior 
rectus  ;  3,  4,  and  6,  third,  fourth,  and  sixth  cranial 
nerves. 


98  AN  INTRODUCTION  TO  ZOOLOGY 

The  second  is  a  pair  of  muscles  arising  together  from  the  inner  an- 
terior corner  of  the  orbit.  One,  the  Obliquus  superior,  runs  obliquely 
upwards  and  backwards,  and  the  other,  the  Obliquus  inferior,  runs 
obliquely  downwards  and  backwards  to  be  inserted  on  the  eyeball. 
Between  them  these  six  muscles,  which  are  to  be  found  in  all  verte- 
brate animals,  can  move  the  eye  in  any  direction  that  the  animal 
wishes.  The  nerve  supply  of  these  muscles,  which  is  also  constant 
in  vertebrates,  has  already  been  indicated.*  In  the  frog  there  is  an 
additional  muscle  present,  called  the  Retractor  bulbi,  which  surrounds 
the  optic  nerve  and  enables  the  whole  eyeball  to  be  drawn  back 
somewhat  into  the  orbit. 

The  senses  of  touch  and  taste  are  served  by  cells  which, 
unlike  those  of  the  other  three  senses,  are  not  aggregated  together 
to  form  fairly  large  well-defined  sense  organs,  but  are  scattered  singly 
or  in  small  groups  over  a  more  or  less  wide  area.  Touch  is  located 
partly  in  the  epidermis,  in  which  nerve  fibres  form  a  fine  network 
with  ends  situated  between  the  cells,  and  for  the  most  part  in  the 
dermis.  The  tactile  and  organs  of  the  dermis  comprise  corpuscles 
of  various  sorts,  but  each  consists  essentially  of  a  much  modified 
cell  or  cells,  in  close  relation  to  which  a  nerve  fibre  breaks  up  into  a 
fine  mesh  work  of  arborisations.  By  means  of  these  different  organs 
the  various  stimuli  that  constitute  "  touch,"  i.e.  pressure,  warmth, 
cold  and  pain,  are  received  and  conveyed  to  the  brain.  It  will  be 
noted  that  the  stimulus  only  reaches  them  in  a  reduced  form  through 
the  epidermis,  with  the  result  that  when  an  accident  removes  a  portion 
of  our  epidermis  all  the  stimuli,  including  pain,  appear  much  mag- 
nified when  anything  is  touched  by  the  injured  spot.  The  gustatory 
end  organs  subserving  taste  are  naturally  limited  to  the  tongue 
and  mucous  membrane  of  the  mouth  and  pharynx.  Generally 
they  consist  of  a  group  of  modified  columnar  cells,  some  of  which 
give  off  branching  processes  that  are  related  to  the  ends  of  the 
nerves  of  the  plexus  underlying  the  epidermis.  The  sense  of  taste 
is  resolvable  into  at  any  rate  four  primary  components  :  Sweetness, 
Bitterness,  Acidity  and  Salinity. 

The  auditory  organ  or  organ  of  hearing  in  the  frog  consists 
of  two  separate  parts,  known  respectively  as  the  inner  ear  and  the 
middle  ear  ;  the  former  is  the  essential  organ  of  hearing,  while  the 
latter  is  concerned  with  receiving  sound  waves  and  conveying 
them  to  the  inner  ear.  The  inner  ear  is  constituted  by  a  somewhat 
complex  structure  known  as  the  membranous  labyrinth,  and  this 
is  lodged  in  the  otic  capsule.  The  labyrinth  consists  of  a  membranous 
sac,  the  vestibule,  divided  into  two  parts.  The  upper  larger  portion 

*  The   muscles  of   the  vertebrate  eye  can    be    studied   more   readily  in 
Scy Ilium,  on  account  of  its  size. 


THE  FROG  99 

is  known  as  the  utriculus,  and  the  lower  smaller  one  is  termed  the 
sacculus.  Connected  with  the  utriculus  are  three  hoop-shaped  mem- 
branous tubes,  the  semicircular  canals.  The  anterior  of  these  lies 
in  a  plane  practically  parallel  with  the  sagittal  plane  of  the  head, 
and  its  lower  front  end  dilates  just  as  it  is  joining  the  utriculus  to 
form  an  ampulla.  The  posterior  semicircular  canal  lies  in  a  trans- 
verse plane  almost  at  right  angles  to  the  former  and,  like  it,  is  dilated 
into  an  ampulla  at  its  lower  end.  The  upper  extremities  of  both 
these  vertical  canals  join  together  to  form  a  common  tube  opening 
into  the  utriculus.  The  third  or 
horizontal  semicircular  canal  is  in  a 
plane  roughly  at  right  angles  to  the  . 
planes  of  the  other  two,  and  has  an 
ampulla  at  its  anterior  end.  From 
the  sacculus  arises  a  small  sac-like 
outgrowth,  the  lagena,  .  which  in 
higher  vertebrates  is  transformed  into 
a  very  complex  spiral  structure,  the 

cochlea,  and  even  in  the  frog  indica-  FlG  34-—  The  right  internal  ear 
.  .  ,  ,  ,  ,  .  r  ,  ,  .  .  i  .  of  the  frog,  removed  from  the 

tions  of  the  beginning  of  this  speciah-  periotic  c*  rtiiage  and  drawn 
sation  are  to  be  seen.  The  utriculus  from  the  outer  surface.  — 
also  gives  rise  to  a  small  tube,  the  From  Marshall  and  Gamble. 

dUCtUS  endolymphatiCUS,  Which  enters  «.,  the  anterior  vertical  semicircular 

,  .    ,  n       t  .  canal;  0.,  its  ampulla;  A.,  the  horizontal 

the  cranial   cavity  and   there   unites     canal  ;  ».,  its  ampulla  ;  p.,  the  posterior 


with  its  fellow  to  form   a   sac,  the 
saccus  endolymphaticus,  filled  by  a 

whitish  fluid  which  owes  its  colour  to  the  presence  of  very  small 
crystals  of  Calcium  carbonate.  A  median  prolongation  from  the 
saccus  runs  back  along  the  dorsal  side  of  the  spinal  cord,  giving  off 
paired  outgrowths  which  pass  out  through  the  intervertebral  fora- 
mina and  form  the  so-called  calcareous  bodies  or  periganglionic 
glands,  visible  as  white  masses  on  each  side  of  the  vertebral 
column. 

The  whole  membranous  labyrinth  is  filled  with  a  fluid,  the  endo- 
lymph,  and  is  closely  invested  by  cartilage  and  bone  ;  the  small 
space  left  between  the  labyrinth  and  the  surrounding  tissues  being 
filled  with  another  fluid,  the  perilymph.  The  endolymph  contains 
a  number  of  calcareous  granules,  the  otoliths,  which  play  a  consider- 
able part  in  the  functioning  of  the  ear. 

The  labyrinth  is  lined  by  an  ectodermal  epithelium  whose  cells 
are  differentiated  to  form  sensory  patches  here  and  there.  The 
patches  contain  pear-shaped  sensory  cells,  each  bearing  a  fine  hair- 
like  process  with  long  columnar  supporting  cells  running  between 
them.  They  may  be  arranged  in  small  ridges  or  cristae  acusticee,  or 


ioo  AN    INTRODUCTION  TO  ZOOLOGY 

take  the  form  of  spots  or  maculae.  Each  ampulla  possesses  a  well- 
marked  crista,  while  the  maculae  acusticae  may  occur  in  the  utriculus, 
sacculus  or  lagena,  in  which  last  part  there  is  a  spiral  band  of 
sensory  cells.  Branches  of  the  eighth  or  auditory  nerve  are  dis- 
tributed to  all  these  sensory  areas. 

The  cavity  of  the  middle  ear  or  tympanic  cavity  is  a  hollow  at  the 
outer  side  of  the  auditory  capsule,  separated  from  the  outside  by  the 
tightly  stretched  tympanum,  which  is  supported  by  a  cartilaginous 
ring,  the  annulus  tympanicus.  To  the  tympanum  is  attached  a  bony 
rod  with  cartilaginous  ends,  the  columella  auris,  which  passes  in- 
wards across  the  roof  of  the  tympanic  cavity.  The  otic  capsule  is 
pierced  by  a  small  aperture,  the  fenestra  ovalis,  which  would  put  the 
perilymph  space  in  communication  with  the  cavity  if  it  were  not 
closed  by  a  cartilaginous  disc,  the  stapes,  on  to  which  the  columella 
fits.  Ventrally,  the  ear  cavity  is  in  communication  with  the  back  of 
the  buccal  cavity,  as  we  have  already  seen,  by  means  of  the  Eusta- 
chian  tube.  In  this  way  the  pressure  on  the  two  sides  of  the  tympa- 
num is  kept  the  same,  and  so  instead  of  being  bulged  either  in  or 
out  with  variations  in  pressure  on  the  two  sides  it  is  kept  flat  and 
taut.  When  sound  waves  strike  the  tympanum  they  cause  it  to 
vibrate,  and  this  vibration  is  given  to  the  columella,  which  in  its 
turn  passes  it  on  to  the  stapes,  thence  it  travels  to  the  perilymph 
and  so  finally  to  the  endolymph,  which  provides  the  actual  stimulus 
for  the  sensory  cells. 

The  function  of  the  ear  is  twofold.  The  more  obvious  is,  that  it 
is  the  organ  of  hearing,  the  other  function,  which  although  not  so 
obvious  is  nevertheless  the  more  primitive,  is  equilibration.  The 
ear  of  the  lower  vertebrates  is  mainly  an  organ  concerned  with  the 
balance  of  the  body,  and  in  some  cases  it  is  doubtful  whether  the 
animals  can  hear  at  all,  as  we  understand  the  term.  As  we  pass  up 
to  the  higher  animals  the  sense  of  hearing  becomes  more  and  more 
important,  and  we  lose  sight  of  the  primitive  function  until  something 
occurs  that  throws  it  out  of  gear.  The  semicircular  canals  are 
mainly  concerned  with  equilibration,  while  the  higher  developments 
of  the  sense  of  hearing  are  dependent  on  the  cochlea. 

In  the  nose,  the  eye  and  the  ear  we  have  had  examples  of  three 
sense  organs,  and  in  spite  of  their  obvious  differences  we  can  find 
certain  underlying  features  that  are  common  to  them  all.  Each  of 
them  is  connected  to  the  brain  by  means  of  its  own  special  nerve, 
each  is  lined  by  an  epithelium  containing  more  or  less  highly  modified 
sensory  cells,  and  this  epithelium,  as  we  shall  see  later,  is  derived 
directly  or  indirectly  from  the  ectoderm  of  the  embryo.  These 
constitute  what  are  termed  the  essential  parts  of  the  organ,  but  in 
addition  we  always  find  a  number  of  other  structures,  the  accessory 


THE  FROG  101 

structures,  which  support  and  nourish  and,  although  they  are  not 
absolutely  essential  for  the  organ  to  do  its  work,  enable  it  to  function 
far  more  efficiently.  These  accessory  parts  may  be  derived  from 
the  ectoderm  also,  as  is  the  lens  of  the  eye,  but  more  often  from 
another  and  deeper  lying  set  of  cells,  the  mesoderm. 

This  then  completes  an  outline  of  the  main  structure  and 
functions  of  the  nervous  system  and  the  sense  organs.  In  the  latter 
we  have  highly  specialised  structures  for  the  reception  of  impulses 
or  stimuli  from  the  outside  world,  which  can  straightway  send  on  the 
messages  to  the  brain.  This  wonderful  organ  forms,  in  the  first  place,  a 
great  co-ordinating  centre,  in  which  all  these  messages,  not  only  from 
without  but  also  from  within,  can  be  received,  appreciated,  and 
brought  into  relation  with  one  another.  It  further  forms  a  place 
in  which  messages  can  be  originated  and  sent,  via  the  nerves,  to  any 
part  of  the  body.  Like  any  other  part  of  the  body,  the  nervous 
system  obtains  the  energy  to  carry  on  its  work  from  the  oxidation 
of  its  own  substance,  and  this  consequently  implies  the  formation 
of  waste  substances.  This  wastage  has  to  be  made  good  by  nutri- 
ment, or  the  whole  organisation  would  break  down.  .  Local  reflexes 
play  some  part  in  the  lives  of  all  animals,  but  in  general  in  complex 
animals  like  vertebrates  any  ordinary  stimulus  needs  more  than  just 
a  local  response  ;  it  needs  a  reply  from  the  animal  as  a  whole,  and 
the  co-ordination  of  the  various  actions  necessary  to  produce  this 
result  is  brought  about  by  the  brain.  These  replies  are  perceived 
in  ourselves,  because  they  are  nearly  all  conscious  replies,  but  in  addi- 
tion to  this,  the  brain  is,  in  a  way  not  noticed  by  ourselves,  or,  as  we 
say,  subconsciously,  controlling  the  various  organs  of  the  body,  ac- 
celerating or  retarding  their  action  to  meet  the  varying  needs  of  life. 
It  will  readily  be  understood  that  an  organ  with  such  an  important 
part  as  the  regulator,  controller  and  co-ordinator  of  the  other  organs 
of  the  body,  is  of  sufficient  interest  to  merit  a  branch  of  study  all  to 
itself.  This  study  of  the  function  of  the  brain  with  all  that  it  implies 
— will,  memory,  perception,  consciousness,  and  so  on — is  termed 
Psychology,  and  is,  save  in  a  very  general  and  vague  way,  outside 
the  scope  of  this  work. 

Ductless  Glands. 

The  organs  included  under  the  present  heading  do  not 
compose  a  system,  as  do  those  treated  of  in  the  immediately  preceding 
pages,  for  they  are  isolated  structurally  and  functionally  independ- 
ently, but  they  all  have  the  one  prominent  feature  in  common  that 
has  given  them  their  name.  Like  all  glands,  they  contain  epithelial 
cells,  but  unlike  other  glands,  in  which  the  secretion  is  poured, 
either  on  to  a  free  surface  or  else  into  an  alveolus,  whence  it  is  carried 


102  ;;A¥  INTRODUCTION  TO  ZOOLOGY 

away  by  ducts,  these  structures  do  not  possess  a  free  surface  nor  a 
duct  to  convey  off  the  secretion.  They  are  so  obviously  glandular 
in  structure  that  their  activity  has  been  generally  assumed,  and  as 
they  are  plentifully  supplied  with  blood-vessels,  it  was  also  assumed 
that  their  secretion  must  be  passed  directly  into  the  blood  stream,  and 
in  consequence  it  is  often  termed  an  Internal  Secretion.  Although 
they  have  long  been  known,  it  is  only  comparatively  recently  that 
the  great  importance  of  some  of  these  organs  has  been  realised,  and 
even  now  their  functions  in  some  cases  have  not  been  definitely 
ascertained.  Some  are  concerned  with  keeping  the  blood  in  proper 
condition,  and  these  are  termed  the  lymphoid  "  glands."  Although 
perhaps  it  is  better  not  to  call  these  glands  at  all,  since  they  do  not 
form  a  secretion,  they  are  included  here  for  convenience.  The 
others,  true  glands,  by  means  of  their  internal  secretions,  which 
contain  one  or  more  enzymes,  termed  hormones,  have  a  far-reaching 
effect  on  the  metabolism  of  the  body  as  a  whole,  or  upon  certain 
parts. 

Spleen. 

This  is  a  small  dark  red  almost  spherical  body,  situated  in 
the  mesentery  close  to  the  anterior  end  of  the  rectum.  Inside  its 
peritoneal  covering  is  a  connective  tissue  capsule,  and  within  this 
again  a  framework  of  connective  tissue,  in  which  is  contained  a  large 
number  of  closely  packed  cells  of  various  kinds,  forming  the  splenic 
pulp.  Some  of  the  cells  are  red  corpuscles  and  different  varieties 
of  leucocytes.  It  is  a  lymphoid  gland,  generally  considered  to  be  a 
centre  in  which  leucocytes  and  probably  red  corpuscles  are  produced, 
and  it  also  serves  as  a  station  for  the  destruction  of  worn-out  cor- 
puscles and  the  removal  of  pigment  and  other  waste  matters  from 
the  blood.  Other  small  lymphoid  glands,  probably  with  similar 
functions,  are  to  be  found.  In  ourselves  one  pair  form  the  tonsils. 

Thymus  Gland, 

A  small  reddish  coloured  oval  body  about  2  mm.  in  length  is 
to  be  found  just  behind  the  annulus  tympanicus  and  beneath  the 
depressor  muscle  of  the  mandible,  this  is  the  thymus  gland.  It  is 
derived  from  the  dorsal  side  of  the  gill  clefts  of  the  embryo,  and  histo- 
logically  appears  much  like  a  lymphoid  gland.  In  the  adult  it  forms 
a  centre  for  lymphocyte  formation,  but  in  the  very  young  animal 
it  appears  to  play  a  considerable  part  in  the  processes  of  nutrition. 

Thyroid  Gland. 

This  is  a  small  compact  oval  mass  lying  just  outside  the 
anterior  end  of  the  posterior  cornu  of  the  hyoid  plate.  It  consists 


THE  FROG  103 

of  a  number  of  small  closed  vesicles  filled  with  fluid,  whose  walls 
are  composed  of  a  single  layer  of  cubical  epithelial  cells.  Between 
these  vesicles  is  a  fibrous  connective  tissue  framework  with  a  rich 
vascular  supply.  It  arises  from  the  floor  of  the  pharynx  of  the 
embryo.  The  secretion  of  this  gland  appears  to  be  a  protein  sub- 
stance rich  in  iodine  (lodothyrin) ,  and  it  exerts  an  influence  over  the 
metabolism  of  the  body.  In  man  a  certain  disease  of  this  organ 
leading  to  its  enormous  enlargement  is  known  as  goitre.  Inefficient 
thyroid  functioning  in  the  young  leads  to  a  condition  known  as 
cretinism,  in  which  both  bodily  and  mental  activities  are  severely 
upset,  giving  rise  to  a  state  of  arrested  growth  and  defective  mental 
development.  In  the  adult  a  condition,  known  as  myxoedema,  ensues 
when  the  thyroid  fails  to  perform  its  function — the  hair  falls  out, 
the  skin  becomes  puffy,  and  there  is  considerable  mental  deteriora- 
tion. A  marked  alleviation  of  these  symptoms  occurs  if  the  patient 
is  fed  with  animal  thyroids  or  injected  with  an  extract  of  the  gland. 
Thus  in  some  way  or  other  the  secretion  of  the  thyroid  gland  pro- 
foundly affects  the  metabolism  of  the  body,  presumable  hindering 
katabolism,  and  so  stopping  the  normal  growth  or  activities  of  its 
parts. 

Certain  other  small  glandular  nodules,  termed  the  Accessory 
thyroids  and  Parathyroids,  are  also  present. 

Supra-Renal  or  Adrenal  Bodies. 

The  position  of  these  glands  on  the  ventral  surface  of  the 
kidneys  has  already  been  pointed  out.  In  the  higher  vertebrates  they 
are  solid  bodies  composed  of  two  layers,  an  outer  or  cortical  layer 
and  an  inner  or  medullary  layer,  and  the  cells  composing  these 
layers  are  quite  different.  Both  kinds  of  cells  are  to  be  found  in 
the  frog,  but  they  are  more  or  less  intermingled  and  without  any 
definite  arrangement.  The  cortical  cells  are  arranged  in  groups 
or  columns  of  various  sizes,  and  the  cells  themselves  are  of  an  epithe- 
lial character  with  large  spherical  nuclei.  The  medullary  cells,  on 
the  other  hand,  do  not  appear  in  such  definite  columns,  have  smaller 
nuclei,  are  more  granular,  and  are  of  the  type  termed  chromophil 
cells.  The  internal  secretion  of  the  supra-renal  bodies  has  been 
isolated,  and  from  it  a  white  powder  termed  adrenalin  can  be  obtained. 
When  a  solution  of  this  is  injected  into  the  blood  stream  it  brings 
about  a  contraction  of  the  non-striate  muscles  in  their  walls,  and  so 
causes  a  great  rise  in  the  blood  pressure  and  at  the  same  time  gives 
tone  to  muscles  as  a  whole.  The  secretion  normally  is  considered 
to  maintain  the  tone  of  the  body,  and  one  of  the  most  marked 
symptoms  of  Addison's  disease,  a  malady  of  these  glands,  is  a  loss 
of  this  tone. 


104  AN   INTRODUCTION   TO  ZOOLOGY 

Pituitary  Body. 

The  hypophysis  cerebri  or  pituitary  body  is,  as  we  have 
seen,  a  saccular  structure  situated  at  the  base  of  the  brain  inside  the 
cranium.  It  is  complex  developmentally  arid  histologically,  •  but 
includes  a  glandular  portion  of  typical  epithelial  cells,  whose  internal 
secretion  influences  the  growth  of  the  body.  Disease  of  the  pituitary 
body  leads  to  a  very  characteristic  condition,  known  as  Acromegaly 
or  gigantism — the  hands,  feet,  jaw  and  bony  parts  of  the  body 
become  much  enlarged  and  exaggerated.  The  disease  does  not  at 
first  lead  to  the  destruction  of  the  gland,  but  on  the  contrary  to 
an  increase  in  size  and  activity,  so  that  the  secretion  appears  to 
control  or  augment  growth,  particularly  that  of  bone. 

In  addition  to  the  above  glands  there  are  to  be  found  included  in 
other  organs  of  the  body  patches  of  cells  with  a  glandular  appear- 
ance, which  presumably  produce  hormones.  Beyond  noting  the  fact 
that  in  the  case  of  the  reproductive  organs  the  secretions  are  probably 
responsible  for  the  development  of  some  of  the  secondary  sexual 
characters,  i.e.  characters  which  are  not  concerned  with  the  actual 
production  or  transmission  of  the  sexual  products,  but  nevertheless 
differentiate  the  sexes  one  from  the  other,  it  is  not  intended  to  enter 
into  any  further  details  of  such  glands  here. 

We  have  already  seen  that  the  central  nervous  system 
takes  its  place  as  the  great  controlling  and  guiding  centre  of  the 
animal,  but  its  action  is  not  all  powerful,  and  it  is  naturally  dependent 
on  the  body  of  which  it  forms  a  part.  The  maintenance  of  the  general 
normal  activities  of  the  body  and  of  the  fitness  of  the  whole  depend 
to  no  small  extent  on  hormones  secreted  by  the  ductless  glands. 

Life  History  of  the  Frog. 

Early  in  the  year,  about  the  middle  or  end  of  March,  the 
frogs  awake  from  their  hibernation  and  seek  water,  where  they 
congregate  in  pairs,  and  the  male,  almost  silent  during  the  remainder 
of  the  year,  commences  to  croak  vigorously.  Reproduction  in  the 
frog  is  sexual,  that  is,  it  involves  the  union  of  an  ovum  produced  by 
an  adult  female  and  a  spermatozoon,  the  product  of  an  adult  male. 
The  male  seeks  the  female  and  clasps  her  tightly  round  the  body 
with  his  fore  limbs,  the  callous  pad  on  the  first  finger  having  become 
enlarged  and  roughened  to  allow  of  his  doing  this  easily,  and  they 
remain  together  during  the  egg  laying.  As  the  eggs  pass  from  the 
oviducts  to  the  cloaca  and  so  to  the  outside  the  male  pours  over 
them  the  spermatic  fluid  containing  the  spermatozoa,  and  they  are 
fertilised  outside  the  body  of  the  female.  The  ovum  is  a  single  cell 
with  a  nucleus,  but  is  very  much  enlarged  by  the  inclusion  of  an 


THE  FROG  105 

enormous  amount  of  food  material,  the  yolk,  in  the  form  of  small 
spheres.  It  is  half  black  and  half  white,  the  upper,  dark,  or  animal 
pole  contains  the  nucleus  with  a  certain  amount  of  accompanying 
cytoplasm,  the  white  or  vegetative  pole  a  much  greater  proportion 
of  inert  yolk.  The  egg  when  it  leaves  the  ovary  is  enclosed  in  a 
very  thin  almost  structureless  membrane,  the  vitelline  membrane, 
and  as  it  passes  down  the  oviduct  it  has  added  to  this  a  thicker  but 
still  relatively  thin  coat  of  a  mucilaginous  substance.  The  semen 
or  spermatic  fluid  contains  in  addition  to  albuminous  substances  a 
countless  number  of  microscopical  cells,  the  sperms.  They  are  also 
single  cells,  and  consist  of  a  head  of  denser  material,  containing  the 
nucleus  and  a  fine  thread-like  tail,  by  means  of  which  they  are  able 
to  swim  actively  for  a  short  time.  When  they  come  into  contact 
with  an  ovum  they  start  swimming  vigorously  into  it  until  one 
penetrates  the  vitelline  membrane,  which  then  becomes  impermeable 
to  any  more.  Once  inside  the  membrane  the  nucleus  from  the  head 
enlarges  and  travels  towards  the  nucleus  of  the  ovum,  and  then  the 
two  nuclei  fuse  together,  this  actually  constituting  fertilisation. 
There  is  produced  as  the  result  of  this  process  a  large  cell  with  but 
a  single  nucleus,  formed  by  the  union  of  the  male  and  female  pro- 
nuclei,  as  they  are  termed,  and  this  constitutes  the  fertilised  ovum 
or  oospore,  the  germ  from  which  the  new  being  will  arise.  It  is 
only  after  fertilisation  that  the  egg  can  grow  up  into  a  frog,  so  that 
we  are  here  dealing  with  an  extremely  important  phenomenon,  and 
it  is  also  a  significant  fact  that  the  product  of  fertilisation  is  a  single 
cell  with  a  single  nucleus.  The  importance  of  these  things  leads  to 
their  frequent  discussion  and  the  use  of  a  special  terminology.  When 
we  wish  to  speak  of  either  a  male  or  female  reproductive  organ 
without  discriminating  between  the  sexes  we  use  the  term  gonad, 
and  in  the  same  way  either  ovum  or  sperm  can  be  called  a  gamete. 
The  union  of  two  gametes,  i.e.  fertilisation,  produces  a  zygote. 

The  egg  when  laid  has  a  diameter  of  about  175  mm.  Immedi- 
ately after  impregnation  the  mucous  coat  absorbs  water  and  starts 
to  swell  up  rapidly,  until  it  forms  a  relatively  large  transparent 
sphere,  in  which  lies  the  black  and  white  fertilised  ovum.  Hundreds 
of  eggs  are  laid  at  the  same  time,  and  when  they  all  swell  they  adhere 
together  to  form  the  large  frothy  masses  common  in  our  ponds  and 
ditches  in  early  spring,  and  known  as  frog's  spawn. 

Soon  after  the  above  processes  have  taken  place  a  groove  appears 
on  the  animal  pole  of  the  egg,  and  this  spreads  until  it  has  encircled 
the  whole  ovum.  Examination  of  sections  shows  that  this  groove 
passes  right  through  the  egg,  and  that  the  original  nucleus  has  divided 
.into  two,  one  in  each  half,  so  that  we  now  have  two  cells  instead  of 
one.  No  sooner  is  this  completed  than  another  furrow  at  right  angles 


io  6 


AN   INTRODUCTION   TO  ZOOLOGY 


to  the  first  starts,  and  in  a  similar  way  cuts  the  two  cells  into  four. 
The  next  furrow  is  at  right  angles  to  both  the  preceding,  and  so  is 
equatorial  and  not  longitudinal.  It  is  situated  nearer  the  upper 
pole,  and  consequently  divides  the  egg  into  four  smaller  and  four 
larger  cells.  This  process  of  dividing  up,  termed  segmentation, 
goes  on  fairly  rapidly  but  less  regularly,  and  as  a  result  there  is  pro- 
duced a  mass  of  cells  very  small  at  the  animal  pole  and  larger  at  the 
vegetative  pole.  Sections  show  that  the  ovum  is  now  hollow  inside, 
the  cells  being  arranged  around  an  excentric  space  known  as  the 
segmentation  cavity  or  blastocoel,  and  the  whole  structure  is  termed 
a  blastula.  The  further  details  of  these  changes  will  be  dealt  with 
again  later,  and  only  the  external  alterations  noted  here.  The  black 


® 


FIG.  35. — -The  life-history  of  a  frog. — After  Brehm. 

1-3,  developing  ova  ;  4,  newly  hatched  forms  hanging  to  water-weed?  ;  5,  6,  stages 
with  external  gills;  7-10,  tadpoles  during  emergence  of  limbs;  u,  tadpoles  with 
both  pairs  of  limbs  apparent ;  12,  metamorphosis  to  frog. 

cells  start  to  grow  down  over  the  others,  and  at  about  the  end  of  the 
fourth  day  the  whole  egg  is  black.  At  the  expiration  of  a  week  the 
embryo,  as  we  may  now  term  it,  is  distinctly  oval,  and  three  or  four 
days  later  a  head,  body  and  tail  portions  are  visible.  After  a  fort- 
night the  young  animal  breaks  from  the  gelatinous  envelope  and 
becomes  a  tadpole.  It  is  now  a  free  swimming  comma-like  creature 
with  a  horseshoe-shaped  structure,  the  sucker,  on  the  ventral  surface 
of  the  head,  by  means  of  which  it  adheres  to  weeds  or  other  objects. 
The  food  required  for  its  growth  so  far  has  been  entirely  derived  from 
the  yolk  stored  in  the  egg  to  start  with,  and  it  is  not  until  a  few 
days  after  hatching  that  a  mouth  with  horny  beak-like  jaws  develops, 
and  the  animal  is  capable  of  feeding  for  itself.  As  a  result  of  this 


THE  FROG  107 

feeding  the  alimentary  canal  grows  rapidly,  and  is  always  to  be  seen 
through  the  semi-transparent  abdominal  wall  as  a  coiled  tube. 
Two  pairs  of  comb-shaped  external  gills  grow  out  from  the  side  of 
the  neck  to  be  followed  shortly  afterwards  by  a  third  pair.  A  fold 
of  skin,  termed  the  operculum,  starts  to  grow  out  from  just  in  front  of 
the  gills  and  then  to  grow  back  over  them.  Between,  in  front,  and 
behind  the  external  gills  slits  appear,  which  lead  through  into  the 
pharynx,  and  on  their  walls  the  internal  gills  are  developed  as  a  series 
of  folds,  and  with  their  coming  the  external  gills  gradually  get  smaller. 
The  operculum  grows  backwards  and  its  hinder  edge  fuses  with  the 
body  wall,  until  at  the  end  of  the  fourth  week  it  is  completely  closed, 
save  for  a  spout-like  opening  on  the  left-hand  side,  which  serves  for 
the  egress  of  the  water  used  in  respiration. 

Tiny  little  projections,  the  limb-buds,  appear  at  the  base  of  the 
tail  during  the  sixth  or  seventh  week ;  they  grow  slowly,  until  a 
fortnight  or  so  later  they  are  miniature  limbs  with  joints  and  digits. 
The  front  limbs  develop  at  the  same  time  as  the  hind  ones,  but  they 
are  hidden  by  the  operculum,  and  do  not  appear  externally  until 
they  are  practically  fully  formed.  One  of  them,  the  left,  protrudes 
through  the  spout  and  the  other  actually  bursts  through  the  oper- 
culum itself. 

Somewhere  about  the  twelfth  week  a  marked  and  deep-seated 
change  takes  place  in  the  tadpole,  which  up  to  now  has  been  in  many 
ways  extremely  fish-like  in  its  internal  anatomy.  The  casting  off 
of  the  skin  allows  the  eyes  to  come  to  the  surface,  the  horny  jaws 
disappear,  and  the  mouth  and  tongue  increase  in  size.  Lungs  develop 
internally,  and  for  a  short  time  the  animal  breathes  by  both  gills  and 
lungs,  but  soon  by  the  latter  alone  ;  a  change  that  necessitates  a 
radical  alteration  in  the  circulatory  system.  The  limbs  become 
stronger  and  more  useful,  and  with  the  disappearance  of  the  tail, 
which  has  been  gradually  getting  shorter  and  shorter,  the  animal 
becomes  a  tiny  frog.  This  last  rather  quick  series  of  changes,  whereby 
the  water-dwelling  fish-like  tadpole  becomes  transformed  into  the 
lung-breathing  land-living  frog,  is  termed  the  metamorphosis.  A 
similar  metamorphosis  is  characteristic  of  all  land-dwelling  Amphibia. 
The  remainder  of  the  year  is  passed  in  feeding  till  the  winter,  when 
this  young  animal  hibernates.  It  comes  forth  the  following  spring, 
but  it  does  not  breed  until  a  year  later.  Thus  the  frog  passes  through 
a  series  of  changes  in  a  regular  sequence,  finally  reaching  the  adult 
stage,  i.e.  not  the  stage  of  full  growth,  but  the  stage  at  which  it  is 
able  to  reproduce,  and  this  development  is  termed  the  life-history 
or  life-cycle.  As  long  as  the  young  animal  is  within  the  egg  envelope 
and  entirely  independent  of  the  outer  world  in  the  matter  of  food  we 
term  it  an  embryo.  When  it  hatches,  however,  and  lives  a  perfectly 


108  AN   INTRODUCTION  TO  ZOOLOGY 

free  life,  obtaining  its  food  for  itself,  but  still  has  not  attained 
the  form  of  the  adult,  that  is  while  it  is  still  a  tadpole,  it  is  called  a 
larva.  The  early  history  of  the  frog  and  the  metamorphosis  of  the 
larva  are  of  importance,  since  they  provide  us  with  an  actual  example 
of  an  animal  changing  from  a  gill-breathing  swimming  form  into  a 
land-dwelling  quadruped.  This  is  a  change  that  must  have  occurred 
in  the  past  when  the  fish-like  Craniates  took  the  all-important  step 
of  forsaking  the  water  for  the  land,  and  it  proved  to  be  the  way  to 
the  evolution  of  the  higher  craniate  forms. 

Animals  and  Plants. 

In  the  frog  we  have  seen  and  studied  in  some  detail  a  fairly 
typical  example  of  a  highly  organised  animal,  and  the  examination 
of  its  fundamental  activities  gives  a  good  general  idea  of  the  charac- 
teristics of  the  vital  phenomena  of  an  animal.  Indeed,  more  than 
this,  the  underlying  principles  are  not  merely  the  property  of  animal 
life,  but  they  are  exhibited  by  all  living  beings,  animal  and  plant. 
Let  us  consider  briefly  these  points,  which  fall  into  four  groups. 

Firstly,  all  the  living  parts  of  an  organism  are  composed  of  a 
very  complex  substance,  Protoplasm,  which  is  a  complicated 
mixture  of  compounds  termed  proteins,  themselves  complex, 
together  with  carbohydrates,  fats,  inorganic  salts  and  water.  Even 
the  lifeless  products  present,  such  as  the  hair,  scales  or  feathers  of 
animals,  or  the  bark  of  trees,  are  the  results  of  the  activities  of  this 
wonderful  substance  protoplasm.  It  is,  in  the  higher  animals  and 
plants  alike,  split  up  into  units,  the  cells,  consisting  of  a  proto- 
plasmic body  controlled  by  a  denser  nucleus  and  modified  in  almost 
every  conceivable  way  to  perform  different  functions.  One  of  the 
main  characteristics  of  protoplasm  is  its  inherent  power  of  con- 
tractility or  motility,  which  enables  it  to  alter  its  shape  and  move. 
This  is  probably  brought  about  by  a  molecular  rearrangement 
accompanied  by  certain  chemical  changes,  and  although  it  is  more 
obvious  in  animals  is  nevertheless  to  be  found  in  plants,  and  is  a 
fundamental  property  of  all  living  matter.  It  is  to  be  noted  that 
the  alteration  is  in  shape  and  not  in  volume,  for  the  protoplasm 
increases  in  one  direction  and  decreases  in  another. 

Secondly,  all  living  beings  exhibit  metabolism,  and  all  that  that 
implies.  They  have  the  power  of  taking  in  dead  or  lifeless  matter 
and  building  it  up  into  protoplasm  by  means  of  a  series  of  changes, 
digestion,  absorption  and  assimilation,  all  implied  in  the  term 
nutrition.  The  energy  required  for  carrying  out  this  work  is  obtained 
from  the  breaking  down  of  the  protoplasm  itself,  a  process  that  is 
dependent  upon  respiration,  the  process  by  which  oxygen  is  supplied 
to  and  utilised  by  the  tissues.  These  processes  result  in  the  formation 


THE  FROG  109 

of  various  waste  products,  which  have  to  be  removed  from  the 
body  by  excretion.  As  has  been  pointed  out  previously,  while  the 
building  up  processes  are  in  the  ascendant  the  organism  increases  in 
size,  i.e.  it  exhibits  the  power  of  growth.  Again,  all  living  beings 
obtain  energy  from  the  oxidation  of  their  tissues,  and  this  energy 
appears  in  the  form  of  heat  motion  and  electrical  changes.  These 
particular  alterations  are  more  striking  in  animals  than  in  plants, 
but,  nevertheless,  are  common  to  all  living  beings,  which  are  there- 
fore capable  of  bringing  about  a  transformation  of  energy,  a  point 
with  which  we  shall  deal  more  fully  later. 

Thirdly,  plants  and  animals  have  the  power  of  replying  to 
messages  from  their  environment.  This  response  to  external 
stimuli  is  not,  as  a  rule,  vague  or  local,  but  definite  and  general,  and 
we  say,  therefore,  that  they  possess  irritability  or  sensitivity. 

Lastly,  we  have  just  seen,  in  the  case  of  the  frog  in  particular, 
that  all  living  beings  pass  through  a  definite  life-cycle.  They 
commence  as  fertilised  ova,  or  as  an  actual  part  of  the  parent,  and 
go  through  a  certain  definite  pre-ordained  series  of  changes,  as  the 
result  of  which  they  become  adult.  When  adult  they  possess  the 
power  of  carrying  on  the  race  by  reproducing  their  like,  and,  after 
a  certain  period  in  the  possession  of  this  power,  long  or  short,  as 
the  case  may  be,  they  pass  on  into  old  age  or  senescence.  They 
finally  die,  for  death  is  also  an  integral  part  of  the  life-cycle. 

Thus  far  we  have  been  considering  the  properties  common  to 
living  beings,  and  although  we  are  here  concerned  with  Zoology,  it 
will  not  be  out  of  place  to  consider  briefly  the  main  differences 
between  animals  and  plants.  The  differences  between  the  higher 
animals,  such  as  the  frog,  dog,  or  cat,  and  the  higher  plants,  such  as 
shrubs  and  trees,  are  so  great  that  they  are  apparent  to  the  most 
superficial  observer,  and  need  not  be  dealt  with  here.  In  the  case 
of  the  lowest  members  of  both  kingdoms,  simple  animals  and  simple 
plants  composed  in  many  cases  of  but  a  single  cell,  the  differences 
become  less  and  less  obvious.  Indeed,  we  actually  find  a  number 
of  organisms,  including,  for  example,  the  slime  fungi,  found  growing 
in  tan  pits  and  on  decaying  wood,  are  claimed  by  both  zoologist  and 
botanist.  It  has  been  proposed  to  give  the  name  Protista  to  this 
group  of  beings,  which  are  a  mixture  of  animal  and  plant,  or  live 
as  an  animal  or  plant  according  to  the  conditions  of  their  environ- 
ment. It  is  probable  that  ah*  living  beings  have  descended  from 
some  such  primitive  organism  possessing  the  potentiality  of  becoming 
either  animal  or  plant,  and  all  exhibiting  to  some  degree  the  various 
vital  powers  we  have  just  discussed. 

The  most  fundamental  difference  between  animals  and  plants 
is  in  the  chemical  constitution  of  the  protoplasm  which,  small  though 


no  AN   INTRODUCTION  TO  ZOOLOGY 

it  is,  manifests  itself  in  a  difference  of  the  chemical  activities  con- 
cerned with  nutrition.  The  typical  plants  are  able  to  utilise  as  food 
Carbon  dioxide,  Ammonium  compounds  and  Nitrates,  all  of  which 
are  generally  diffused  over  the  surface  of  the  globe  in  the  soil  or 
carried  about  by  the  air  and  the  water.  From  these  simple  sub- 
stances, with  the  aid  of  radiant  energy  trapped  from  the  sunlight, 
the  plant  is  able  to  build  up  or  synthesise  its  protoplasm,  a  method 
of  feeding  termed  holophytic,  or  completely  plant-like.  The  typical 
animal,  on  the  other  hand,  is  unable  to  do  this,  and  must  have  a 
fairly  large  proportion  of  proteins  in  its  food.  Such  substances  are 
not  found  distributed  generally,  but  only  in  other  living  beings,  so 
that  the  food  of  an  animal  must  be  living  or  dead  organisms ;  a  mode 
of  feeding  termed  holozoic,  or  completely  animal-like.  The  plant 
can  be,  and  usually  is,  quite  stationary,  and  is  generally  modified  in 
such  a  way  as  to  expose  the  greatest  amount  of  surface  to  the  air 
and  the  water  in  the  soil,  and  at  the  same  time  attain  stability.  To 
do  this  the  plant  has  developed  a  spreading  root  to  pick  up  the 
nitrogenous  substances,  and  a  trunk,  by  means  of  which  a  number  of 
flat  leaves  are  exposed  to  the  air,  from  which  they  obtain  Carbon 
dioxide.  The  stability  is  attained  in  some  measure  by  the  spreading 
of  the  root,  which  acts  as  an  anchor,  but  largely  by  the  development 
of  a  strengthening  substance,  usually  Cellulose,  or  a  closely  allied 
carbohydrate  compound.  Therein  lies  one  of  the  most  striking 
differences  between  plant  and  animal  histology,  in  the'' plant  the  cell 
is  practically  always  enclosed  in  a  moderately  thick  cellulose  wall, 
while  in  the  animal  the  cell  generally  has  no  cell  wall,  and  cellulose 
and  similar  substances  are  absent  from  the  majority  of  animals. 
As  the  food  substances  of  the  plant  are  already  in  a  drffusable  form, 
there  is  no  need  for  an  elaborate  digestive  mechanism,  but  it  can 
at  once  start  to  build  them  up  into  proteins  in  most  cases  by  the  aid 
of  a  green  colouring  matter,  chlorophyll,  that  is  not  found  in  typical 
animals. 

The  animal,  on  the  other  hand,  has  to  search  for  food,  and  so  is 
modified  for  motility,  and,  as  a  rule,  possesses  locomotor  organs  and 
a  body  adapted  for  movement.  The  food,  too,  generally  consists 
of  solid  particles,  and  not  liquids  or  gases,  and  so  a  temporary  or 
permanent  aperture  is  present  in  the  form  of  a  mouth  or  its  equiva- 
lent. Before  the  food  can  be  in  jested  it  is  frequently  necessary  for 
it  to  be  caught,  killed  or  broken  up,  and  so  we  find  a  whole  series  of 
mechanisms,  claws,  teeth,  etc.,  to  serve  these  ends.  Even  when 
the  food  is  acquired  it  is  not  in  a  form  in  which  it  can  be  assimilated, 
and  so  a  simple  or  complex  digestive  system  is  provided  to  make  it 
available  for  the  body.  With  the  food  a  certain  amount  of  in- 
digestible matter,  supporting  structures,  etc.,  is  taken  in,  and  has 


THE  FROG  in 

to  be  passed  out  again  through  a  temporary  or  permanent  aperture 
for  egestion,  the  anus.  To  co-ordinate  these  various  parts  a  central 
nervous  system  is  developed,  and  in  connection  with  the  movements 
a  series  of  sense  organs,  which  put  the  animal  into  touch  with  the 
outside  world.  Practically  all  the  striking  characters  of  an  animal 
are  concerned  with  the  question  of  food,  and  its  whole  structure  is  a 
complex  combination  of  parts,  enabling  it  to  obtain  its  food  readily. 
In  many  cases  this  main  object  is  modified,  and  sometimes  to  a  large 
extent  by  another  aim,  and  that  is  to  enable  the  animal  itself  to 
escape  being  utilised  as  food  by  another  animal.  All  these  structural 
modifications  that  are  for  some  useful  purpose  we  term  adaptations. 
One  further  striking  difference  between  animals  and  plants 
remains  to  be  noticed,  and  that  is,  their  gaseous  exchanges  with  the 
atmosphere.  In  the  case  of  all  organisms,  save  certain  lowly  plants, 
respiration,  i.e.  the  exchange  of  carbon  dioxide  for  the  oxygen  of 
the  air  occurs,  and  it  is  more  rapid  in  animals  than  in  plants.  Those 
plants  that  possess  chlorophyll  or  an  allied  substance,  in  the  presence 
of  sunlight,  take  in  from  the  air  carbon  dioxide,  from  which  they 
remove  the  carbon,  setting  free  the  oxygen  into  the  air  again.  This 
second  exchange,  which  does  not  occur  in  animals,  is  by  far  the 
larger  of  the  two  in  green  plants,  and  almost  completely  masks  the 
other. 

All  the  vital  manifestations  of  both  animals  and  plants  are  in 
reality  manifestations  of  energy,  and  so  it  will  be  well  to  return 
to  the  second  phenomenon  characteristic  of  living  beings,  and 
examine  quite  generally  the  way  in  which  energy  is  obtained,  stored 
and  transformed  by  organisms.  Two  kinds  of  energy  are  dis- 
tinguishable :  one  is  kinetic  energy,  that  is,  energy  that  is  mani- 
fested in  the  form  of  motion,  heat,  light  or  chemical  or  electrical 
changes  ;  the  other,  termed  potential  energy,  is  energy  that  is 
stored  up  in  a  quiescent  condition,  only  needing  some  stimulus  to 
release  it  and  allow  it  to  become  transformed  into  kinetic  energy. 
Potential  energy  in  the  living  being  is  stored  up  in  a  series  of  fairly 
complex  chemical  compounds.  Such  energy  is  stored  as  the  result 
of  a  complicated  sequence  of  chemical  reactions,  which  lead  to  the 
formation  of  compounds  of  higher  and  higher  chemical  complexity, 
until  we  reach  that  highly  organised  substance,  or  intimate  mixture 
of  substances,  which  we  term  protoplasm.  We  use  the  term 
Anabolism  to  include  all  these  constructive  changes  culminating  in 
the  building  up  of  protoplasm,  and  this  represents,  as  it  were,  the 
credit  side  of  the  account,  the  storage  of  energy.  On  the  other  side 
we  have  the  debit  account,  the  expenditure  of  this  reserve  in  the 
form  of  kinetic  energy  brought  about  by  the  breaking  down  of  the 


112 


AN   INTRODUCTION   TO  ZOOLOGY 


complex  chemical  substances  within  the  organism,  and  to  this  whole 
destructive  series  of  changes  we  apply  the  term  Katabolism.  If  we 
wish  to  include  both  sets  of  reactions,  that  is,  energy-storing  and 
energy-releasing,  we  apply  the  word  Metabolism. 


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The  first  and  simplest  stages  of  anabolism  are  encountered  only 
in  the  green  plant,  which  is  able,  as  pointed  out  above,  to  utilise 
the  radiant  energy  of  sunlight  in  such  a  way  that  from  the  relatively 
simple  inorganic  salts  in  the  soil,  or  in  the  air,  it  can  build  up  com- 


THE   FROG  113 

pounds  known  as  carbohydrates  that  are  more  or  less  chemically 
complex,  and  which  we  term  organic.  This  power  is  termed  photo- 
synthesis, since  the  building  up  is  dependent  on  light.  Further 
changes  then  occur,  probably  involving  the  utilisation  of  energy 
obtained  by  the  oxidation  of  previously  manufactured  organic 
compounds,  and  these  result  in  the  final  production  of  substances 
termed  proteins,  from  which  protoplasm  itself  is  built,  and  they  are 
among  the  most  complex  bodies  known.  The  typical  animal,  as 
we  have  seen,  requires  for  food  organic  matter  in  the  form  of  carbo- 
hydrates or  proteins.  These  it  breaks  up  by  digestion  into  some- 
what simpler  compounds,  by  means  of  the  chemically  active 
enzymes  ;  but  afterwards,  by  means  of  energy  obtained  by  oxida- 
tion, it  recombines  them  into  proteins  and  protoplasm.  The  building 
up  processes  in  both  animals  and  plants,  involving  the  manipulation 
of  organic  compounds  and  their  actual  incorporation  into  the  living 
protoplasm,  are  distinguished  as  assimilation. 

The  constitution  of  living  protoplasm  is  not  known  with  any 
certainty,  since  as  soon  as  we  start  to  analyse  it  we  must  kill  it,  and 
then  we  have  only  a  mixture  of  proteins,  etc.  It  may  possibly 
always  be  a  system  of  such  unstable  compounds  interacting  one  upon 
another,  and  is  certainly  in  a  state  of  continual  molecular  change, 
which  forms  the  chemical  basis  of  the  vital  phenomena,  for  when  they 
stop  life  also  ceases. 

The  katabolic  or  energy-release  changes  are  probably  brought 
about  in  the  main  by  the  action  of  enzymes,  and  result  in  the  break- 
ing down  of  either  the  carbohydrate  food  reserves  or  the  proteins, 
with  the  consequent  production  of  waste  substances,  and  these 
reactions  are  ultimately  dependent  upon  the  oxygen  obtained  by 
respiration.  The  dissimilation  of  the  proteins  takes  place  in  succes- 
sive stages,  which  we  may  for  convenience  distinguish  as  the  forma- 
tion of  the  first  katastates  (i.e.  peptones,  albumoses,  amino- acids, 
etc.),  and  the  production  of  the  intermediate  bodies  like  urea,  etc.. 
still  moderately  complex  chemically  and  still  termed  organic. 
These  occur  in  both  animals  and  plants.  Just  as  we  find  plants 
at  the  beginning  of  the  energy-building  series,  so  they  come  in  again 
at  the  end,  and,  as  Bacteria,  etc.,  are  mainly  responsible  for  the 
disintegration  of  the  intermediate  bodies  to  form  simple  inorganic 
end-products  (e.g.  CO2,  NH3,  P2O6,  H2S,  CH4,  etc.)  ready  to  be  distri- 
buted again  in  the  water  and  air,  and  serve  as  food  for  a  new 
generation  of  green  plants. 

This  wonderful  cycle  of  metabolic  changes  is  infinitely  more 
complex  than  has  been  outlined  above,  and  in  spite  of  the  vast 
amount  of  work  that  has  been  done  upon  it,  many  of  its  details  still 
remain  to  be  discovered.  The  general  idea,  however,  that  underlies 
it  all  may  be  expressed  somewhat  crudely  in  a  diagramatic  manner. 

I 


ii4  AN   INTRODUCTION  TO  ZOOLOGY 

Classification. 

The  frog,  whose  anatomy,  histology,  physiology  and,  to  a  lesser 
extent,  development  we  have  now  studied,  will  also  serve  to  introduce 
another  important  branch  of  Biology  which  follows  after  these, 
namely,  Taxonomy  or  Classification.  When  we  have  learned  what 
we  can  of  a  new  animal  it  is  necessary  to  give  it  a  name,  and  to 
ascertain  its  position  with  regard  to  other  animals.  The  first  object, 
the  naming,  follows  from  the  second,  and  cannot  be  done  arbitrarily, 
for  the  object  of  a  natural,  as  opposed  to  an  artificial,  classification 
is  to  arrange  the  animals  in  groups  that  show  their  relationships 
with  one  another,  and  then  the  name  given  is  one  that  will  indicate 
the  position  of  the  animal  in  these  groups.  The  theory  of  descent 
is  the  great  key  to  this  problem  of  classification,  for  it  teaches  that 
structural  similarity  or  homology  indicates  a  community  of  descent, 
and  therefore  a  degree  of  affinity,  and  we  utilise  this  theory  to 
bring  animals  together  in  such  a  way  as  to  make  clear  their  relation- 
ships. It  is  obviously  necessary,  in  order  to  carry  out  such  a  scheme, 
to  have  some  recognisable  unit  to  form  its  basis,  and  so  we  associate 
together  all  animals  that  possess  certain  fairly  definite  and  constant 
characters  of  structure  and  form,  and  this  assemblage  we  call  a 
species.  This  term  is  hard  to  define  accurately,  for  it  has  no 
absolute  criteria  or  definite  limits,  but,  as  ordinarily  used,  implying 
a  group  of  beings  resembling  one  another,  closely  and  constantly, 
and  easily  recognisable,  as  belonging  to  one  type,  it  is  a  very  useful 
concept,  in  spite  of  its  vagueness.  There  is  a  certain  amount  of 
variation  within  the  limits  of  each  species,  and  each  animal  has  its 
own  individual  peculiarities,  but  they  are  not  so  great  usually  as  to 
obscure  the  type  to  which  the  individual  belongs.  The  constancy 
that  has  been  referred  to  is  only  a  relative  matter,  for  although, 
as  far  as  human  remembrance  or  records  go,  the  species  of  animals 
now  on  the  globe  are  constant,  it  will  be  seen  that,  when  we  bear  in 
mind  the  phenomenon  of  evolution,  the  constancy  is  more  apparent 
than  real,  for  they  must  all  have  arisen  from  pre-existing  different 
types.  The  rate  of  change,  however,  is  so  slow  that  this  alteration 
is  lost  sight  of.  The  actual  differences  separating  one  species  from 
another  are  often  small,  merely  differences  of  colour  or  size  and 
proportions,  as,  for  example,  those  between  the  lion  and  the  tiger, 
and  yet  members  of  one  species,  as  a  rule,  cannot,  or  do  not,  breed 
with  those  of  another. 

When  a  number  of  species  resemble  one  another,  closely  possess- 
ing certain  characters  in  common,  we  associate  them  together  in  a 
larger  group  that  is  termed  a  Genus.  A  species  then  is  a  number  of 
individuals  possessing  in  common  certain  specific  characters,  while 
genera  are  groups  of  one  or  more  species  with  common  generic 
characters.  It  sometimes  happens  that  a  species  is  so  distinct  from 


THE  FROG  115 

other  species  that  none  other  can  be  regarded  as  closely  allied  to  it, 
and  in  such  cases  the  genus  is  composed  of  that  species  alone.  From 
these  two  categories  the  name  of  the  animal  is  derived,  and  a  double 
name  results,  so  that  this  method  of  naming  is  known  as  the  system 
of  binomial  nomenclature.  This  system  was  introduced  by  the 
Swedish  naturalist,  Linne,  or  Linnaeus,  in  the  eighteenth  century. 
It  was  a  great  advance  on  the  previous  haphazard  way  of  naming, 
and  it  is  so  useful  that  it  is  now  universally  adopted,  and,  indeed, 
it  is  hard  to  see  how  the  enormous  number  of  species  we  now  know 
could  have  been  coped  with  but  for  its  use.  The  two  names  are 
usually  of  Greek  or  Latin  derivation,  sometimes  curious  mixtures  of 
both,  and  in  some  measure  correspond  to  a  Christian  and  a  surname, 
save  that  in  the  animal  the  order  is  reversed,  and  the  generic  precedes 
the  specific  name. 

The  name  of  the  common  English  frog  is,  as  we  have  seen,  Rana 
temporaria,  indicating  in  the  first  place  that  it  belongs  to  the  genus 
Rana.  As  pointed  out  in  the  opening  chapter,  another  frog,  Rana 
esculenta,  is  common  on  the  continent,  and  yet  another  frog,  Rana 
tigrina,  is  common  in  Northern  India.  If  there  is  no  doubt  as  to 
the  animal  we  intend,  it  is  often  customary  to  reduce  the  generic 
name  to  its  initial  letter,  thus  our  frog  becomes  R.  temporaria. 
These  frogs  just  mentioned  are  alike  in  their  essential  features,  and 
so  we  find  them,  with  a  number  of  others  from  various  parts  of  the 
world,  included  in  the  one  genus  Rana.  They  differ  much  among 
themselves  in  colour,  markings  and  size :  the  English  frog  is  the 
smallest,  the  edible  frog  a  little  larger,  and  the  Indian  frog  con- 
siderably larger.  It  is,  therefore,  necessary  to  separate  them  from 
one  another  into  separate  species,  each  with  its  own  specific  name. 

This  is  a  step  on  the  way  to  classification,  but  if  we  stopped  here 
we  should  still  have  a  large  number  of  genera  closely  related,  or 
widely  separated,  from  one  another,  but  not  in  any  sort  of  order. 
To  obviate  this  we  associate  allied  genera  together  into  Families  ; 
these  into  larger  groups  termed  Sub-classes  or  Orders  ;  these  into 
still  larger  ones  termed  Classes.  The  classes  that  show  a  certain 
common  fundamental  unity  of  structure,  and,  therefore,  descent 
from  a  common,  if  remote,  ancestor,  together  form  a  Phylum,  one 
of  the  major  divisions  of  the  animal  kingdom. 

To  take  the  case  of  the  frog : — 

Phylum    "...       ...  ..  ..     Chordata. 

Class           ...        . , :\:  . .  . .     Amphibia. 

Sub-class  or  Order  . .  . .     Anura. 

Family        . .         . .  . .  . .     Ranidae. 

Genus         . .         . .  . .  ..     Rana. 

Species        . .         . .  '. .  .V*  Temporaria. 


n6  AN   INTRODUCTION   TO  ZOOLOGY 

The  important  thing  to  bear  in  mind  is  that  too  much  weight 
must  not  be  given  to  the  classification  of  any  particular  animal,  as 
it  merely  represents  the  present  state  of  our  knowledge,  and  if  fresh 
facts  come  to  light  might  have  to  be  revised.  The  animal  itself  is 
the  thing  with  a  real  existence,  and  the  various  groups,  species, 
genera,  etc.,  are  merely  convenient  abstractions  that  enable  the 
zoologist  to  deal  with  and  arrange  an  enormous  mass  of  material. 

Let  us  now  consider  the  way  in  which  the  animal  kingdom 
is  classified.  It  has  been  noted  above  that  the  most  primitive 
animals  consist  of  one  cell,  and  these  simple  forms  are  grouped 
together  in  the  Phylum  Protozoa,  the  unicellular  animals,  and 
separated  from  all  the  remaining  forms,  which  are  termed  the 
Metazoa  or  multicellular  animals.  The  Protozoa  are  not,  strictly 
speaking,  unicellular,  for  although  the  majority  of  them  consist  of 
only  one  cell,  a  large  number  are  composed  of  many  cells  joined 
together.  In  this  case,  however,  the  cells  are  all  more  or  less  similar 
and  co-equal,  and  they  stand  in  marked  contrast  to  the  cells  of  the 
Metazoa,  which  are  always  differentiated  to  form  tissues.  The  two 
groups  may  be  distinguished  as  cell  animals  and  tissue  animals. 

The  lowest  members  of  the  Metazoa  are  animals  composed  of  two 
layers  of  cells  :  an  outer  layer,  the  ectoderm,  forming  the  outside 
covering  of  the  animal ;  and  an  inner  layer,  the  entoderm,  which 
forms  the  lining  of  the  one  internal  cavity  of  the  gut  or  enteron. 
They  are  classified  together  in  the  Phylum  Ccelenterata.  The  remain- 
ing Metazoa  have  not  only  the  two  layers,  ectoderm  and  entoderm, 
but  a  third  layer  of  cells,  the  mesoderm,  is  also  present  between  them. 
The  most  typical  forms  have  not  only  an  enteron,  but  also  another 
cavity  situated  in,  and  surrounded  by,  the  mesoderm,  termed  the 
coelom,  and  hence  they  are  distinguished  as  the  Coelomata. 

This  division,  the  Coelomata,  is  in  itself  subdivided  into  two  groups : 
the  Invertebrata  and  Vertebrata,  the  former  composed  of  a  number 
of  separate  Phyla.  The  term  Vertebrata,  of  course,  actually  implies 
that  the  animal  'possesses  a  dorsal  column  of  separate  vertebra, 
like  the  frog,  and  this  was  how  the  term  was  originally  employed. 
It  was  subsequently  found  that  this  segmented  vertebral  column  is 
always  preceded  in  development  by  a  long  unsegmented  supporting 
rod  of  peculiar  structure,  called  the  Notochord  or  Chorda  dorsalis. 
Moreover,  some  of  the  lower  members  of  the  group,  i.e.  Amphioxus, 
and  some  of  the  primitive  fishes  retain  a  notochord  throughout  their 
entire  life,  and  never  reach  the  stage  of  development  in  which  it  is 
replaced  by  segmented  vertebrae.  The  important  distinction 
between  the  two  groups  is  then  the  presence  of  a  notochord,  and  so, 
strictly  speaking,  the  term  Chordata  should  be  given  to  the  group 
and  Vertebrata  reserved  for  those  members  of  it  with  the  spinal 


THE  FROG 


117 


column  composed  of  vertebrae.  Although  this  is  sometimes  done, 
those  animals  that  never  have  a  notochord  are  almost  invariably 
classed  together  as  the  Invertebrata,  and  hence,  for  contrast,  some 
authors  retain  the  term  Vertebrata  for  the  remaining  group,  and 
include  in  it  all  the  Chordata.  In  the  widest  use  of  the  cerm 
Invertebrata  it  can,  of  course,  include  Protozoa  and  Ccelenterata, 
but  it  is  convenient  to  restrict  it  to  invertebrate  ccelomates.  These 
two  groups  differ  greatly  in  relative  value,  for  the  Phylum  Chordata 
forms  what  may  be  termed  a  natural  group,  since  all  its  members 
are  related  to  one  another  by  community  of  descent,  and  all  save 
certain  degenerate  forms  conform  to  one  fundamental  plan.  The 
Invertebrata,  on  the  other  hand,  are  simply  all  the  Ccelomates  that 
are  not  Chordates.  It  includes  various  Phyla,  each  a  unit  in  itself, 
but  not  closely  or  obviously  related  to  one  another,  and  following 
different  plans  of  organisation.  The  relations  of  the  main  divisions 
of  the  animal  kingdom,  omitting  certain  small  groups,  can  be 
expressed  schematically  as  follows  : — 

/PROTOZOA.  PHYLA. 

Protozoa.  Amoeba,  Par amoecium, 

Monocystis,  Malaria,  etc. 

CCELENTERATA.  Ccelenterata.  Hydra,  Obelia,  etc. 

ANIMAL      /  /  Platyhelminthes.    Tape  -  worms, 

KINGDOM.  1  Liver-flukes,  Flat-worms,  etc. 

Nemertea.  Ribbon-worms. 
I  Nemathelminthes.         Round - 

worms. 

Annulata.  Lumbricus,  other  seg- 
,  INVERTE-    I       mented  worms. 
BRATA.       (  Mollusca.  Shell-fish,  Octopus,  etc. 
Echinodermata.    Star-fish,    Sea- 
urchins,        Sea-lilies,         Sea- 
cucumbers,  etc. 

Afthropoda.  Lobsters.  Crabs, 
Spiders,  Centipedes,  Insects, 
etc. 

\CHORDATA  Chordata.  Amphioxus,  Sea- 
(VERTE-  squirts,  Scyltium,  Rana,  Rep- 
BRATA).  tiles,  Birds,  Lepus,  Canis,  etc. 

In  the  following  chapters  we  shall  deal  with  representative 
forms  from  some  of  these  Phyla ;  their  position  in  the  above  table 
is  indicated  by  the  names  in  italics. 


METAZOA. 


CCELO- 


CHAPTER  V 

THE   PROTOZOA 
•  t  {^^ 

Free  living  Protozoa,  Amoeba  and  Paramaecium — Parasitic  Protozoa, 
Monocystis  and  Plasmodium. 

WE  have  already  seen  that  the  Protozoa  comprise  the 
simplest  animal  forms.  In  their  adult  stage  they  are  composed  of  a 
single  cell  with  one  or  more  nuclei ;  or,  if  of  more  than  one  cell, 
then  they  form  an  aggregation  of  equivalent  cells.  They  are  for  the 
most  part  very  small,  invisible  to  the  naked  eye  and  requiring  a 
microscope  to  make  them  out  ;  but  in  spite  of  their  small  size  and 
simplicity  each  one  is  a  complete  living  unit.  Each  possesses  all  the 
characteristics  of  living  matter,  and  each  is  capable  of  exhibiting  all 
the  vital  phenomena.  A  certain  amount  of  specialisation  within 
the  limits  of  the  cell  is  nearly  always  met  with,  and  sometimes  quite 
a  number  of  cell  organs  are  produced  each  serving  some  special 
function.  They  are  widely  spread  over  the  whole  surface  of  the 
globe  from  the  arctic  regions  to  the  tropics,  in  fresh  and  salt  water, 
as  well  as  in  or  on  the  soil,  but,  as  a  rule,  a  fair  amount  of  moisture 
is  necessary  for  them  to  keep  alive.  A  very  large  number  of  them 
live  on  and  in  other  animals,  and  they  bring  about  a  diseased  con- 
dition in  the  animal  in  which  they  live.  Those  producing  diseases 
in  man  are  naturally  of  much  importance  from  the  medical  point  of 
view.  For  convenience  of  study  we  may  divide  the  Protozoa  with 
which  we  are  immediately  concerned  into  free-living  and  parasitic 
forms,  although  this  is  by  no  means  a  natural  classification.  To  start 
off  with  we  will  take  Amceba,  which  is  a  lowly  and  little  specialised 
form. 

Free-Living  Protozoa — i.  Amoeba. 

The  genus  Amceba  contains  a  number  of  species,  some  even 
parasitic,  and  one  of  the  largest  and  most  common  is  A .  proteus,  the 
"  proteus  animalcule  "  as  it  was  termed  by  the  older  naturalists  on 
account  of  one  of  its  most  striking  characteristics,  that  of  constantly 
altering  its  shape.  It  is  found  on  the  surface  of  the  mud  and 
decaying  vegetable  matter  in  ponds  and  ditches.  The  animal  is 

if* 


THE   PROTOZOA 


119 


very  tiny,  one  of  large  size  reaching  only  about  0*25  mm.,  and  so 
it  needs  a  lens  to  see  it  at  all. 

Under  the  microscope  Amoeba  appears  as  a  mass  of  fairly  trans- 
parent granular  jelly  continually  changing  in  shape  by  the  pro- 
trusion of  blunt  processes,  the  pseudopodia,  as  they  are  called. 
In  many  ways  it  recalls  the  leucocyte  of  the  frog's  blood,  but  it  is 
much  larger  and  more  active.  Its  protoplasm,  which  is  not  pro- 
tected by  any  form  of  skeletal  structure,  is  clearly  marked  off  into  an 
outer  and  an  inner  portion.  The  outer  layer,  the  ectoplasm,  is  thin 
and  transparent,  and  although  soft,  is  firmer  than  the  underlying 
substance,  so  forming  a  protective  covering.  Inside  this,  and  form- 
ing the  bulk  of  the  cell,  is  the  endoplasm,  of  a  more  fluid  consistency 


FIG.   37. — •Different  forms  assumed  by  Amoeba  proteus.     Photographs 
from  preparations. — From  Calkin. 

and  less  transparent,  owing  to  the  presence  in  it  of  a  number  of 
refractive  granules.  All  the  vital  organs  of  the  cell  lie  in  it,  and  it 
flows  about  freely  within  the  ectoplasm  as  if  confined  in  a  sort  of 
bag.  Somewhere  near  the  middle  of  the  cell  is  the  nucleus,  a 
spherical  structure  not  readily  seen  until  the  animal  is  killed  and 
stained.  The  nucleus  is  clearly  delimited  from  the  remaining 
endoplasm  by  a  very  thin  homogeneous  membrane,  the  nuclear 
membrane.  The  protoplasm  within  the  membrane  is  practically 
indistinguishable  from  that  outside,  but  contains  a  large  number  of 
minute  granules  of  an  important  substance  termed  chromatin, 
because  of  the  readiness  with  which  it  takes  up  certain  basic  stains. 
This  chromatin,  which  is  in  Amoeba  fairly  evenly  distributed,  thus 


120  AN   INTRODUCTION  TO  ZOOLOGY 

producing  a  stippled  appearance  in  the  stained  nucleus,  is  a  constant 
and  essential  constituent  of  all  nuclei  in  plants  and  animals  alike. 
The  endoplasm  also  contains  certain  spaces  with  a  more  or  less  fluid 
content  termed  the  vacuoles.  When  the  food  is  taken  in  it  imme- 
diately becomes  surrounded  by  fluid,  thus  constituting  a  food 
vacuole  in  which  the  food  is  carried  freely  round  in  the  endoplasm 
while  it  gradually  becomes  digested.  Certain  other  of  these  spaces 
appear  to  contain  nothing  but  a  watery  fluid,  and  so  are  called  the 
water  vacuoles.  Finally,  we  have  a  vacuole  that  is  concerned  with 
the  elimination  of  the  liquid  waste  of  the  body.  This  may  appear 
quite  small,  but  as  we  watch  it,  it  gradually  gets  larger  and  larger, 
a  process  termed  diastole,  until  a  limit  is  reached.  At  this  point  it 
suddenly  expels  its  contents  to  the  outside  by  a  sharp  contraction 
known  as  systole,  and  disappears  temporarily  to  reappear  later  and 
repeat  the  process.  From  this  constant  and  fairly  regular  diastole 
and  systole  it  is  known  as  the  contractile  vacuole,  and  it  discharges  its 
contents  through  a  temporary  break  in  the  ectoplasm. 

The  various  granules  in  the  endoplasm  are  probably  minute 
quantities  of  digested  food  on  their  way  to  being  built  up  into  proto- 
plasm, or  stores  of  reserve  material,  or  particles  of  protoplasm  that 
are  being  broken  down  into  waste  matter.  Collectively  they  may  be 
called  the  metaplasmic  granules. 

When  the  environmental  conditions  become  unfavourable, 
Amoeba  assumes  a  different  form.  The  pseudopodia  are  withdrawn, 
the  contractile  vacuole  is  often  much  reduced,  and  the  animal  rounds 
itself  off  and  secretes  around  itself  a  tough  cyst  of  chitin  or  a  closely 
allied  substance.  Inside  the  cyst  the  ectoplasm  is  more  marked 
than  when  it  is  moving  about  freely.  The  Amoeba  remains  quiescent 
until  upon  the  restoration  of  favourable  conditions  the  cyst  wall 
ruptures  and  the  animal  comes  out  again.  This  encystment  is  to 
be  regarded  as  a  protective  adaptation  enabling  the  animal  to  tide 
over  periods  of  stress,  etc.,  and  incidentally  in  this  condition  it  can 
be  more  readily  conveyed  from  place  to  place. 

Let  us  turn  now  to  consider  the  physiological  activities  of 
the  Amoeba.  Movement  takes  place  by  the  formation  of  pseudo- 
podia,  a  process  that  can  easily  be  watched  in  the  living  animal. 
The  ectoplasm  bulges  out  to  form  a  small  knob,  which  gets  larger 
and  larger,  and  then  all  at  once  the  endoplasm  bursts  into  it.  This 
streaming  movement  may  continue  until  the  whole  protoplasm 
has  flowed  into  the  pseudopodium,  and  thus  a  certain  amount  of 
ground  has  been  covered.  On  the  other  hand,  after  pushing  out  the 
pseudopodium  a  short  way,  it  may  be  withdrawn  again.  Certain 
of  the  different  species  of  Amoeba  can  be  recognised  by  the  shape  of 
their  pseudopodia,  which  may  be  single  or  many,  short  and  blunt, 


THE   PROTOZOA  121 

or  long  and  thread-like.  Several  pseudopodia  may  be  put  simul- 
taneously in  A.  proteus,  and  so  the  animal  in  life  presents  a  very 
characteristic  motile  appearance,  and,  further,  its  mode  of  movement 
is  characteristic  of  a  number  of  other  forms  of  protozoa,  and  is 
described  as  amoeboid.  Thus  Amceba  exhibits  very  completely 
the  power  of  motility. 

If  we  watch  a  living  specimen  we  shall  see  that  it  moves  freely 
from  place  to  place  without  any  apparent  cause.  This  form  of 
movement  in  ourselves  or  the  higher  animals  we  should  term 
voluntary,  but  in  Amceba  it  is  called  automatic  or  spontaneous.  A 
few  simple  experiments  will  show  that  another  sort  of  movement  is 
also  possible.  For  example,  if  the  slide  is  jarred,  or  certain  chemicals 
are  acbied,  the  animal  will  withdraw  its  pseudopodia,  or  perhaps 
move  away  from  the  chemical,  thus  acting  in  response  to  an  external 
stimulus.  Such  movements  clearly  indicating  the  power  of  irrita- 
bility or  sensitivity  are  termed  induced  as  opposed  to  spontaneous. 
It  is,  of  course,  impossible  to  draw  a  hard-and-fast  distinction 
between  them,  for  we  have  no  means  of  knowing  what  stimuli  are 
playing  on  the  animal,  and  so  we  cannot  be  certain  what  are  spon- 
taneous movements.  Some  authors  go  so  far  as  to  claim  that  all 
movements  of  Amceba  can  be  explained  in  terms  of  environmental 
stimuli.  Changes  in  temperature  undoubtedly  play  a  great  part 
in  determining  the  animal's  activities,  as  we  can  easily  test  by  con- 
trolling the  temperature  carefully.  At  25°  C.  Amceba  exhibits  its 
greatest  activity,  and  so  this  is  termed  the  optimum  temperature  ; 
above  and  below  this  it  becomes  more  and  more  sluggish,  movement 
finally  ceasing  altogether  when  the  temperature  gets  very  low,  near 
freezing-point,  or  reaches  35°  C.  At  40°  C.  the  protoplasm  undergoes 
striking  changes  and,  as  we  say,  coagulates,  in  the  same  way  as  the 
white  of  an  egg  on  cooking,  and  the  animal  is  thereby  killed. 

Feeding  in  Amceba  is  a  very  simple  process,  the  animal  flows 
along  until  it  encounters  a  particle  of  food,  a  piece  of  organic  debris 
or  a  diatom,  etc.  It  goes  straight  on  and  engulfs  or  ingests  the  piece 
which  is  passed  through  the  ectoplasm  into  the  endoplasm,  and 
there  has  formed  around  it  a  food  vacuole.  It  is  necessary  that  the 
food  should  be  organic  matter  of  some  sort,  for  Amceba  is  holozoic 
in  its  feeding  and  cannot  utilise  inorganic  substances.  Within  the 
food  vacuole  the  organic  matter  is  digested  until  nothing  but  an 
indigestible  residue  remains.  It  is  not  yet  possible  to  examine  the 
substances  in  the  fluid  of  the  vacuoles,  although  it  appears  to  be 
acid  to  start  with,  and  later  becomes  alkaline.  Therefore,  by  analogy 
we  should  expect  it  to  contain  enzymes,  which  are  similar  in  action 
to  those  in  the  digestive  juices  of  the  frog,  since  the  composition  of 
the  organic  food  matter  is  similar  in  the  two  animals.  When  it  is 


122  AN   INTRODUCTION  TO  ZOOLOGY 

time  to  get  rid  of  the  indigestible  residue  the  Amceba  simply  flows 
on  and  leaves  the  useless  particles,  termed  the  faeces,  behind,  a 
process  known  as  egestion  or  defaecation.  Digestion  results  in  the 
breaking  down  of  the  food  into  comparatively  simple  substances, 
and  in  order  that  they  may  be  utilised  by  the  cell  they  have  to  be 
assimilated.  This  assimilation  consists  of  a  series  of  building  up 
processes  whereby  the  simpler  compounds  are  once  again  trans- 
formed into  complex  protoplasm,  and  these  constructive  changes 
are  collectively  termed  anabolism.  Interesting  light  has  been 
thrown  on  these  processes  by  a  number  of  experiments  in  which  the 
Amceba  was  cut  into  two  pieces,  one  with  and  one  without  a  nucleus. 
The  part  without  a  nucleus,  although  capable  of  living  for  some 
days,  loses  all  power  of  digesting  or  assimilating  food  ;  therefore, 
we  look  to  the  nucleus  as  the  centre  producing  the  enzymes  required 
for  digestion,  and  as  the  origin  of  the  chemical  activities  concerned 
in  assimilation.  The  digestion  of  food  in  Amceba  takes  place  in  a 
way  markedly  different  from  that  in  Rana.  In  the  latter  case  the 
whole  of  the  process  is  gone  through  in  the  alimentary  canal,  and 
the  food  does  not  pass  into  the  cells  of  the  mucosa  until  it  has  been 
rendered  soluble :  the  digestion  takes  place  in  a  cavity  lined  by 
cells,  and  is  termed  intercellular.  In  Amceba  the  raw  food  is  taken 
straight  into  the  cell  within  which  the  digestion  occurs,  and  this  in 
consequence  is  described  as  intracellular. 

There  is  no  special  organ  in  Amceba  which  corresponds 
functionally  to  the  lungs,  but  the  exchange  of  Oxygen  for  Carbon 
dioxide  takes  place  by  diffusion  over  the  general  surface  of  the 
body.  In  this  way  the  animal  obtains  the  oxygen  required  to  oxidise 
its  protoplasm  and  gets  rid  of  the  carbon  dioxide,  one  of  the  waste 
products  resulting  from  this  process. 

In  order  to  perform  its  vital  functions  it  is  necessary  for  Amceba 
to  obtain  kinetic  energy,  and  this  it  does  by  the  oxidation  of  its 
tissues  in  the  way  just  indicated.  Again,  arguing-  from  analogy 
with  the  frog,  we  should  expect  certain  nitrogenous  waste  matters 
such  as  urea  to  be  produced,  and  these  need  to  be  eliminated  from 
the  body.  It  is  practically  certain  that  these  substances  are  removed 
through  the  agency  of  the  contractile  vacuole,  and  although  the 
amount  of  fluid  it  discharges  at  a  time  is  very  small,  the  presence  of 
uric  acid  crystals  in  it  has  been  demonstrated.  The  vacuole  is, 
therefore,  an  organ  of  excretion  corresponding  in  function  with  the 
frog's  kidney,  and,  like  it,  may  be  concerned  with  other  functions 
such  as  the  removal  of  superfluous  water,  etc. 

The  phenomenon  of  growth  is  clearly  exhibited  by  Amceba,  and  it 
affords  us  an  interesting  example  of  the  way  this  occurs  in  living 
matter.  As  anabolism  proceeds  new  matter  is  added  throughout 


THE   PROTOZOA  123 

the  whole  of  the  cell  at  the  same  time,  a  process  of  growth  known  as 
growth  by  intussusception,  which  is  extremely  characteristic  of  living 
things,  and  entirely  different  from  that  in  inorganic  substances.  A 
crystal  grows,  as  noted  previously,  but  only  by  the  deposition  of  new 
material  on  its  surface ;  a  method  known  as  accretion.  Furthermore, 
a  crystal  only  grows  in  a  solution  of  the  same  chemical  composition 
as  itself,  and  it  is  quite  unable  to  synthesise  such  a  compound  from 
other  substances.  If  any  of  the  digested  food  remains  after  the 
waste  of  the  body  has  been  made  good  and  growth  provided  for,  it  is 
formed  into  the  reserve  material,  scattered  as  we  have  already  noted 
in  the  form  of  granules  throughout  the  cell. 

After  a  more  or  less  prolonged  period  of  growth  the  animal 
reaches  its  maximum  size,  and  if  the  food  is  still  plentiful  it  divides 
into  two  by  a  simple  process  known  as  binary  fission.  It  is  not  quite 
clear  exactly  what  determines  the  point  at  which  division  occurs, 
but  some  light  is  thrown  on  the  matter  by  a  consideration  of  the 
relation  of  Amaba  to  its  environment.  The  respiration  of  the  animal 
takes  place  all  over  the  general  surface,  and  so  in  a  small  specimen 
can  readily  be  carried  on.  As  growth  proceeds  we  shall  find,  as  in 
all  solid  objects,  that  while  the  volume  varies  as  the  cube  of  the 
diameter  the  surface  only  varies  as  the  square.  Roughly  speaking, 
then,  when  the  Amoeba  has  grown  to  eight  times  its  original  bulk  it 
has  only  four  times  its  original  surface.  If  we  assume  that  the 
need  for  gaseous  interchange  depends,  in  the  main,  on  the  volume 
of  the  protoplasm,  then  it  becomes  increasingly  difficult  to  satisfy 
as  the  animal  grows  larger.  Division  into  two  cells  would,  of 
course,  restore  the  proper  ratio  again.  Other  factors,  such  as  the 
relative  size  of  the  nucleus  to  the  cytoplasm  and  the  surface  tension 
of  the  protoplasm,  are  also  concerned  in  the  process  of  fission. 

The  actual  process  of  division  in  some  species  of  Amoeba  is  very 
simple.  The  nucleus  elongates,  becomes  dumb-bell  shaped,  and, 
finally,  divides  into  two,  a  good  example  of  direct  nuclear  division. 
Closely  following  this  a  similar  division  of  the  cytoplasm  takes 
place,  and  so  there  result  two  daughter  Amaba. 

In  A.  proteus  the  proceedings  are  slightly  more  complex.  The 
chromatin  granules  in  the  nucleus  become  rearranged  and  take  up 
a  median  position  transversely  to  the  long  axis  of  the  elongating 
nucleus.  They  then  divide  up  into  two  groups,  one  going  to  each 
end  of  the  nucleus,  so  that  when  the  two  daughter  nuclei  are  pro- 
duced each  contains  a  set  of  granules  which  are  subsequently 
scattered  about  as  in  the  original  nucleus.  This  furnishes  a  very 
simple  example  of  indirect  nuclear  division,  a  process  that  becomes 
much  more  elaborate  in  the  higher  animals,  and  is  by  far  the  more 
common  method  of  nuclear  division  met  with  in  living  beings. 


I24 


AN   INTRODUCTION   TO  ZOOLOGY 


The  division  of  the  nucleus  is  followed  by  the  division  of  the  proto- 
plasm. A  somewhat  similar  process  of  division  occurs  in  the  egg 
of  the  frog  which  divides  into  two,  and  each  of  these  again  into  two, 
and  so  on,  but  here  all  the  cells  produced  remain  together  and  do  not 
separate  as  in  Amoeba. 

In  this  production  of  two  new  individuals  it  should  be  noted  that 
only  one  parental  organism  is  concerned.    There  is  no  question  of  a 


FIG.  38. — Division  in  Amceba. 

A,  B,  C,  D,  four  successive  stages  in  division  ;  C.G.,  chromatin  granules  ; 
C.V.,  contractile  vacuole  ;  EC.,  ectoplasm  ;  En.,  endoplasm ;  F.V.,  food  vacuole  ; 
N.,  daughter  nucleus  ;  P.,  pseudopodia. 

male  and  female  parent  as  in  the  case  of  Rana,  and  so  we  distinguish 
it  as  Asexual  Reproduction. 

Another  striking  contrast  with  the  reproduction  in  the  frog  is 
also  presented.  When  the  frog's  eggs  have  been  laid  and  fertilised 
both  the  parents  remain  the  same  individuals  as  before.  This  goes 
on  time  after  time  until  death  by  old  age  or,  much  more  probably, 
by  mischance  intervenes.  _  In  Amoeba,  however,  the  mother  organism 
does  not  die ;  it  ceases  to  exist  as  an  individual,  but  it  passes  on  as 
two  new  beings.  Hence  it  is  that  some  writers  speak  of  an  Amceba 
as  potentially  immortal,  for  we  have  no  evidence  to  show  that  it  ever 
dies  of  old  age  as  do  higher  forms. 

We  have  seen,  then,  that  Amoeba  is  a  tiny  living  unit  which,  in 
spite  of  its  small  size  and  great  simplicity  of  structure,  exhibits  all 


THE   PROTOZOA 


125 


T/flCHOCYQTS 


the  vital  phenomena  characteristic  of  all  plants  and  animals,  namely, 
contractility,  irritability,  metabolism,  growth  and  reproduction. 

Free- Living  Protozoa — ii.  Paramcecium. 

The  genus  Paramcecium,  like  the  genus  Amceba,  contains  a  number 
of  separate  species  of  which  P.  aurelia  and  P.  caudatum  are  the 
commonest,  but  these  only 
differ  from  one  another  in 
comparatively  small  points, 
and  a  general  description  will 
serve  equally  well  for  either 
species.  From  its  somewhat 
fanciful  resemblance  to  a  slip- 
per, the  animal  receives  its 
popular  name  of  the  slipper  ******* 
animalcule.  It  is  to  be  found 
plentifully  in  ponds  and 
ditches,  and  belongs  to  that 
class  of  the  Protozoa  known 
as  the  Infusoria,  from  the  fact 
that  they  appear  in  infusions 
of  organic  matter  that  are  ex- 
posed to  the  air.* 

Paramcecium  differs  con- 
siderably from  Amceba  in 
several  important  respects  :  it 
is  larger,  reaching  a  length  of 
0-3  mm.,  and  so  is  just  visible 
to  the  naked  eye  as  a  tiny 
whitish  speck  ;  it  has  a  definite 
shape ;  it  is  not  a  creeping 
form,  but  swims  about  actively, 
and  altogether  it  is  a  higher 
form,  having  reached  a  con- 
siderable degree  of  structural 
complexity.  In  shape  it  is 
somewhat  like  a  cigar,  one  FIG. 
end  is  bluntly  rounded,  and  as 
it  lies  foremost  in  locomotion 
is  the  anterior  end.  The  opposite  or  posterior  end  is  more  pointed 

*  A  very  simple  infusion  can  be  made  by  putting  chopped  hay  into  water, 
bringing  it  to  the  boil  and  then  allowing  it  to  cool.  If  this  be  placed  aside  in  a 
cool  spot  in  the  open  air  Paramcecium  of  several  species  will  appear  in  it 
after  a  few  days,  and  subsequently  an  enormous  number  will  be  found. 


CONTRACTILE 


39. — Diagram  of  structures  of 
Paramcecium  caudatum. 


126  AN   INTRODUCTION   TO  ZOOLOGY 

and  is  slightly  sharper  in  P.  caudatum  than  in  P.  aurelia.  An 
asymmetrical  groove,  the  peristome  groove,  commences  as  a  fairly 
wide,  shallow  depression  near  the  anterior  end,  runs  backwards  with 
a  slight  spiral  twist,  getting  deeper  and  narrower  until  about  the 
middle  of  the  body  it  finally  leaves  the  surface  and  runs  in  as  a 
funnel-shaped  structure.  The  hole  where  the  groove  leaves  the 
surface  is  the  cell  mouth  or  cytostome,  and  the  funnel-shaped  con- 
tinuation, termed  the  gullet,  oesophagus  or  cytopharynx,  ends  blindly 
in  the  endoplasm  of  the  cell.  Thus  we  are  able  to  distinguish  not 
merely  anterior  and  posterior  ends,  but,  the  side  on  which  the  mouth 
is  situated  being  ventral,  we  have  dorsal  and  ventral  surfaces,  and  so 
also  right  and  left  sides.  The  animal  thus  has  a  definite  shape  which 
does  not  change,  although  it  is  elastic  enough  to  allow  it  to  squeeze 
through  an  aperture  slightly  smaller  than  itself,  and  also  to  bend 
round  any  obstacles  that  lie  in  its  path.  It  is  enabled  to  swim 
comparatively  rapidly  and  uniformly  by  means  of  a  well-developed 
locomotor  apparatus  in  the  form  of  a  covering  of  minute  motile 
hair-like  processes,  the  cilia.  They  are  fairly  evenly  distributed 
over  the  whole  surface,  over  the  peristome  groove,  and  even  in  a 
modified  form  down  the  cytopharynx.  Owing  to  the  way  in  which 
they  strike  the  water  and  the  twist  at  the  front  end  Paramcecium 
rotates  on  its  own  axis  as  it  moves  forward  much  in  the  same  way 
as  a  bullet  from  a  rifle  or  a  shell  from  a  gun. 

Under  the  high  powers  of  the  microscope  further  details  of  its 
structure  can  be  made  out.  The  protoplasm,  like  that  in  Amceba, 
is  clearly  differentiated  into  an  outer  layer,  the  ectoplasm  or  cortex, 
strongly  marked  off  from  the  more  granular  endoplasm  or  medulla. 

The  ectoplasm  itself  is  divided  into  two  distinct  layers.  The 
outer  layer  is  a  thin,  tough  elastic  membrane  known  as  the  cuticle, 
formed  by  the  modification  of  the  outer  ectoplasm,  and  giving 
to  the  animal  its  definite  shape.  The  surface  of  the  cuticle  exhibits 
a  characteristic  sculpturing,  being  divided  up  by  series  of  fine  grooves 
into  a  number  of  minute  hexagons,  from  the  centre  of  each  of  which 
a  cilium  arises.  Each  cilium  is  a  minute  protrusion  of  the  ectoplasm 
perforating  the  cuticle,  and  it  can  be  traced  into  its  deeper  layers 
where  it  takes  its  origin  in  a  tiny  speck,  the  basal  granule, 

The  remaining  deeper  part  of  the  cortex  under  the  cuticle  is  far 
thicker  and  appears  transversely  striated  owing  to  the  presence  in  it 
of  a  large  number  of  very  minute  spindle-shaped  bodies,  the  tri- 
chocysts,  set  at  right  angles  to  the  surface.  Even  under  the  highest 
powers  of  the  microscope  the  trichocysts  only  appear  as  small  simple 
rods-,  in  which  no  details  of  their  internal  structure  can  be  made  out. 
When  the  animal  is  treated  with  an  irritant  fluid  such  as  very  dilute 
acetic  acid,  however,  the  trichocysts  each  discharge  a  very  fine 


THE   PROTOZOA  127 

thread,  very  much  longer  than  the  cilia,  and  so  clothe  the  animal  in 
a  matted  coating  of  very  fine  hairs.  The  exact  use  of  these  tri- 
chocysts  is  almost  as  hard  to  understand  as  their  structure  is  difficult 
to  determine.  They  are  generally  thought  to  be  weapons  of  offence 
or  defence,  but  have  not  been  ob- 
served in  use,  and  are  generally  only 
seen  when  the  animal  is  killed.  In  the 
deeper  layers  of  the  ectoplasm  below 
the  trichocysts  are  a  number  of  deli- 
cate threads  disposed  parallel  to  the 
surface,  the  myoneme  fibrillae,  which 
are  highly  contractile  and  bring  about 
the  bending  movements  of  the  animal. 

In  one  species  of  Paramcecium,  namely,  FlG-  4°;— Surface  view  of  cuti- 
-n  7  .  ,,  ,  ,  .  cle  of  Paramcecium,  adapted 

P.  bursana,  the  ectoplasm  contains  a         from  Butchli. 

number  of  minute  green  corpuscles  E.G.,  basal  granule;  c.,  ciiium; 
containing  chlorophyll,  the  colouring  ^i5;ochstxagonal  depressed  area ;  T- 
matter  of  the  leaves  of  plants,  and 

similar  to  those  we  shall  describe  more  fully  in  the  case  of  Hydra. 
They  give  the  individuals  of  this  species  a  green  colour. 

Two  further  structures  are  to  be  considered  as  ectoplasmic, 
although  they  project  deeply  into  the  endoplasm,  and  these  are  the 
pulsating  vacuoles.  They  are  constant  in  position,  being  situated 
about  one-third  of  the  way  from  each  end  and,  not  moving  about  like 
the  contractile  vacuoles  in  Amoeba,  they  discharge  always  at  the 
same  spots.  When  full  they  appear  as  spheres  containing  a  clear 
fluid,  and  if  they  are  watched  they  will  be  seen  to  contract  suddenly 
and  almost  disappear.  Careful  examination  shows  that  in  their 
place  is  left  a  small  central  spot  from  which  radiate  a  series  of  from 
five  to  ten  fine  lines.  These  lines  are  the  collecting  canals,  and  they 
soon  begin  to  swell  up  with  the  accumulation  of  fluid  within  them, 
presenting  a  very  characteristic  rosette-like  appearance.  Finally, 
they  discharge  into  the  central  dot,  and  ultimately  they  all  become 
merged  in  the  one  central  vesicle.  This  process  of  diastole  and 
systole  is  continued  in  a  very  regular  manner  as  long  as  the  animal 
lives,  and  occupies  a  little  less  than  half  a  minute.  Defaecation  in 
Paramcecium  always  takes  place  at  one  spot  on  the  ventral  surface, 
but  there  does  not  appear  to  be  any  permanent  opening.  In  some 
forms  allied  to  Paramcecium,  e.g.  Nyctotherus,  which  lives  and  is  almost 
always  to  be  found  in  the  frog's  rectum,  a  permanent  cell  anus  or 
cytoproct  is  present  at  the  hinder  end. 

The  endoplasm  differs  from  the  ectoplasm  in  being  far  more 
granular  and  more  fluid,  although  there  is  no  sharp  line  of  demarca- 
tion between  the  two.  The  endoplasm  is  constantly  moving  round 


128  AN   INTRODUCTION  TO  ZOOLOGY 

with  a  slow  circulatory  movement  termed  cyclosis,  which  can  readily 
be  seen  in  the  living  animal.  The  granules  in  it  are  of  several  sorts. 
The  reserve  food  material  takes  the  form  of  small  particles  of  animal 
starch  or  glycogen,  and  can  readily  be  stained  with  Iodine  which 
turns  them  a  wine  red.  Some  of  the  granules  of  excretory  matter 
appear  to  take  the  form  of  Calcium  phosphate.  The  remaining 
granules  are  mostly  the  indigestible  remains  of  the  food.  Food 
vacuoles  similar  to  those  in  Amoeba  containing  a  certain  amount  of 
fluid  and  food  in  a  more  or  less  digested  state  are  also  present,  and 
render  the  cyclosis  very  distinct. 

Near  the  middle  of  the  cell,  not  far  from  the  cytopharynx,  is  a 
fairly  large  ovoidal  body  that  stains  uniformly  and  very  intensely 
with  basic  stains.  This  is  the  meganucleus  or  macronucleus,  and 
near  it  closer  examination  will  reveal  the  presence  of  a  much  smaller 
and  more  lightly  staining  granule,  the  micronucleus.  Thus,  unlike 
Amoeba,  the  nuclear  matter  in  Paramcecium  is  contained  in  two 
separate  nuclei,  each  with  its  own  particular  function. 

It  has  already  been  noted  that  • Paramcecium  is  a  free- 
swimming  animal  moving  relatively  quickly  by  means  of  its  cilia. 
As  it  moves  forward  it  not  only  rotates  on  its  own  axis,  but  the  path 
it  traces  is  similar  to  a  spiral  line  drawn  on  a  cylinder.  On  striking 
a  solid  object  it  is  able  suddenly  to  reverse  the  action  of  its  cilia 
for  a  very  brief  period,  and  so  it  recoils  a  short  distance.  When  it 
is  necessary  to  change  the  direction  of  its  course  it  appears  to  be  able 
to  keep  the  posterior  end  relatively  still  while  the  anterior  end  rotates 
in  a  small  circle.  It  then  moves  off  at  an  angle  to  its  former  course, 
and  several  such  movements  are  required  to  reverse  completely  the 
direction  of  its  progress.  As  in  A  mceba,  its  movements  maybe  divided 
into  spontaneous  and  induced,  and,  indeed,  it  shows  very  marked 
reactions  to  mechanical,  chemical,  thermal  and  electrical  stimuli. 

The  cilia  covering  the  anterior  end  are  arranged  in  curved  rows 
pointing  into  the  peristome  groove,  so  that  food  particles,  consisting 
mainly  of  bacteria,  are  gradually  driven  towards  the  cytostome. 
In  the  cytopharynx  the  cilia  are  arranged  in  a  special  way.  There 
is  a  row  of  long  modified  cilia  partially  fused  so  as  to  form  a  ribbon- 
like  structure  known  as  the  undulating  membrane.  This,  by  its 
wave-like  movements,  takes  the  food  particles  down  to  the  endoplasm 
where  they  accumulate  in  small  masses  which,  when  they  reach  a 
certain  size,  become  surrounded  by  a  vacuole  and  pass  into  the 
general  circulation  brought  about  by  the  cyclosis  of  the  endoplasm. 
Within  the  food  vacuole  digestion  takes  place  in  a  manner  doubtless 
similar  to  that  in  Amoeba.  A  very  instructive  idea  of  the  part 
played  by  the  macronucleus  in  the  activities  of  the  cell  can  be 
gathered  by  over-feeding  the  animal.  Under  this  treatment  its 


THE   PROTOZOA 


129 


cytoplasm  becomes  loaded  up  with  dark  granules  ;  in  this  surfeited 
condition  the  animal  becomes  very  sluggish  and  unable  to  continue 
its  ordinary  activities,  and  we  say  it  is  in  a  state  of  depression.  If  a 
small  quantity  of  certain  Potassium  salts  be  added  to  the  water  the 
dense  coloration  gradually  disappears  and  normal  activities  are 
resumed.  It  is  noteworthy  that  the  protoplasm  first  starts  to  get 
clear  in  the  neighbourhood  of  the  macronucleus,  whence  it  spreads 
outwards,  indicating  that  this  body  is  the 
centre  of  the  chemical  changes  that  have 
taken  place  in  the  cell.  When  all  the 
digestible  part  of  the  food  has  been  as- 
similated the  residue  is  egested  via  the 
anal  spot  towards  the  posterior  end  of 
the  animal. 

Respiration  occurs  all  over  the  general 
surface  of  the  body,  and  the  function  of 
nitrogenous  excretion  is  carried  out  by  the 
pulsating  vacuoles  which  discharge  a  fluid 
containing  uric  acid  in  some  form  ;  both 
are  very  like  the  same  processes  in  Amceba. 
Reproduction  is  also  somewhat 
similar,  and  takes  place  by  means  of 
binary  fission,  the  plane  of  division  lying 
transversely  to  the  long  axis  of  the  ani- 
mal. Just  prior  to  the  division  of  the 
cytoplasm,  division  of  the  nuclei  occurs. 
The  meganucleus  elongates,  becomes 
dumb-bell  shaped,  and  then  separates  into 
two  parts,  one  travelling  to  each  end  of 
the  body  in  a  way  quite  typical  of  direct 
nuclear  fission.  The  micronucleus  divides 
slightly  later  than  the  meganucleus,  and 
by  a  form  of  indirect  division.  It  first 
enlarges,  and  the  chromatin  within  it 
after  passing  through  a  reticular  or  net- 
like  stage  breaks  up  into  a  large  number  of  definite  small  rod- 
shaped  bodies  known  as  the  chromosomes,  which  pass  towards  the 
middle  of  the  nucleus.  At  each  end  of  the  nucleus,  by  this  time  some- 
what elongated,  appears  a  modified  protoplasmic  area  derived  from 
the  division  of  a  sort  of  nucleolus-like  body  termed  the  nucleo- 
centrosome.  The  areas,  from  their  position  at  the  poles  of  the 
nucleus,  are  called  the  polar  plates,  and  they  appear  to  play  an 
important  part  in  the  processes  connected  with  division.  Between 
them  appears  a  number  of  fine  fibrillae  forming  the  spindle,  in  the 

K 


FIG.  41. — Diagram  of  a 
longitudinal  section  of 
a  dividing  Paramcecium, 
adapted  from  a  photo- 
graph of  Calkins. 

C.,  cilia  ;  F.V.,  food  vacuole  ; 
M.  and  M1.,  divided  macronu- 
clei ;  Mi.,  micronucleus  ;  T., 
exploded  trichocysts. 


130     .  •         AN   INTRODUCTION  TO  ZOOLOGY 

middle  of  which  lie  the  chromosomes  forming  the  equatorial  plate. 
The  nucleus  now  elongates  very  markedly,  and  a  group  oi  chromo- 
somes pass  towards  each  end.  ^  Finally,  the  nuclear  membrane 
separates,  and  thus  two  daughter  nuclei  are  formed  which  take  up 
positions  near  their  corresponding  macronuclei.  Shortly  after  this 
a  circular  furrow  appears  in  the  central  region  of  the  body,  and 
soon  we  have  in  the  place  of  the  one  parent  two  daughter  Para- 
mcecia.  Each  grows  until  it  attains  the  maximum  size,  and  is  then 
ready  to  divide. 

The  process  of  indirect  nuclear  division  in  the  micronucleus  is 
more  complex  than  in  the  nucleus  of  Amoeba  proteus,  and  is  more 
advanced  in  that  definite  chromosomes,  polar  plates  and  a  spindle 
are  produced  in  it,  but  it  must  be  borne  in  mind  that  even  here  the 
whole  series  of  changes  occurs  within  the  nuclear  membrane,  a  marked 
difference  from  the  indirect  division  that  we  meet  in  the  higher 
animals,  as  we  shall  see  later. 

The  normal  method  of  reproduction,  then,  is  this  asexual  one 
by  simple  fission,  a  process  taking  from  half  an  hour  to  two  hours, 
according  to  the  temperature,  and  from  the  one  individual  a  large 
number  are  ultimately  produced  by  repeated  divisions.  It  has  been 
found  that  this  process  can  go  on  for  a  long  time,  and  then  it  gradually 
slows  down  and  ceases.  Change  of  food  will  start  it  off  afresh,  and 
an  American  observer  was  able  to  keep  the  process  maintained  for 
nearly  two  years,  during  which  742  generations  were  passed  through, 
but  after  that  no  stimulus  proved  of  any  avail,  and  the  animals 
died  off.  Multiplication  by  fission  can  continue  for  some  time, 
but  ultimately  a  limit  is  reached,  and  no  change  of  diet  will  produce 
the  desired  effect,  and  the  Paramoecia,  unless  they  are  allowed  to 
conjugate,  will  die  out.*  The  act  of  conjugation  appears  to  bring 
about  a  rejuvenescence,  and  after  it,  the  two  individuals  concerned 
serve  as  the  starting  points  for  new  series  of  divisions.  If  Para- 
mcecia  be  kept  in  an  infusion  it  will  be  seen  that  every  now  and  then 
nearly  all  the  animals  come  together  in  pairs,  and  what  has  been 
termed  an  epidemic  of  conjugation  sets  in.  Wha^  brings  about  this 
impulse  to  conjugate  is  not  known,  but  the  changes  that  accompany 
it  have  been  studied  fairly  fully.  The  details  vary  in  different 
species,  but  the  essential  processes  are  the  same,  and  we  will  now 
consider  them  in  P.  caudatum. 

*  More  recent  observations  show  that  a  race  of  Paramcecia  can  be  kept 
going  almost  indefinitely  without  conjugation,  but  in  this  case  the  individuals 
undergo  a  series  of  regenerative  nuclear  changes  within  themselves.  This 
process  of  endomyxis,  as  it  is  termed,  is  somewhat  similar  to,  and  under  some 
conditions  can  undoubtedly  take  the  place  of  conjugation.  It  is  to  be  borne 
in  mind,  however,  that  this  occurs  under  laboratory  conditions  which  are 
practically  inconceivable  in  the  natural  surroundings  of  the  animal. 


THE   PROTOZOA 


The  two  individuals  that  are  going  to  conjugate,  and  may 
therefore  be  termed  the  conjugants,  do  not  differ  structurally  from 
other  individuals,  but  perhaps  they  are  slightly  smaller.  They 
come  together  by  their  oral  or  ventral  surfaces  and  adhere  by  their 
anterior  halves.  The  micronucleus  in  each  enlarges  and  undergoes 
indirect  division  similar  to  that  in  binary  fission.  The  daughter 
nuclei  similarly  divide  so  that  four  micronuclei  are  produced  in  each 
cell,  one  of  which,  the  one  that  happens  to  lie  nearest  the  peristoirte, 
persists,  while  the  three  remaining  ones  break  up  and  are  gradually 
absorbed  by  the  cytoplasm.  While  they  are  disappearing  the 
remaining  micronucleus  again  divides  into  two.  The  one  lying 


FIG.  42. — Diagram   of   conjugation   in   Paramcecium,   to    show   the  nuclear 
changes.     The  position  and  size  of  the  nuclei  are  purely  diagrammatic. 

A.  B.(  conjugants  ;  C.  D.,  ex-conjugants. 

further  from  the  cytostome  we  distinguish  as  the  stationary  nucleus, 
because  it  remains  behind  in  the  cell  that  produced  it.  The  other, 
nearer  to  the  cytostome,  is  the  migratory  nucleus,  and  is  destined 
very  shortly  to  pass  over  into  the  other  conjugant.  When  this 
happens  each  individual  possesses  two  micronuclei,  one  descended 
from  its  own  original  micronucleus  and  the  other  that  has  migrated 
into  it.  These  two  nuclei  fuse  together  and  give  rise  to  the  conjuga- 
tion nucleus,  and  then  the  two  conjugants  separate  from  one  another. 
We  have  thus  as  the  essential  part  of  conjugation  the  fusion  of 
nuclear  material  derived  from  two  distinct  individuals,  and  it  will 
be  remembered  that  a  somewhat  similar  fusion  is  characteristic 
of  the  phenomenon  known  as  fertilisation  in  the  Metazoa. 


132 


AN   INTRODUCTION  TO  ZOOLOGY 


The  macronucleus  takes  no  part  in  these  changes  at  all,  but 
remains  unaltered.  After,  the  separation,  or  perhaps  before, 
however,  it  breaks  up  into  smaller  and  smaller  fragments  which  are 
gradually  absorbed  and  play  no  further  part  in  the  activities  of  the 
cell. 

The  mouth  which  has  disappeared  in  both  individuals  is  now 

re-formed,  and  so  each  ex- 
conjugant  consists  of  a  more 
or  less  normal  free-swim- 
ming and  feeding  Para- 
mcecium,  save  that  it  only 
possesses  one  nucleus,  the 
conjugation  nucleus.  This 
nucleus  undergoes  three  suc- 
cessive divisions,  giving  rise 
to  eight  nuclei,  four  of 
which  migrate  to  the  ante- 
rior and  four  to  the  posterior 
end  of  the  animal.  Those 
at  the  anterior  end  grow 
and  turn  into  macronuclei, 
while  of  those  at  the  other 
end,  three  disappear  alto- 
gether and  the  fourth  re- 
mains as  the  micronucleus. 
The  ex-conjugant,  a  form 
with  five  nuclei,  is  now 
ready  to  undergo  fission, 
which  it  soon  does  if  con- 
ditions are  favourable.  The 
micronucleus  divides  indi- 
rectly and  the  macronuclei 
are  distributed  equally  to 
each  daughter  individual, 
thus  leaving  each  with 
three  nuclei.  Another  simi- 
lar division  follows,  so  that 
the  ex-conjugant  gives  rise 
to  four  daughter  individuals 

before  the  ordinary  arrangement  of  one  micro-  and  one  macronucleus 
is  restored,  and  these  then  serve  as  starting  points  for  new  series 
of  asexual  divisions. 

From  the  description  that  has  just  been  given  it  will  be  seen  that 
conjugation  is  not  strictly  speaking  a  method  of  reproduction,  because 


FIG.  43. — Diagram  of  the  nuclear  changes 
leading  to  the  production  of  four  typical 
Paramcecia  from  an  ex-conjugant.  The 
position  and  relative  size  of  the  nuclei 
are  purely  diagrammatic. 

A.,  ex-conjugant  after  the  first  division  of  theyion- 
jugation  nucleus  ;  B.-E.,  normal  Paramaecia. 


THE   PROTOZOA  133 

only  two  individuals  result  from  the  union  of  two.  It  is,  however, 
closely  bound  up  with  multiplication,  since  two  divisions,  resulting 
in  the  production  of  four  individuals,  must  necessarily  intervene 
before  the  typical  nuclear  arrangement  is  restored.  The  main 
function  seems  to  be  a  rejuvenating  process,  so  preventing  the  extinc- 
tion of  the  race  owing  to  exhaustion.  It  differs  from  sexual  repro- 
duction in  the  higher  forms  in  the  absence  of  the  formation  of  visibly 
differentiated  gametes.  A  further  difference  is  to  be  seen  in  the 
fact  that  in  the  higher  animals,  after  one  or  a  limited  number  of 
reproductive  periods,  the  parent  forms  die.  In  other  words,  in  the 
Metazoa  the  somatic  or  body  tissues  are  perishable  and  only  the 
germ  cells  are  potentially  immortal.  In  a  similar  way  Paramcecium 
and  Amoeba,  too,  for  that  matter,  may  be  regarded  as  immortal  for, 
excepting  accident  or  disease,  death  does  not  appear  to  come  into 
the  life  cycle  of  these  animals. 

The  conjugation  is  termed  partial  because  the  whole  of  the  two 
animals  does  not  fuse ;  it  is  merely  a  fusion  of  muclear  material, 
and  as  far  as  we  can  see,  no  fusion  of  the  cytoplasm  of  the  conjugants 
occurs.  The  union,  too,  can  hardly  be  spoken  of  as  fertilisation,  as 
no  gametes  are  produced.  The  micronuclei  are  the  important 
factors  in  the  process,  and  the  essential  part  of  conjugation  consists 
in  the  union  of  the  two  nuclei  to  form  the  conjugation  nucleus. 

Although  the  nuclei  play  such  an  important  part,  the 
meganucleus  is  not  concerned  at  all,  and  disintegrates  during  the 
process,  thus  remotely  recalling  the  fate  of  the  soma  or  body  of  the 
higher  animals.  The  micronucleus  is  alone  concerned,  and  to 
distinguish  the  parts  played  by  the  two  nuclei  the  larger  is  often 
termed  the  tropho-nucleus,  since  it  is  concerned  with  the  feeding  or 
trophic  functions,  and  the  smaller,  the  gono-nucleus,  since  it  is 
related  to  reproduction.  By  some  authorities  they  are  regarded  as 
one  nuclear  apparatus  in  which  the  trophochromatin  is  separated 
from  the  idiochromatin  or  reproductive  chromatin,  while  generally, 
as  in  Amceba,  the  two  varieties  of  chromatin  are  indistinguishably 
mixed. 

Another  interesting  point  is  the  elimination  of  a  certain  amount 
of  nuclear  material  from  the  micronucleus  in  a  way  that  recalls,  as 
we  shall  see  later,  the  rejection  of  nuclear  matter  from  the  ovum  of 
the  Metazoa  preparatory  to  fertilisation. 

The  two  nuclei  remaining  from  the  divisions  of  the  original 
micronucleus  do  not  differ  in  appearance,  but  because  one  of  them 
migrates  into  the  other  individual  in  the  way  the  nucleus  of  the 
sperm  penetrates  the  ovum  it  is  sometimes  called  the  male  pro- 
nucleus.  In  the  same  way  the  stationary  one  is  distinguished  as  the 
female  pro-nucleus.  The  divisions  of  the  micronuclei  are  of  a 


134  AN   INTRODUCTION   TO  ZOOLOGY 

primitive  kind  of  indirect  division,  but  all  the  changes  take  place 
entirely  within  the  persistent  nuclear  membrane,  and  no  extra 
nuclear  spindle  or  centrosomes  are  formed. 

If  we  compare  briefly  Amceba  and  Paramcecium  we  see 
that  the  latter  is  a  more  complex  animal  than  the  former.  In 
Amceba  we  have  a  comparatively  simple  mass  of  protoplasm,  almost 
an  ideal  primitive  cell,  that  is  capable  of  exhibiting  all  the  vital 
phenomena,  almost  the  only  specialised  part  being  the  contractile 
vacuole.  Paramcecium,  on  the  other  hand,  has  a  definite  protective 
and  supporting  cuticle,  a  locomotor  apparatus  in  the  form  of  cilia, 
excretory  pulsating  vacuoles,  contractile  myoneme  fibrillge,  a 
cytostome,  cytopharynx,  trichocysts,  and  so  on.  All  of  these  are 
definite  structures  which  can  perform  special  functions  that  in 
Amceba  are  carried  out  by  the  general  protoplasm.  Here  we  have 
an  example  of  a  very  important  principle  underlying  animal  organisa- 
tion, namely,  the  division  of  physiological  labour,  accompanied  by 
the  production  of  special  parts  to  subserve  definite  functions,  or,  as 
we  say,  by  a  differentiation  of  structure.  The  higher  we  pass  in  the 
scale  of  animal  life  the  more  complete  is  the  division  and  the  more 
elaborate  the  specialisation.  The  frog  affords  an  illustration  of  this, 
for  not  merely  are  certain  parts  separated  off  for  set  purposes,  but 
the  one  function  itself  may  be  subdivided.  Thus  the  function  of 
digestion  carried  out  in  Amceba  and  Paramcecium  by  the  food 
vacuoles,  is  delegated  to  the  alimentary  system  in  Rana,  and  this 
in  itself  is  composed  of  a  number  of  separate  organs,  each  with  a 
limited  part  to  play.  One  result  of  this  is  that  the  various  bodily 
activities  can  be  carried  out  with  the  maximum  efficiency.  On  the 
other  hand,  however,  when  a  tissue  has  become  so  highly  specialised, 
it  can  do  one  thing  only  and  nothing  else,  so  that  the  maintenance  of 
life  depends  on  the  activity  of  large  separate  parts.  If  any  one 
of  these  ceases  to  function,  life  stops,  and  so  we  find  a  number  of 
"  vital  organs,"  the  elimination  of  any  one  of  which  by  accident 
or  disease  brings  about  the  death  of  the  whole  animal.  A  com- 
parison is  often  drawn  between  this  separation  of  functions  in  the 
animal  body  and  the  division  of  an  industrial  community  into 
different  trades  and  occupations  of  varying  utility  to  the  remainder. 
The  analogy,  although  not  perfect,  is  fairly  close,  and  here  again  the 
complete  cessation  of  work  on  the  part  of  certain  classes  of  operatives 
would  bring  the  whole  industry  to  a  standstill.  This  would  only 
be  temporary,  however,  since  men  are  not  so  highly  specialised  that 
they  are  incapable  of  doing  any  other  sort  of  work,  and  a  fresh  set 
of  workmen  could  soon  be  trained.  In  the  community,  as  in  the 
animal,  increased  efficiency  is  purchased  at  the  price  of  increased 
vulnerability. 


THE  PROTOZOA  135 

Parasitic  Protozoa. 

The  two  protozoa  just  considered  are  free-living  forms,  that 
is  to  say,  they  move  about  freely  from  place  to  place  finding  and 
catching  their  own  food.  On  the  other  hand,  a  large  number  of 
the  Protozoa  do  not  do  this,  but  live  on,  and  in,  other  animals  whose 
tissues  and  juices  they  use  as  food.  Animals  living  in  this  manner 
are  termed  parasites,  and  the  form  upon  which  they  live  is  called  the 
host.  One  whole  class  of  the  Protozoa,  namely,  the  class  SPOROZOA, 
is  composed  entirely  of  parasitic  species.  The  Sporozoa  possess 
certain  characters  in  common.  They  all  live  inside  the  bodies  of 
their  hosts  and  so  are  internal  or  endoparasites,  some  living  in  the 
cavities  of  the  viscera,  but  others  actually  inside  the  individual  cells  ; 
the  latter  we  distinguish  as  intracellular,  and  the  former  as  inter- 
cellular parasites.  As  a  general  rule,  each  species  is  limited  to  a 
definite  species  of  host  to  which  in  many,  perhaps  the  majority  of 
cases,  they  appear  to  bring  no  harm,  and  so  are  described  as  harmless 
parasites.  In  other  cases  they  are  very  harmful,  setting  up  a 
diseased  or  pathological  condition  which  produces  great  bodily 
disturbances,  and  may  even  prove  fatal.  They  all  live  on  fluid 
food  which  is  absorbed  by  osmosis  over  the  general  surface  of  the 
body,  so  that  as  a  result  they  lack  a  mouth  and  a  pharynx,  either 
permanent  or  temporary,  also,  as  it  is  food  already  digested  and  even 
assimilated  by  the  host,  they  have  no  food  vacuoles  and  no  con- 
tractile vacuoles.  There  is  no  need  for  them  to  move  actively  to 
find  nutriment,  so  that  locomotor  organs  are  absent.  The  name 
of  the  class  is  derived  from  another  of  their  principal  characteristics, 
and  that  is  that  reproduction  takes  place  by  the  formation  of  spores. 
In  the  process  of  spore  formation,  or  sporulation,  the  parent  indi- 
vidual breaks  up  into  a  large  number  of  minute  nucleated  fragments 
of  protoplasm,  often  protected  in  some  way  or  other,  which  serve 
for  the  dissemination  of  the  species  and  the  spreading  of  the  parasites 
to  new  hosts.  Two  members  of  the  class  we  shall  now  study  in  some 
detail,  namely,  Monocystis  and  Plasmodium. 

Parasitic  Protozoa — i.  Monocystis. 

Two  species  of  Monocystis,  namely,  M.  magna  and  M.  agilis, 
are  to  be  found  in  some  stage  of  their  life  history  in  practically  every 
ordinary  earthworm,  and  so  serve  conveniently  for  purposes  of 
study.  They  are  both  to  be  discovered  in  the  sperm  sacs  of  the 
earthworm,  large  conspicuous  whitish  sacs  lying  in  the  ninth  to 
twelfth  segments  of  its  body,  in  which  the  sperms  undergo  part  of 
their  development  and  are  stored  until  required  for  use.  M.  magna 
in  the  mature  condition  may  reach  a  length  of  5  mm.,  and  so  is 


136  AN   INTRODUCTION   TO  ZOOLOGY 

visible  to  the  naked  eye.  It  is  to  be  found  inside  the  sperm  sacs 
attached  to  the  rosette-shaped  funnels  of  which  there  is  a  pair  in 
segments  10  and  n,  and  which  lead  from  the  sacs  to  the  sperm 
ducts  or  vasa  deferentia.  M.  agilis,  on  the  other  hand,  is  a  far 
smaller  form,  only  reaching  a  length  of  2  mm.  when  fully  grown. 
It  floats  freely  in  the  fluid  contents  of  the  sperm  sac,  and  can  be 
found  by  smearing  these  on  a  slide  and  examining  the  smear  under  a 
microscope.  The  two  species  are  very  similar  save  in  size,  and  pass 
through  practically  similar  life  histories,  so  that  one  description  will 
apply  almost  equally  well  to  either. 

The  adult  organism  is  of  an  elongated  spindle  shape,  somewhat 
flattened,  and  contains  near  the  middle  a  fair-sized  nucleus.  Its 
protoplasm  is  divided  into  a  fairly  clear  firm  ectoplasm,  outside  which 
is  a  very  thin  but  distinct  cuticle  marked  with  delicate  longitudinal 
striations  and  a  more  fluid  internal  endoplasm.  In  the  deeper  layers 
of  the  ectoplasm  are  a  number  of  myoneme  fibrillse  which  branch  and 
anastomose,  forming  a  fine  but  somewhat  feeble  contractile  network 
by  means  of  which  the  movements  of  the  body  are  brought  about. 
The  endoplasm  is  opaque,  owing  to  the  presence  in  it  of  a  large 
number  of  granules  of  reserve  food  material  composed  of  a  Carbo- 
hydrate substance  allied  to  starch.  The  opacity  partly  hides  the 
nucleus  which,  however,  cannot  even  be  rendered  conspicuous  by 
ordinary  stains.  The  nucleus,  which  is  spherical  with  a  definite 
nuclear  membrane,  contains  a  clear  nuclear  sap  wherein  float  several 
deeply  staining  nuclear  corpuscles.  These  bodies  are  composed  of  a 
basis  of  a  substance  plastin,  impregnated  with  chromatin,  and  are 
termed  karyosomes  in  order  to  distinguish  them  from  true  nucleoli, 
which  contain  plastin  alone.  The  animal  just  described  is  in  its 
feeding  or  trophic  stage,  and  is  in  consequence  called  a  trophozoite. 
Its  movements  are  very  restricted,  it  may  bend  slightly  arid  is  only 
able  to  move  quite  slowly.  When  it  progresses  a  wave  of  contraction 
passes  from  one  end  of  the  body  to  the  other,  followed  by  another 
wave,  and  so  on.  This  produces  a  very  characteristic  form  of  motion 
termed  euglenoid  movement,  since  it  is  exhibited  in  a  typical  manner 
by  the  protozoon  Euglena,  one  of  the  MASTIGOPHORA.  As  has 
already  been  pointed  out,  feeding  takes  place  by  osmosis,  proteid 
material,  built  up  by  its  host,  the  earthworm,  being  absorbed  from 
its  surroundings.  In  the  same  way  respiration  and  nitrogenous 
excretion  occur  all  over  the  surface,  and  no  special  organs  are 
developed.  Reproduction  is  more  complicated  than  in  Amceba  or 
Paramcecium,  and  forms  parts  of  a  noteworthy  cyclical  series  of 
changes. 


THE   PROTOZOA  137 

Life  History. 

When  it  first  reaches  the  sperm  sac,  Monocystis  is  a  very 
minute  form,  and  it  bores  its  way  into  a  sperm  mother  cell  becoming 
an  intracellular  parasite.  The  sperm  mother  cell  divides  into  a 
number  of  daughter  cells  that  become  arranged  to  form  a  mulberry- 
like  mass,  the  sperm  morula,  around  a  central  mass  of  non-nucleated 
protoplasm,  the  cytophore.  The  young  trophozoite  lives  in  the 
cytophore,  which  it  gradually  consumes.  The  cells  of  the  morula, 
after  a  certain  number  of  divisions,  give  rise  to  numerous  spermatozoa 
which  assume  the  typical  form  with  a  head  and  long  thread-like 
tail.  By  this  time  the  parasite  has  consumed  the  central  protoplasm, 
and  it  now  attacks  the  heads  of  the  sperms,  leaving  the  tails  un- 
touched. It  is  now  fairly  well  grown,  and,  with  all  the  tails  of  the 
spermatozoa  adhering  to  it,  looks  as  if  it  were  covered  with  a  coating 
of  long  cilia.  For  a  while  it  lives  in  the  spermatic  fluid,  thus 
becoming  an  intercellular  parasite. 

When  full  grown  and  mature  two  of  these  trophozoites  come 
together  lengthwise  and  adhere.  They  shorten  considerably  and 
secrete  around  themselves  a  very  tough  double-layered  spherical 
cyst,  composed  of  a  rigid  outer  coat  or  epicyst  and  a  softer  endocyst. 
As  these  cells  are  destined  to  produce  the  gametes,  they  may  now  be 
spoken  of  as  the  garnet ocytes.  The  nucleus  in  each  passes  through 
a  certain  series  of  changes,  resulting  in  the  elimination  of  a  quantity 
of  nuclear  substance  that  is  absorbed  by  the  cytoplasm,  and  then 
it  undergoes  repeated  division,  producing  many  daughter  nuclei. 
These  take  up  a  position  around  the  periphery  of  the  cell,  and  the 
superficial  cytoplasm  breaks  up  into  an  equal  number  of  small 
masses,  each  enclosing  a  nucleus  and  attached  to  a  central  lump  of 
residual  protoplasm.  When  they  afterwards  become  free  we -can 
distinguish  these  minute  nucleated  bodies  as  the  gametes,  and  they 
soon  enter  a  brief  motile  stage.  The  double  layer  of  cuticle  that 
came  between  the  two  original  gametocytes  breaks  down  and  the 
gametes  move  about  and  come  together  in  pairs,  the  two  in  each 
couple  most  probably  being  derived  from  different  parents.  These 
pairs  and  their  nuclei  fuse  to  form  a  single  cell,  the  zygote  or  sporo- 
blast.  Each  zygote  secretes  around  itself  a  very  tough  resistant 
cyst,  the  sporocyst,  made  of  a  substance  allied  to  chitin  and  of  a 
characteristic  elongated  lemon  shape.  From  its  resemblance  to 
one  of  the  unicellular  plants,  a  diatom  Navicella,  in  its  tuin  so  named 
from  its  boat  shape,  the  sporocyst  has  long  been  known  to  naturalists 
as  the  pseudonavicella.  The  nucleus  of  the  sporoblast  undergoes 
three  successive  divisions,  giving  rise  to  eight  daughter  nuclei,  which 
take  up  a  peripheral  position  near  the  middle  of  the  length  of  the 


138 


AN   INTRODUCTION   TO  ZOOLOGY 


sporocyst.  Curved  pieces  of  protoplasm  are  separated  off  and 
enclose  each  nucleus,  leaving  a  small  mass  of  residual  protoplasm. 
The  eight  minute  bodies  so  formed  are  termed  the  sporozoites  or 
falciform  young,  the  last  name  being  bestowed  on  them  from  their 


FIG.  44.' — Diagram  of  life  history  of  Monocystis,  adapted  from 
various  authors. 

A.,  trophozoite  ;     B.,  two  individuals,  conjugants  or  gametocytes,  encysted  ; 

C.,  formation  of  gametes  ;    D.,  union  of  gametes  to  form  zygotes  or  sporoblasts  ; 

E.,  mother  cyst  with  sporocysts  ;  F.,  one  sporocyst  under  higher  magnification  to 
show  sporozoites. 

somewhat  fanciful  resemblance  to  a  sickle  blade.  Thus  we  have 
inside  the  original  mother  cyst  a  large  number,  at  least  fifty,  and 
probably  many  more,  of  sporocysts,  each  containing  eight  sporo- 
zoites, and  this  is  the  stage  most  commonly  found.  No  further 


THE  PROTOZOA  139 

development  can  take  place  until  these  sporozoites  are  transferred 
to  another  worm.  The  eaxct  method  in  which  this  takes  place  is 
not  known,  but  it  is  plausibly  suggested  that  when  the  earthworm 
dies  and  disintegrates  the  cysts  are  left  unaffected  in  the  soil ;  or, 
again,  when  a  worm-eating  bird  swallows  the  earthworm  it  is  able  to 
digest  it,  but  not  to  injure  the  resistant  spores,  which  pass  through 
the  alimentary  canal  and  are  scattered  on  the  ground  with  the 
excrement.  Another  worm  swallows  the  soil  containing  some  of 
the  spores,  and  so  in  a  chance  manner  becomes  infected.  The 
sporozoites  are  probably  released  in  the  alimentary  canal,  whose 
digestive  juices  are  able  to  dissolve  the  tough  wall  of  the  sporocyst, 
but  it  is  not  yet  known  how  they  reach  the  sperm  sacs.  Once 
here  they  enter  the  sperm  mother  cell  and  the  cycle  begins 
all  over  again. 

We  see  a  considerable  difference  between  the  method  of 
reproduction  in  Monocystis  as  compared  with  either  Amceba  or 
Paramcecium.  In  the  latter  two  forms  the  multiplication  always 
takes  place  by  binary  fission,  i.e.  division  of  the  nucleus  into  two, 
followed  by  a  corresponding  splitting  of  the  cytoplasm.  The  former 
animal,  when  it  divides  either  as  a  sporoblast  or  as  a  gametocyte, 
behaves  differently.  The  nucleus,  first  by  repeated  indirect  division, 
gives  rise  to  eight  or  a  large  number  of  daughter  nuclei,  and  not 
until  the  nuclear  divisions  are  quite  complete  and  the  daughter 
nuclei  have  migrated  to  the  periphery  does  any  separation  of  the 
cytoplasm  occur.  Then  as  many  separate  small  cells  as  there  were 
nuclei  are  formed  simultaneously.  This  type  of  multiplication  is 
distinguished  as  multiple  fission,  as  opposed  to  binary  fission. 
Another  interesting  difference  is  to  be  found  in  the  life  histories, 
for  in  Monocystis  we  first  encounter  in  a  simple  form  the  phenomenon 
known  as  "  alteration  of  generations  "  or  metagenesis.  In  the  first 
place  we  have  the  sexual  generation  ending  with  a  large  number  of 
gametes  which  unite  to  form  zygotes,  and  in  the  second  place  the 
zygotes  themselves  multiply  asexually  to  produce  eight  sporozoites. 
It  is  to  be  noted,  however,  that  once  the  sporozoites  have  been  formed 
they  do  not  divide  any  further,  as  commonly  occurs  in  other  allied 
forms,  even  when  they  enter  into  their  feeding  stage  in  the  sperm 
sacs.  This  particular  sort  of  life  history  with  its  alternation  is 
termed  digenetic,  in  contradistinction  to  the  simple  one  in  Amceba, 
which  is  said  to  be  monogenetic. 

The  two  gametocytes  each  produce  a  large  number  of  gametes 
which  fuse  in  pairs  to  form  zygotes  in  a  way  suggestive  of  the  fertili- 
sation of  the  ovum  by  the  sperm  in  the  higher  animals.  The  two 
gametes,  however,  are,  as  far  as  we  can  see,  exactly  the  same  in  size 
and  structure,  and  we  cannot  distinguish  a  male  and  a  female  form 


140  AN   INTRODUCTION   TO  ZOOLOGY 

as  in  some  Sporozoa.     For  this  reason  they  are  termed  Isogametes, 
and  the  process  of  their  union  is  spoken  of  as  Isogamy. 

To  all  appearances  Monocystis  is  quite  a  harmless  parasite,  and 
does  not  seerii  to  have  any  evil  effect  upon  its  host ;  indeed,  it  is  so 
widely  spread  that  almost  all  worms  are  infected  to  a  greater  or 
less  extent.  The  only  parts  affected  are  the  sperms,  and  these  are 
produced  in  such  quantities  that  even  in  the  case  of  a  heavy  infection 
there  are  still  sufficient  healthy  sperms  to  do  the  work  of  fertilisation. 

Parasitic      Protozoa — ii.      PJasmodium,      the      Malarial 
Parasite. 

Three  species  of  the  genus  Plasmodium,  causing  in  man 
three  distinct  diseases,  malaria  and  two  kinds  of  ague,  are  known  : 
P.  immaculatum  (or  P '.  falciparum) ,  producing  pernicious  or  tropical 
malaria  ;  P.  vivax,  producing  tertian  ague  ;  and  P.  malaria,  re- 
sponsible for  quartan  ague.  The  diseases  are  very  widespread  over 
the  tropical  and  temperate  parts  of  the  world,  and  were  at  one  time 
common  in  the  fenlands  and  low-lying  districts  of  England,  where 
now,  fortunately,  they  have  practically  disappeared.  Often  tracts 
of  country  are  devastated  by  their  ravages,  for  they  may  be  deadly, 
as  England  found  to  her  cost  in  the  Walcheren  expedition  in  1809. 
In  this  force,  out  of  39,219  men,  4175  died,  the  number  who  suffered 
from  the  disease  was  nearly  27,000,  and  even  when, the  troops 
were  recalled  and  reached  England,  11,500  were  suffering  from 
"  Walcheren  sickness,"  as  it  came  to  be  called.  It  was  long  asso- 
ciated with  very  damp  soils,  hence  its  name  of  marsh-fever,  and  also 
the  word  malaria,  mat' aria,  which  means  the  bad  air  of  marshy 
places,  and  which  was  supposed  to  be  poisonous. 

The  discovery  in  1882  that  the  disease  was  caused  by  minute 
sporozoa  in  the  red  blood  corpuscles  we  owe  to  A.  Laveran,  a 
French  military  medical  officer,  and  the  satisfactory  working  out 
of  its  complex  life  history  may  well  be  regarded  as  one  of  the  bio- 
logical triumphs  of  the  end  of  the  nineteenth  century.  The  life 
history,  practically  the  same  in  the  three  species,  is  more  com- 
plicated than  that  of  Monocystis,  since  it  is  intimately  connected 
with  two  distinct  animals,  man  and  a  blood-sucking  mosquito  of 
the  genus  Anopheles.  We  term  the  species  in  which  the  sexual 
part  of  the  life  cycle  is  gone  through,  in  this  case  the  mosquito,  the 
principal  host,  and  that  in  which  the  asexual  period  occurs,  here  a 
man,  the  secondary  host.  These  two  terms,  principal  and  secondary, 
are  applicable  generally  to  the  life  histories  of  all  parasites,  but  in 
the  case  of  malaria  and  certain  other  diseases  it  is  often  customary, 
especially  in  medical  works,  to  entirely  reverse  the  terms.  In 


THE  PROTOZOA 


141 


dealing  with  malaria  man  is  termed  the  principal  host,  since  he 
suffers  a  disease,  while  the  mosquito  is  the  secondary  host,  for  the 
parasite  is  quite  innocuous  to  it,  but  this  usage  of  the  terms  is  not 
strictly  biological. 

The  parasite,  when  it  is  introduced  into  the  blood  stream  of  a  man 
by  the  stab  of  an  infected  female  mosquito,  has  the  form  of  an 
extremely  minute  spindle-shaped  sporozoite.  The  sporozoite  im- 
mediately attaches  itself  to  a  red  blood  corpuscle,  into  which  it 
bores  its  way,  and  when  inside  enters  into  the  trophozoite,  or  feeding 


FIG.  45. — A  diagram  illustrating  the  stages  of  the  life  cycle  of  Plas- 
modium  vivax  found  in  human  blood. — 'From  Bourne. 

I, -IX.  show  the  schizogonous  cycle.  In  7.  a  sporozoite  is  boring  its  way  into  a  red  corpuscle  ; 
//.,  young  amoeboid  phase;  ///.,  a  vacuole  has  appeared  near  the  nucleus  giving  the  characteristic 
ring  form  ;  IV.  and  V.,  pigment  (melanin)  granules  are  deposited  in  the  cytoplasm,  the  parasite 
has  increased  in  size  and  exhibits  active  pseudopodial  movements  ;  VI.,  nucleus  with  an  equatorial 
ring  of  chromatin  granules  ;  VII.  and  VIII.,  successive  stages  of  nuclear  division  ;  IX.,  segmenta- 
tion of  the  cytoplasm  round  the  nuclei  to  form  the  merozoites  which  are  shown  at  mer  escaping 
into  the  blood  plasma,  spz.,  sporozoites  ;  p.,  pigment  granules  ;  gam.,  a  young  game tocyte  ;  tf  , 
a  male  gametocyte  (microgametocyte)  and  2  a  female  gametocyte  (macrogametocyte)  of  P.  xivax, 
In  this  species  and  in  P.  malaria  the  gametocytes  are  not  crescent-shaped  as  they  are  in  P.  im- 
maculatum.  (Somewhat  diagrammatic  after  Scbaudinn.) 

stage,  assuming  an  amoeboid  form.  At  first  very  tiny,  it  gradually 
grows  until  it  almost  fills  the  corpuscle  ;  it  acquires  a  large  vacuole, 
and  granules  of  a  dark  pigment  termed  melanin  are  formed  in  it. 
When  fully  grown  the  amoeboid  movements  slow  down,  the  vacuole 
disappears,  and  the  animal  rounds  itself  off  preparatory  to  entering 
into  a  reproductive  phase.  The  form  of  multiplication  undergone 
is  termed  Schizogony,  and  the  individual  about  to  pass  through  it  is 
consequently  known  as  the  Schizont.  The  nucleus  divides  indirectly 
a  number  of  times,  giving  rise  to  from  twelve  to  sixteen  daughter 
nuclei.  As  in  all  multiple  fission  the  cytoplasm  next  divides  into  the 


142  AN   INTRODUCTION  TO  ZOOLOGY 

same  number  of  separate  minute  bodies,  known  as  the  merozoites. 
These  discrete  masses  are  arranged  in  a  radiating  manner  around  a 
central  mass  of  residual  protoplasm,  in  which  all  the  melanin  has 
accumulated,  and  so  a  very  characteristic  form,  known  as  the 
"  rosette  "  stage,  is  entered  upon.  The  corpuscle  then  breaks  down 
and  the  merozoites  are  set  free  to  attack  a  fresh  set  of  corpuscles,  and 
the  melanin  is  discharged  into  the  blood.  The  merozoites  behave  in 
precisely  the  same  way  as  the  sporozoites,  and  a  fresh  cycle  is  again 
gone  through.  In  the  recurrent  forms  of  the  sickness  a  day  of  high 
fever  alternates  with  non-feverish  intervals,  and  it  has  been  shown 
that  this  fever  corresponds  with  the  end  of  the  schizogonous  cycle 
and  the  discharge  of  the  fresh  lot  of  merozoites  into  the  blood.  In 
tertian  ague  (P.  vivax)  this  occurs  every  third  day,  and  in  quartan 
ague  (P.  malaria)  every  fourth  day.  The  schizogony  in  P.  immacu- 
latum  is  not  so  regular,  and  the  merozoites  are  discharged  constantly 
into  the  blood,  the  result  being  either  a  continuous  fever  or  one 
recurring  at  quite  irregular  intervals. 

It  will  be  seen  that  if  this  process  goes  on  an  increasing  number  of 
red  corpuscles  are  infected  and  destroyed,  and  after  a  comparatively 
few  schizogonous  cycles  an  enormous  number  will  have  been 
attacked  even  from  a  small  original  infection.  The  patient  becomes 
anaemic,  the  pigment  is  deposited  in  the  brain  capillaries  and  certain 
of  the  viscera,  the  spleen  becomes  enlarged  and  congested,  and  general 
cachexia  ensues.  This  may  prove  fatal,  but  if  it  does  the  parasites 
are  destroyed  with  their  host  and  not  able  to  reinfect,  so  that  the 
death  of  the  host  is  obviously  an  undesirable  event  from  the  point 
of  view  of  the  parasite.  We  find  generally  that  after  a  number  of 
schizogonous  cycles  have  been  completed  the  trophozoite  does  not 
turn  into  a  schizont.  On  the  other  hand,  probably  as  the  result 
of  chemical  changes  in  the  blood,  it  does  not  reach  such  a  large  size, 
but  rounds  off  slightly  earlier  and  turns  into  a  gametocyte.  These 
are  of  two  forms,  generally  easily  distinguishable  by  the  distribution 
of  the  melanin  within  them.  The  male  form,  or  microgametocyte, 
has  a  large  nucleus,  lightly  staining  protoplasm,  and  small  pigment 
granules  more  or  less  evenly  distributed,  while  the  female  form,  or 
macrogametocyte,  has  a  smaller  nucleus,  more  deeply  staining 
cytoplasm,  and  coarser  granules  aggregated  around  the  nucleus. 
In  P.  immaculatum  we  find  the  same  differences  in  nucleus  cytoplasm 
and  melanin,  but  the  animals  take  on  a  characteristic  bent  sausage- 
shape  within  the  corpuscle,  being  then  termed  the  "  crescents,"  and 
so  are  clearly  distinguished  from  trophozoites  and  schizonts  in  all 
stages  of  growth.  No  further  changes  are  undergone  and  no  develop- 
ment is  possible  on  the  part  of  the  gametocytes  until  they  are  trans- 
ferred to  the  next  host,  a  mosquito. 


THE   PROTOZOA  143 

The  next  thing,  then,  is  to  see  how  the  mosquito  becomes  infected. 
The  female  mosquito  lives  by  sucking  the  warm  blood  of  mammals, 
including  man,  and  in  order  to  do  this  is  furnished  with  a  very 
complex  instrument,  the  proboscis.  This  is  composed  of  seven 
distinct  parts,  all  modified  from  the  structures  which  in  other  insects 
constitute  the  jaws  and  accessory  organs,  and  together  they  form  a 
most  efficient  piercing  stylet,  as  we  soon  learn  to  our  discomfiture 
when  we  go  into  a  mosquito-infected  district.  Such  an  apparatus 
is  absent  in  the  male,  which  cannot,  therefore,  suck  blood.  Not 
only  does  it  form  a  piercing  organ,  but  it  is  so  constructed  that  when 
it  reaches  a  small  blood-vessel  the  parts  can  be  closed  together  and 
form  a  tube  leading  to  the  mouth.  Behind  the  mouth  is  a  suctorial 
pharynx,  by  means  of  which  a  quantity  of  blood  is  drawn  up  into  the 
stomach  of  the  mosquito.  Should  the  female  stab  a  person  infected 
with  malaria  it  automatically  sucks  up  with  the  blood,  corpuscles 
containing  the  parasites  in  different  phases  of  development.  In  the 
stomach  all  stages  of  the  parasite  are  digested  with  the  blood  save 
only  the  gametocytes,  and  these  are  not  merely  left  undigested,  but 
actually  stimulated  to  activity  and  assume  a  motile  amoeboid  form. 
In  this  country  we  have  a  species  closely  related  to  the  mosquito 
Anopheles,  namely,  the  gnat  Culex  pipiens,  also  capable  of  stinging 
human  beings.  C.  pipiens  is  the  intermediary  in  a  malaria  fever, 
avian  malaria,  confined  entirely  to  birds.  So  marvellously  exact 
is  the  inter-relationship  between  the  parasite  and  its  hosts  that  on  the 
one  hand  the  gnat  cannot  be  infected  with  human  malaria  or  ague, 
nor  on  the  other  hand  can  the  mosquito  carry  avian  malaria, 
although  the  parasites  in  the  two  cases  are  practically  identical  in 
appearance  and  life  history.  Each  species  is  capable  of  spreading 
only  the  particular  disease  that  affects  either  the  man  or  the  bird. 

Still  within  the  stomach  of  the  mosquito  the  nucleus  of  the 
macrogametocyte  undergoes  an  unequal  division,  and  the  smaller 
daughter  nucleus  surrounded  by  a  minute  quantity  of  protoplasm 
is  extruded  from  the  cell  as  the  so-called  "  polar  body."  After 
this  process  of  maturation,  or  ripening,  the  cell  is  ready  for  fertilisa- 
tion, and  so  is  now  a  macrogamete. 

The  changes  in  the (microgamejgTare  more  complex.  Its  nucleus 
enlarges,  and  after  the  cell  has"  passed  through  a  short  phase  of 
activity,  putting  forth  and  withdrawing  pseudopodia,  it  breaks  up 
by  a  kind  of  multiple  fission  into  six  or  eight  masses.  Each  of  these 
consists  of  a  karyosome  surrounded  by  chromatin  granules.  They 
take  up  a  position  near  the  periphery  of  the  protoplasm,  which  gives 
rise  to  usually  six  long  actively  moving  filiform  processes.  Into 
each  of  these  a  karyosome  migrates,  occupying  a  central  position, 
while  its  attendant  chromatin  grains  are  scattered  along  its  length. 


144  AN   INTRODUCTION  TO  ZOOLOGY 

The  remaining  protoplasm  with  the  pigment  and  odd  karyosomes 


en. 


G. 


K. 


J. 


H. 


FIG.  46. — Figures  illustrating  the  stages  of  the  life  cycle  of  the  malarial 
parasite  found  in  Anopheles. — From  Bourne. 

/4.,  crescent-shaped  gametocytes  of  Plasmodium  immaculatum  of  pernicious  tropical  malaria; 
cf  ,  the  microgametocyte.;  9  ,  the  macrogametocyte.  B.,  further  stages  in  the  development  of  d*  the 
microgametocyte  and  9  the  macrogametocyte  of  Plasmodium  vivax  ;  the  nucleus  of  the  macro- 
gametocyte has  divided  unequally  to  form  a  polar  body,  pb.  C.,  the  nucleus  of  the  microgametocyte 
has  broken  up  into  eight  karyosomes,  ky.,  each  surrounded  by  a  ring  of  chromatin  granules  ;  the 
polar  body  has  separated  from  the  macrefgametocyte.  D.,  formation  of  microgamctes  from  the 
microgametocyte-  £.,  a  single  microgamete  ;  ky.,  the  central  karyosome.  F.,  fertilisation  of  the 
macrogamete  'by  the  microgamete  ;  n.  tf  ,  male  pronucleus  ;  n.  ?  ,  female  pronucleus.  G.,  the 
motile  zygpte  or  ookinete  ;  few.,  fertilisation  nucleus.  H.,  the  ookinete  (oocyst)  surrounded  by 
a  very  delicate  cyst  wall,  at  rest  in  the  tissues  of  the  wall  of  the  stomach  of  the  mosquito  ;  n., 
nucleus  ;  p.,  pigment.  /.,  multiplication  of  nuclei  in  the  oocyst.  K.,  the  protoplasm  of  the  oocyst 
has  divided  into  numerous  sporoblasts,  sp.  bl.,  each  containing  a  nucleus.  L.,  early  formation 
of  sporozoites  from  the  sporoblasts.  M.,  a  ripe  oocyst  full  of  minute  sporozoites,  spz  ,  which  arc 
escaping  by  the  bursting  of  the  cyst. ;  rp.,  residual  protoplasm  containing  an  abortive  nucleus 
(B.-G.  after  Schaudinn,  the  remaining  figures  after  Grassi.  The  figures  are  not  all  drawn  to  the 
same  scale). 

constitutes  the  residual  protoplasm,  and  plays  no  further  part  in 
the  life  history.     The  filiform  bodies  so  formed  are  the  microgametes, 


THE   PROTOZOA  145 

and  they  move  about  actively  when  they  become  free.  When  one 
approaches  a  macrogamete  this  cell  puts  out  a  small  protuberance, 
the  "  cone  of  reception,"  generally  from  that  part  of  the  circum- 
ference nearest  the  nucleus,  and  to  this  the  microgamete  adheres. 
Gradually  the  microgamete  is  absorbed,  and  its  male  pro-nucleus 
travels  to  a  position  near  the  female  pro-nucleus.  Shortly  after 
this  fertilisation  is  completed  by  the  fusion  of  the  two  pro-nuclei 
to  form  the  synkaryon  or  fertilisation  nucleus.  A  single  cell  thus 
results,  which  is  the  zygote,  but,  as  it  enters  into  a  motile  stage  in 
Plasmodium,  it  is  frequently  spoken  of  as  the  ookinete.  This  leaves 
the  alimentary  canal  of  the  mosquito  by  boring  through  the  epithelial 
lining  of  the  wall  of  the  stomach  and  embedding  itself  in  the  sub- 
mucosa.  Here  it  secretes  around  itself  a  fairly  soft  cyst,  the 
oocyst.  and  becomes  truly  parasitic  on  the  mosquito,  feeding  on  its 
juices  and  increasing  in  size  until  it  forms  a  knob  as  large  as  a  grain 
of  millet  projecting  on  the  outside  of  the  stomach.  The  formation 
of  the  zygote  in  Plasmodium  differs  considerably  from  that  in 
Monocystis,  for  it  results  from  the  union  of  two  gametes  very 
different  in  appearance  ;  a  male  or  microgamete,  and  a  female  or 
macrogamete.  Thus  it  constitutes  an  example  of  Anisogamy  or 
the  fusion  of  dissimilar  gametes,  Anisogametes. 

After  a  growth  period  the  ookinete  enters  on  a  phase  of  asexual 
reproduction,  leading  to  the  formation  of  sporozoites.  Its  nucleus 
and  cytoplasm  divide  up,  producing  a  number  of  masses  known  as 
the  sporoblasts,  not  completely  isolated  from  one  another,  but  united 
by  fine  protoplasmic  strands  and  leaving  a  small  portion  of  residual 
protoplasm.  The  sporoblast  in  turn  undergoes  multiple  fission  ;  a 
very  large  number  of  daughter  nuclei  are  formed,  which  take  up  a 
peripheral  position,  and  a  corresponding  number  of  delicate  pro- 
cesses are  given,  into  each  of  which  a  nucleus  migrates.  The  residual 
protoplasm  breaks  down  in  each  case,  setting  free  an  enormous 
number  of  very  minute  sporozoites.  Finally,  the  oocysts  burst, 
releasing  the  sporozoites  into  the  blood  stream  of  the  mosquito, 
where  they  are  carried  about  until  they  come  to  the  salivary  glands. 
They  leave  the  blood  at  this  point  and  accumulate  in  vast  numbers 
in  the  gland. 

Now  when  the  mosquito  bores  into  a  small  vessel  to  suck  the 
blood  it  injects  a  drop  of  saliva  through  the  proboscis  into  the 
wound,  and  this  has  the  effect  of  preventing  the  blood  from  coagu- 
lating. It  is  very  obvious  that  if  the  Anopheles  happens  to  be 
infected  a  great  many  sporozoites  get  poured  into  the  wound  with 
the  saliva,  and  so  the  man  becomes  infected  in  this  manner.  Thus 
the  whole  cycle  is  ready  to  start  all  over  again.  An  interesting  point 
in  this  connection  is  that  the  asexual  production  of  the  sporozoites 

L 


146  AN   INTRODUCTION   TO  ZOOLOGY 

in  the  mosquito  takes  some  days,  and  so  if  a  person  is  stabbed  inside 
this  time,  unless  the  mosquito  was  previously  infected,  no  harm 
follows,  and  this  has  been  shown  experimentally. 

Various  precautionary  measures  are  adopted  to  prevent  the 
spread  of  these  diseases.  Firstly,  the  number  of  mosquitos  in  an 
area  may  be  reduced  by  removing  all  small  puddles  and  accumula- 
tions of  water  in  old  cans,  barrels,  etc.,  for  it  is  only  in  still  water 
that  the  eggs  of  the  mosquito  are  laid  and  the  larvae  live.  Larger 
areas  of  stagnant  water  can  often  be  dealt  with  by  pouring  a  little 
petrol  upon  them  ;  this  spreads  out  over  the  surface  and  forms  a 
thin  film  that  effectively  prevents  the  mosquito  larva  or  pupa  from 
coming  to  the  surface  to  breathe,  and  so  kills  it.  Various  ointments 
may  be  smeared  over  the  exposed  parts,  which  tend  to  reduce  the 
number  of  bites,  and,  most  important  of  all,  an  infected  man  must 
be  isolated  at  once  and  kept  in  curtained  rooms  from  which  mosquitos 
are  rigidly  excluded. 

The  two  parasitic  protozoa  we  have  just  studied  illustrate  several 
of  the  main  characteristics  of  parasites  in  general.  In  the  first  place, 
owing  to  the  peculiar  conditions  under  which  they  live,  they  are  for 
the  most  part  devoid  of  all  adaptations  for  a  free-living  and  food- 
seeking  type  of  life :  they  are  inactive,  save  for  certain  limited 
periods,  and  consequently  lack  organs  of  locomotion  ;  their  food 
is  already  in  an  assimilable  form,  so  they  do  not  possess  cytostome, 
cytopharynx  or  food  vacuoles.  The  result,  therefore,  is  a  greater 
or  less  degree  of  simplification,  or  better,  perhaps,  degeneration,  of 
structure  as  compared  with  free-living  forms.  Lastly,  their  immo- 
bility, although  satisfactory  in  some  ways,  is  a  distinct  bar  to  their 
reinfecting  fresh  hosts,  and  so  maintaining  the  species.  In  Mono- 
cystis  the  transference  of  the  spores  to  another  worm  is  largely  a 
matter  of  pure  chance,  and  even  in  Plasmodium  chance  plays  a  large 
part,  and  it  is  necessary  to  ensure  that  any  mosquito  biting  an 
infected  man  should  itself  become  infected,  and  should  subsequently 
reinfect  in  turn.  Thus  it  is  not  merely  necessary  to  reproduce,  but 
also  desirable  that  the  mode  of  multiplication  should  also  serve  as  a 
means  of  dispersal.  The  element  of  chance  iit  both  cases  is  met  by  a 
typically  parasitic  phenomenon,  the  production  of  an  enormous 
number  of  young.  In  Monocystis  also,  in  order  to  withstand  the 
weather  changes,  the  spores  are  provided  with  a  tough  resistant 
envelope  that  can  protect  the  sporozoites  and  keep  them  alive. 
Some  such  protected  stage  is  often  met  with  in  parasites,  but  not 
invariably,  for  in  some,  as  in  Plasmodium,  where  the  parasite  never 
lives  outside  the  body  of  its  hosts,  the  second  host  that  has  been 
acquired  obviates  the  necessity  of  providing  against  the  inclemencies 
of  the  weather. 


THE   PROTOZOA  147 

In  Plasmodium,  moreover,  two  different  forms  have  been  evolved, 
which  have  been  termed  the  multiplicative  and  the  propagative  forms 
respectively ;  the  former  serves  to  multiply  in  the  host,  and  so  bring 
about  a  thorough  and  continuous  infection,  and  the  latter  serves  to 
infect  new  hosts.  The  trophozoites  in  the  blood  of  the  vertebrate 
by  multiple  fission  produce  merozoites  or  agametes,  serving  ex- 
clusively for  self-infection.  Such  part  of  the  life  history  is  termed 
schizogony,  and  the  trophozoites  known  as  schizonts.  The  game- 
tocytes  in  the  stomach  of  Anopheles  produce  well-differentiated 
anisogametes,  and  these  by  syngamy  form  zygotes.  The  multiple 
fission  of  the  zygotes  leads  to  the  formation  of  sporozoites,  and  these 
are  utilised  for  cross-infection.  We  distinguish  this  part  of  the  life 
cycle  as  sporogony,  and  the  gametocytes  may  be  termed  sporonts. 
Plasmodium  then  is  also  digenetic,  exhibiting  in  a  marked  form 
"  alternation  of  generations/'  one  sexual  alternating  with  an 
indefinite  number  of  asexual  generations. 


CHAPTER  VI 
THE   CCELENTERATA 

A  Simple  Ccelenterate,  Hydra — -A  Compound  Coelenterate,  Obelia. 

FROM  the  single-celled  Protozoa  we  now  ascend  the  first 
step  in  the  scale  of  animal  life  and  turn  to  the  Metazoa  or  multi- 
cellular  forms.  These  are  animals  whose  many  cells  are  arranged 
in  at  least  two  layers,  which  are  differentiated  in  structure  and 
function.  The  first  phylum  of  the  Metazoa  with  which  we  are  now 
concerned  is  the  Phylum  Ccelenterata  and  Hydra,  a  fairly  widely 
spread  genus,  furnishes  a  good  example  of  a  simple,  little  specialised 
type. 

A  Simple  Ccelenterate — Hydra. 

The  various  members  of  the  genus  Hydra  are  all  inhabitants 
of  fresh  water,  hence  their  name  of  "  fresh  water  polyps,"  and  are 
to  be  found  adhering  to  aquatic  plants  and  other  submerged  objects 
in  our  ponds,  ditches  and  streams.  Three  species,  not  differing  m 
essentials  but  only  in  details  of  structure,  are  commonly  to  be  met 
.with  in  this  country  :  H.  viridis  is  of  a  bright  green  colour  ;  H.  vul- 
garis  (or  H.  grisea)  is  of  a  pale  greyish  colour ;  and  H.  oligactis  (or 
H.fusca)  is  dark  yellow  or  brown.  An  examination  with  a  hand  lens 
is  sufficient  to  show  that  the  colouring  matter  in  the  first  and  last 
is  in  the  inner  parts,  and  that  the  outer  layer  of  the  body  wall  is 
transparent  and  practically  colourless.  The  animals  vary  greatly 
in  appearance,  according  to  the  extent  they  are  stretched  out.  In 
a  fully  expanded  condition  they  appear  as  long,  slender,  cylindrical 
threads,  from  6  to  7  mm.  in  length,  attached  by  one  end  and  bearing  at 
the  other  a  circlet  of  much  finer  long  filiform  processes,  the  tentacles, 
varying  from  six  to  ten  in  number,  and  these  may  extend  as  far 
again  beyond  the  body.  The  Hydra  adheres  by  a  flattened  plate- 
shaped  foot  or  basal  disc,  and  at  its  distal  end,  within  the  whorl  of 
tentacles,  is  a  low  conical  projection,  the  "  oral  cone  "  or  hypostome, 
in  whose  centre  is  a  circular  opening,  the  mouth.  The  whole  is 
exceedingly  contractile  and  extensile  and  very  sensitive,  the 
slightest  shock  causing  it  to  contract  both  its  body  and  tentacles 

148 


THE  CCELENTERATA 


149 


until  it  appears  as  a  small  jelly-like  blob.  If  left  undisturbed  it  will 
gradually  expand  again,  throwing  out  its  tentacles,  which  wave 
about  in  the  water  in  search  of  prey.  These  movements  are  not 
the  only  ones  exhibited,  for  although  usually  fixed  it  is  able  to  move 
from  place  to  place.  When  it  does  so  it  lengthens  out  and  bends 
over  to  one  side,  attaching  itself  to  the  substratum  by  means  of  its 
tentacles  or  mouth  some  way  from  the  foot.  The  latter  is  then 
released  and  brought  up  nearer  to  the  tentacles,  where  it  becomes 
fixed,  so  that  the  animal  has  thus 
moved  a  short  distance.  This  may 
be  repeated  several  times,  and  so 
slow  progress  is  possible  in  a  manner 
recalling  that  of  a  looper  caterpillar. 
All  sorts  of  intermediate  conditions 
between  the  extremes  of  extension 
and  contraction  may  be  seen,  some- 
times limited  to  one  or  other  end  of 
the  body,  so  that  the  cylinder  is 
not  of  equal  thickness  throughout. 
Individuals  may  sometimes  be  seen 
in  the  process  of  budding,  with  tiny 
daughter  forms  growing  from  them, 
or  several  knob-like  projections,  the 
reproductive  organs,  may  be  present 
on  the  top  two-thirds  of  the  body. 

A  longitudinal  section  shows  that 
the  mouth,  the  sole  external  aper- 
ture, leads  into  a  hollow  cavity  co- 
extensive with  the  body,  which  is 
therefore  a  very  minute  tube.  The 
tentacles  are  also  hollow,  being  but 
long  attenuated  outgrowths  from  the 
body  wall.  This  central  hollow  is 


FIG.  47. — 'Hydra,  with  bud  and 
gonads. 


the  gut  cavity  or  enteron,  termed  B.,  basal  disc;  H.,hypostome;  o., ovary ; 
also  the  ccelenteron,  to  indicate 

that  it  corresponds  to  both  the  body  cavity  (ccelom)'  and  gut 
cavity  (enteron)  of  the  higher  animals,  and  this  particular  type 
of  structure  is  fundamentally  characteristic  of  the  whole  phylum, 
hence  its  name.  Another  common  feature  of  the  Coelenterata  is 
exemplified  by  Hydra  in  the  simplicity  of  its  structural  plan.  If  its 
main  axis  be  marked  out  by  an  imaginary  line  drawn  through  the 
mouth  and  the  centre  of  the  basal  disc,  we  find  that  the  tentacles 
are  arranged  around  it  radially,  i.e.  they  are  related  to  the  axis,  as 
are  the  radii  to  the  centre  of  a  circle.  This  is  a  condition  we  term 


150  AN   INTRODUCTION  TO  ZOOLOGY 

radial  symmetry,  which  is  not  only  general  in  Coelenterata,  but  also 
tends  to  be  assumed  by  all  animals  adopting  a  fixed  mode  of  life. 
We  cannot  speak  of  anterior  and  posterior  ends  or  dorsal  and  ventral 
sides,  but  only  of  oral  surface,  i.e.  on  the  side  of  the  mouth,  and 
aboral,  i.e.  on  the  side  opposite  to  the  mouth.  This  type  stands 
strongly  contrasted  with  that  in  ourselves  and  the  frog,  in  which  the 
parts  are  only  symmetrically  disposed  with  regard  to  one  plane 
passing  through  the  long  axis,  cutting  the  body  into  right  and  left 
halves,  which  in  consequence  we  term  bilateral  symmetry. 

Hydra  can  easily  be  kept  under  observation  in  water  in  a  watch- 
glass,  and  if  some  "  water-fleas  "  (small  Crustacea,  Daphnids)  be 
added  their  capture  can  be  watched.  The  tentacles  are  widely 
spread  out,  forming  a  primitive  kind  of  net,  and  sooner  or  later  a 
water-flea  comes  into  contact  with  one  of  them.  Immediately  it 
does  so  it  comes  to  a  standstill  and  all  movement  ceases,  as  if  it  were 
paralysed,  and  it  remains  adhering  to  the  tentacle.  This  gradually 
shortens  down  and  bends  over  so  that  the  animal  is  brought  to  the 
mouth,  which  enlarges  and  takes  it  in,  removing  it  from  the  tentacle. 
It  is  passed  inside  into  the  enteron,  and  for  some  time  causes  a 
distinct  swelling  in  the  body  of  the  Hydra.  Here  it  is  digested,  and 
later  the  shell  and  other  indigestible  residue  are  ejected  through  the 
mouth. 

The  animal  is  so  small  that  its  structure  cannot  be  made  out  by 
dissection,  as  in  the  case  of  the  frog,  and  it  has  to  be  studied  by  means 
of  sections  and  by  isolating  the  cells.  A  transverse  section  of 
Hydra  shows  that  the  body  wall  is  composed  of  two  layers  of  cells. 
The  outer  layer,  or  ectoderm,  is  thinner,  and  consists  of  a  number  of 
cells  tightly  packed  together,  forming  a  very  efficient  covering  for 
the  animal.  These  cells,  as  we  shall  see  later,  are  more  or  less  cone- 
shaped,  with  their  bases  outwards,  and  so  a  series  of  spaces  are  left 
between  their  inner  ends.  The  interstices  are  not  left  empty, 
however,  but  are  filled  with  smaller  cells.  Inside  the  ectoderm,  and 
so  forming  the  lining  of  the  gastral  cavity,  is  the  second  layer  of 
cells,  the  endoderm,  or  better,  the  entoderm.  This  is  composed  of 
much  larger,  more  columnar  cells,  but,  like  those  of  the  ectoderm, 
they  are  radially  arranged.  Between  the  ectoderm  and  entoderm 
is  a  layer  of  structureless  jelly-like  substance,  the  mesoglea,  forming 
a  sort  of  strengthening  sheath,  the  supporting  lamella.  Cells  may 
occasionally  be  found  in  it,  but  it  should  be  borne  in  mind  that  such 
cells  do  not  originate  in  it,  they  migrate  into  it  from  the  other  layers. 
It  cannot  then  be  regarded  as  a  cell  layer,  but  simply  as  a  sheet  of 
jelly  secreted  by  the  ectoderm  and  entoderm.  In  Hydra  it  remains 
quite  thin,  but  in  the  "  jelly-fish  "  it  is  enormously  thickened,  form- 
ing the  main  bulk  of  the  body.  This  type  of  body  wall,  consisting  of 


THE  CGELENTERATA  151 

only  two  cellular  layers,  ectoderm  and  entoderm,  is  designated  two- 
layered  or  diploblastic,  and  is  characteristic  of  ccelenterates.  In 
order  to  examine  the  structure  of  the  layers  more  fully  we  may 
either  crush  an  individual  under  a  cover  slip  by  means  of  a  tap  with  a 
pencil,  or,  better  still,  immerse  it  for  a  short  while  in  a  macerating 
fluid.* 

We  have  already  seen  that  the  ectoderm  is  composed  of  two 
groups  of  cells,  those  forming  the  main  epithelium  and  those  in  the 
interstices,  and  we  can  now  studv  them  more  in  detail.  The  larger 


N.D. 

FIG.  48. — Diagram  of  a  transverse  section  of  part  of  body  wall  of  Hydra. 

E.,  entoderm  ;  EC.,  ectoderm  ;  I.,  Interstitial  cells  ;  M.,  mesoglea  ;  N.,  nematocyst ; 
N.D.,  discharged  nematocyst ;  V.,  vacuole  ;  Z.,  zoochlorella?. 

cells  are  known  as  the  epithelio-muscular  cells,  and  are  roughly  cone- 
shaped.  Their  outer  sides,  i.e.  the  bases,  are  very  firmly  joined 
together,  so  that  it  is  a  matter  of  difficulty  to  isolate  them  save  in 
groups,  and  they  are,  moreover,  covered  by  a  thin  homogeneous 
cuticle.  Their  inner  ends,  the  apices,  are  blunt  and  give  off  fairly 
long  processes,  running  transversely  to  the  long  axis  of  the  cell. 
These  protoplasmic  filaments  are  the  portions  of  the  cell  especially 
set  aside  for  contraction,  and  hence  distinguished  as  the  muscle 
processes.  They  lie  embedded  in  the  mesoglea,  and  careful  examina- 
tion reveals  the  presence  in  them  of  minute  myofibrillee,  the  special- 
ised contractile  elements.  Thus  the  ectoderm  furnishes  Hydra  with 
an  external  layer  of  longitudinally  running  muscles,  by  the  contraction 
of  which  the  shortening  of  the  body  is  brought  about.  The  body  of 

*  E.g.  Schneider's  fluid,  one  part  of  '02  %  solution  of  osmic  acid  and  four 
parts  of  5  %  solution  of  acetic  acid  mixed  together.  The  animal  requires  to 
be  immersed  from  three  to  five  minutes  and  then  handled  with  care,  for  it 
disintegrates  readily. 


152  AN   INTRODUCTION  TO  ZOOLOGY 

the  cell  is  composed  of  a  fairly  clear  vacuolated  cytoplasm,  in  which 
are  a  few  coarse  granules  and  a  network  of  delicate  fibres,  continued 
through  the  apex  of  the  cell  into  the  myofibrils  of  the  muscular 
processes.  A  large  spherical  vesicular  nucleus  with  well-marked 
chromatin  threads  and  one  or  two  distinct  nucleoli  is  situated  near 
the  middle  of  the  cell.  In  the  basal  disc  the  ectoderm  cells  are 
slightly  modified,  are  columnar  in  shape,  and  fitting  close  together 
leave  no  interstices.  They  are  very  granular,  containing  minute 

droplets  of  a  sticky  secre- 
tion, which,  when  passed 
out,  enables   the  foot  to 
adhere  to  the  substratum. 
These  cells  are,  therefore, 
not      merely     musculo  - 
epithelial,  but  also  gland- 
FIG.  49.— Epithelio-muscular  cells  of  Hydra.      ular   m   character.       The 
M.,myoneme  fibrill* ;  N.,  nucleus.  CCtodermal     Cells     in     the 

tentacles  also  are  a  little 

flatter  than  in  the  body  wall,  and  have  very  markedly  developed 
muscular  processes.  They  are  so  closely  set  together  that  there  are 
no  spaces  between  their  inner  ends. 

The  remaining  cells  of  the  ectoderm,  termed  from  their  position 
the  interstitial  cells  or  sub-epithelial  cells,  are  small  and  round  and 
tightly  packed  together.  They  form  as  it  were  a  reserve  of  im- 
specialised  cells,  from  which  other  cells  of  the  ectoderm  can  be 
replaced,  and  they  give  rise  to  four  very  distinct  types. 

i.  The  most  numerous  and  striking  cells  arising  from  the  inter- 
stitial ones  are  the  wonderful  structures  known  as  the  "  nettle  cells," 
or  cnidoblasts,  in  each  of  which  is  enclosed  a  complicated  highly- 
refractive  capsule,  the  "  stinging  capsule/'  or  the  nematocyst. 
These  capsules  contained  in  their  parent  cells  are  to  be  found 
distributed  generally  in  the  ectoderm  with  the  exception  of  the  basal 
disc.  In  the  tentacles  they  are  particularly  numerous,  being 
arranged  in  groups,  the  so-called  nematocyst  batteries,  which  give 
the  tentacles  a  knobby  appearance.  The  cnidoblast  is  not  found  free 
in  the  ectoderm,  but  completely  embedded  in  one  of  the  musculo- 
epithelial  cells — a  cell  within  a  cell,  recalling  in  some  respects  a 
parasite. 

The  cnidoblast  consists  of  a  thin  layer  of  protoplasm  enclosing 
the  capsule,  and  in  it  is  situated  a  nucleus.  From  the  outermost 
part  of  the  cell  wall  comes  off  a  short  slender  bristle-like  projection, 
the  cnidocil  or  trigger  process.  It  pierces  the  musculo-epithelial 
cell  and  cuticle  and  sticks  out  beyond  the  body  wall.  It  recalls  the 
process  of  a  sense  cell,  and,  like  it,  is  receptive  and  capable  of 


THE   CCELENTERATA 


153 


conveying  a  message  into  the  cell.  The  nematocyst  is  formed  within 
the  cnidoblast  as  a  metaplasmic  product,  and  takes  the  shape  of  a 
refringent  oval  capsule  with  a  tough,  probably  double  wall.  At  its 
outermost  point  the  capsular  wall  is  pushed  in  or  introverted  to 
form  a  narrow  tubular  ingrowth  which  extends  nearly  the  length  of 
the  nematocyst.  It  then  quickly  narrows  down  and  is  continued 
as  a  very  fine  thread  spirally  coiled  around  the  tubular  ingrowth. 
The  remainder  of  the  inside  of  the  capsule  is  filled  with  a  semi- 
gelatinous  fluid  which  renders  the  whole  tense  and  turgid.  Under 
suitable  conditions  of  hunger,  etc.,  if  a  small  water  animal  touches 
the  cnidocil  it  causes  the  nematocyst  thread  to  be  shot  out  with 
explosive  violence.  The  semi-gelatinous  fluid  is  apparently  very 


Cn. 


A.  B.  C. 

FIG.  50. — Cnidoblasts  of  Hydra. 

A.,  large  variety  undischarged  ;  B.,  large  variety  discharged  ;  C.,  small  variety  discharged. 
Note  small  size,  absence  of  barbs  and  thickness  of  thread. 

Ba.,  barbs;  C.B.,  cnidoblast  cell;  Cn.,  cnidocil  or  trigger  process;  N.,  nematocyst;  Nu., 
nucleus  of  cnidoblast ;  N.T.,  nematocyst  thread. 

hygroscopic,  and  the  slight  disturbance  of  the  cnidocil  in  some  way 
or  other  allows  it  to  absorb  water,  with  the  result  that  a  pressure  is 
produced  practically  instantaneously  and  the  thread  is  ejected,  being 
turned  inside  out  in  the  process.  When  discharged  the  nematocyst 
appears  as  an  empty  capsule  on  one  side  of  which  is  a  projection 
like  the  short  handle  of  a  whip,  often  bearing  several  barbs  large 
and  small,  continued  on. as  a  delicate  thread-like  lash  many  times 
longer  than  the  cell  itself.  These  serve  as  offensive  and  defensive 
weapons,  and,  as  we  have  seen,  there  is  force  enough  in  the  thread 
when  discharged  for  it  to  penetrate  into  the  body  of  the  water-flea. 
The  consequent  paralysis  of  the  animal  is  presumably  brought  about 
by  the  thread  carrying  with  it  a  small  quantity  of  an  irritant  poison. 
The  presence  of  this  poison  has  not  been  shown  in  Hydra,  but  some 
of  its  allies,  certain  sea-anemones  and  jelly-fish,  when  they  sting  a 


154 


AN   INTRODUCTION  TO  ZOOLOGY 


human  being,  produce  a  result  similar  to  the  sting  of  a  nettle,  only 
it  may  be  a  great  deal  more  severe. 

When  once  discharged  the  nematocyst  thread  cannot  be  intro- 
verted again,  and  so  is  useless  and  shed.  They  must,  therefore,  be 
replaced  constantly,  and  so  we  can  find  in  the  interstitial  cells, 
especially  in  the  distal  parts  of  the  body,  many  that  have  capsules 
in  different  stages  of  formation  within  them.  They  are  used  up  most 
rapidly  on  the  tentacles  where,  however,  there  are  no  interstitial 
cells,  and  it  is  not  quite  clear  how  the  replacement  is  effected.  It  is 
almost  certainly  brought  about  by  the  migration  of  the  fairly  young 
cnidoblasts,  although  this  has  not  been  demonstrated.  Two 
varieties  of  nematocyst  are  met  with  in  Hydra  ;  one,  the  larger,  is 
of  an  oval  form  similar  to  that  described  above,  and  the  other  is 
smaller,  a  longer  oval,  and  the  thread,  when  it  is  discharged,  is 
thicker,  shorter  and  without  barbs  at  the  base. 

The  nematocyst  is  a  structure  confined  to  the  Ccelenterata,  but 
widely,  if  not  universally,  distributed  in  that  phylum,  and  retaining 

throughout  the  same 
fundamental  structure, 
although  the  size,  pro- 
portion of  its  parts,  and 
the  presence  or  absence  of 
barbs  varies  from  species 
to  species.  It  forms,  as 
can  readily  be  seen,  a  very 
efficient  weapon. 

2.  The  second  type  of 
cell  produced  by  the  in- 
terstitial cells  is  the  nerve 
element.  All  the  cells  of 
Hydra  appear  to  be  very 
sensitive,  but  the  nerve 
cells  are  those  in  which 
this  property  is  most 

highly  developed.  They  are  small,  bipolar  or  multipolar  primitive 
ganglion  cells,  and  all  their  processes  may  branch  freely,  but  do  not 
appear  to  be  structurally  differentiated  into  axons  and  dendrons. 
Around  the  mouth  they  are  rather  more  numerous  than  elsewhere, 
but  even  here  they  do  not  form  ganglia  or  aggregations,  being  merely 
scattered  about  in  the  mesoglea.  In  this  way  they  form  a  loose 
network  of  a  nervous  nature,  serving  to  keep  all  parts  of  the  animal 
in  communication  with  one  another,  and  so  co-ordinating  the 
activities  of  the  animal  as  a  whole. 

3.  A  few  cells  on  the  hypostome  and  the  basal  disc  appear  to  b 


FIG.  51. — -Primitive  nerve  cells  of  Hydra, 
adapted  from  Schneider. 


THE  CCELENTERATA  155 

of  a  sensory  nature.  They  are  long  thread-like  cells  situated 
between  the  epithelio-muscular  cells  and  branching  out  at  their 
inner  ends  like  the  nerve  cells,  to  which  no  doubt  they  are  closely 
related. 

4.  Lastly,  it  is  to  the  interstitial  cells  that  we  have  to  look  for  the 
primitive  germ  cells.  Hydra  is  hermaphrodite,  that  is  to  say,  the  one 
individual  produces  both  spermatozoa,  the  male  cells,  and  ova,  the 
female  cells.  We  express  the  same  idea  when  we  term  it  monoecious, 
i.e.  of  one  sexual  form  as  opposed  to  most  of  the  higher  animals, 
which  are  dioecious,  i.e.  of  two  forms,  male  and  female.  The 
spermatozoa  are  aggregated  in  large  numbers,  producing  one  or 
more  enlargements,  the  testes,  lying  in  the  oral  third  of  the  body 
beneath  the  ring  of  tentacles.  The  ova  when  fully  ripe  are  very 
large  single  isolated  cells,  which  also,  produce  swellings,  the  ovaries, 
situated  about  the  middle  of  the  body  below  the  testes. 

The  entoderm  of  Hydra  is  composed  in  the  main  of  three  types  of 
cells : — 

1.  Again,  the  main  part  is  composed  of  musculo-epithelial  cells 
which  are,  however,  much  larger  and  more  columnar  than  those  of 
the-  ectoderm.     Their  muscular  processes  are  also  embedded  in  the 
mesoglea,  but  are  arranged  transversely  to  the  long  axis  of  the  body, 
i.e.  circularly,  and  are  not  so  strongly  developed.     The  edge  of  the 
cell  towards  the  gastral  cavity  is  of  indefinite  shape  and  capable  of 
putting  forth  amoeboid  processes  which  can  seize  small  pieces  of  the 
food.     The  cytoplasm  is  very  vacuolated,  sometimes  containing  one 
large  vacuole,  and  also  granular  with  tiny  particles  of  food  in  it. 
In  H.  viridis  and  H.  oligactis  there  are  also  present  in  the  cell 
numerous  minute  ovoidal  capsules  of  a  bright  green  or  yellow  colour, 
according  to  the  species,  lying  towards  the  basal  part  of  the  cell. 
They  are  really  minute  plants,  Algae,  that  have  made  these  cells 
their  dwelling-place.     A  noticeable  vesicular  nucleus  with  a  distinct 
nucleolus  is  also  present,  and  usually  more  or  less  basally  situated. 

2.  The  second  type  of  cell  is  secretory,  and  such  glandular  cells 
are  to  be  found  fairly  generally  scattered,  but  most  commonly  in 
the  neighbourhood  of  the  hypostome.     They  are  much  like  the  other 
cells,  but  contain  large  droplets  of  a  slimy  or  albuminous  nature. 

3.  Lastly,  we  find  in  the  entoderm  a  certain  number  of  inter- 
stitial cells  wedged  in  between  .the  bases  of  the  epithelio-muscular 
cells,  but  they  are  not  nearly  so  plentiful  as  in  the  ectoderm,  and 
only  occur  here  and  there. 

The  limited  movement  of  Hydra  as  a  whole,  and  that  of 
the  tentacles,  has  already  been  mentioned.  Its  feeding  is  of  an 
interesting  type.  When  the  prey  reaches  the  gastral  cavity  it  comes 
under  the  action  of  the  secretion  of  the  gland  cells  which  contain 


156  AN   INTRODUCTION   TO   ZOOLOGY 

enzymes  allied  to  trypsin.  As  a  result  of  this  activity  the  prey 
disintegrates,  and  as  the  small  pieces  of  it  float  about  in  the  cavity 
they  are  seized  by  the  epithelio-muscular  cells  and  engulfed  in  the 
same  manner  as  Amoeba  ingests  its  food.  They  are  then  digested  in 
the  cell.  Thus  we  find  two  distinct  types  of  digestion  within  the 
body  of  Hydra  ;  firstly,  intercellular  digestion  in  a  cell-lined  gastral 
cavity,  the  only  kind  met  with  in  the  higher  metazoa  ;  and  secondly, 
the  intracellular  digestion,  in  which  the  food  is  broken  up  within  the 
cell  itself  as  in  the  Protozoa.  The  only  phenomenon  allied  to  this 
latter  type  in  the  higher  animals  is  that  we  have  already  dealt  with 
in  the  case  of  the  white  blood  corpuscles  of  Rana,  termed  phagocytes, 
for  in  this  case  the  bacteria  are  swallowed  and  eaten  by  the  cells. 
Any  insoluble  residue,  including  starch,  for  apparently  Hydra  does 
not  secrete  a  starch-splitting  enzyme,  is  voided  through  the  mouth, 
which  therefore  functions  also  as  an  anus. 

Another  phenomenon  calls  for  notice  here,  and  that  is  the 
relation  between  the  tiny  green  plants  and  the  entoderm  cells. 
These  green  algae,  when  they  are  included  in  an  animal  cell,  as  they 
are  in  certain  protozoa  and  worms,  are  termed  in  general  Zoo- 
chlorellae,  and  the  particular  species  particularly  associated  with 
H.  viridis  is  Chlorella  vulgaris.  They  are  in  no  sense  parasites,  for 
they  are  not  detrimental  either  to  the  structure  or  function  of  Hydra  ; 
in  fact  they  are  quite  the  reverse.  The  animal  not  only  lodges  and 
protects  the  algae,  but  produces  as  waste  products  carbonic  acid  gas 
and  various  nitrogenous  matters  which  serve  the  plant  as  food. 
On  the  other  hand,  the  plant  removes  these  waste  materials  for  the 
animal,  and  in  its  turn  makes  Oxygen,  which  is  of  use  to  Hydra,  and 
perhaps  also  certain  carbohydrates.  Certain  it  is  that  a  H.  viridis 
freed  of  its  Chlorellcz  does  not  live  so  vigorously  as  with  them.  Thus 
we  have  a  living  together  which  results  in  a  mutual  benefit  to  both 
parties  concerned,  and  this,  which  we  term  Symbiosis,  is  to  be  sharply 
distinguished  from  parasitism. 

Respiration  and  excretion  in  Hydra  other  than  that  just  dealt 
with  appear  to  be  carried  out  as  in  Amoeba,  by  diffusion  over  the 
general  surface  of  the  body,  and  there  is  no  special  organ  for  the 
performance  of  either  function. 

Reproduction  in  Hydra  takes  place,  as  we  have  seen,  by 
two  methods,  by  budding  and  by  the  production  of  gametes.  In 
asexual  multiplication  there  is  first  of  all  an  increase  in  the  inter- 
stitial cells,  and  then  the  two  layers  of  the  body  wall  grow  out  into 
a  tiny  knob-like  projection  whose  internal  cavity  is  continuous  with 
that  of  the  parent.  At  first  it  is  rounded,  but  a  series  of  tiny  sprouts 
at  the  distal  extremity  mark  the  beginning  of  the  tentacles,  which 
soon  grow.  In  the  middle  of  these  appears  an  opening,  the  mouth, 


THE   CCELENTERATA  157 

and  so  we  have  produced  a  tiny  Hydra  growing  out  from  the  body  of 
the  parent  with  which  its  layers  and  cavity  are  directly  continuous. 
When  conditions  are  favourable,  more  than  one  bud  may  be  formed ; 
indeed,  specimens  are  sometimes  met  with  in  which  the  buds  them- 
selves have  budded,  so  that  temporary  associations  are  formed. 
Sooner  or  later,  however,  the  daughter  individuals  drop  off  and  lead 
an  independent  life.  Hydra  exhibits  another  phenomenon  some- 
what allied  to  this  production  of  buds,  and  although  it  can  hardly 
be  considered  as  an  ordinary  means  of  reproduction,  doubtless  plays 
its  part  in  cases  of  injury.  If  an  individual  be  cut  into  a  number  of 
pieces,  provided  they  are  not  too  small,  and  contain  fairly  repre- 
sentative parts  of  both  ectoderm  and  entoderm,  each  piece  is  capable 
of  regrowing  into  a  complete  animal.  This  is  a  power  we  term 
Regeneration,  which  is  fairly  common  among  lowly  organised  beings, 
but  becomes  more  and  more  limited  as  we  ascend  the  animal  scale. 
Sexual  reproduction  also  occurs  as  a  normal  method  in  Hydra, 
but  no  alternation  or  definite  relation  between  it  and  the  asexual 
has  been  shown  to  exist,  and  it  is  the  more  uncommon  of  the  two. 
The  testes  at  first  consist  of  a  mass  of  interstitial  cells  which  at  a 
certain  stage  are  termed  the  spermatogonia.  These  divide,  producing 
two  spermatocytes,  and  each  of  these  in  turn  gives  rise  to  four  sperms. 
The  adult  spermatozoa  are  composed  of  a  small  oval  head  to  which 
is  attached  a  fairly  long  vibratile  tail.  They  are  aggregated  in 
large  numbers  between  the  epithelio-muscular  cells  of  the  ectoderm, 
and  so  form  noticeable  colourless  swellings,  the  testes.  When  quite 
ripe  the  ectoderm  cells  split,  releasing  the  free-swimming  sperms  into 
the  water.  Like  the  testes,  the  ovaries  also  commence  as  a  collection 
of  intestitial  cells  which  form  the  oogonia,  but  only  one  of  these 
is  destined  to  become  the  sex  cell.  This  divides  up,  and  one  of  the 
daughter  cells,  i.e.  the  obcytes,  so  produced  assumes  a  central 
position  in  the  mass.  It  sends  out  pseudopodia  and  feeds  upon  its 
sister  cells  much  in  the  same  way  as  an  Amceba  ingests  its  food. 
The  result  of  this  feeding  is  that  the  cell  grows  to  a  relatively 
enormous  size,  and  the  products  of  digestion  are  stored  up  within 
it  in  the  form  of  numerous  tiny  spherical  masses,  the  yolk  spheres  or 
deutoplasts.  This  amoeboid  stage  is  not  at  all  common  in  developing 
ova  among  animals,  and  when  it  is  ended  the  oocyte  withdraws  its 
pseudopodia  and  rounds  off.  It  then  undergoes  two  successive 
unequal  divisions,  giving  off  two  tiny  masses  of  protoplasm,  each 
with  a  certain  amount  of  nuclear  material,  the  first  and  second  polar 
bodies,  and  so  becomes  a  ripe  ovum  ready  for  fertilisation.  By  this 
time  it  has  been  surrounded  by  a  thin  gelatinous  layer,  and  increased 
so  much  in  size  that  it  has  caused  the  ectoderm'cells  to  split,  exposing 
part  of  its  surface  to  the  water  ready  for  the  sperm.  Fertilisation 


158 


AN   INTRODUCTION  TO  ZOOLOGY 


is  effected  in  the  normal  manner  by  the  penetration  of  a  single 
spermatozoon,  and  the  fusion  of  the  two  pronuclei  to  form  a  zygote 
with  its  segmentation  nucleus. 

The  process  of  cleavage  or  segmentation  now  sets  in,  and 


FIG.  52. — Development  of  Hydra,  adapted  from  Brauer, 

A.,  the  mature  ovum  full  of  yolk  granules  and  attached  to  the  parent ;  B.,  section  through  the 
blastula,  some  of  whose  cells  are  already  beginning  to  pass  inwards  ;  C.,  section  through  gastrula 
showing  the  irregular  entoderm  cells  commencing  to  fill  the  blastocoel ;  D.,  the  embryo  within 
protective  sheath  flattening  out  and  the  entoderm  cells  arranged  to  form  a  layer  around  the  enteron; 
E.,  embryo  escaping  from  sheath  ;  D.  and  E.,  less  highly  magnified  than  remainder. 

bl.,  blastocoel ;  ec.,  ectoderm  ;  en.,  entoderm. 

it  is  during  this  period  in  H.  viridis  that  the  ovum  becomes  infected 
with  the  green  algal  cells.  Two  meridional  divisions  cut  the  original 
cell  into  four,  and  a  subsequent  equatorial  split  into  eight,  cells 


THE  CCELENTERATA  159 

surrounding  a  central  cavity.  Ultimately  a  hollow  sphere  of  cells 
is  produced,  the  blastula,  in  which  is  a  large  central  space,  the 
blastoccel  or  segmentation  cavity.  The  blastoccel  is  next  filled  by 
cells  derived  from  the  periphery  in  two  ways.  Some  of  the  outside 
cells  divide  tangentially,  and  the  more  centrally  situated  of  the  two 
daughter  cells  pass  inwards,  a  process  called  multipolar  delamination, 
because  it  occurs  at  many  points  ;  others  of  the  cells  from  different 
parts  migrate  into  the  centre,  and  this  is  termed  multipolar  im- 
migration. Finally,  as  a  result  of  these  two  processes,  the  sphere 
becomes  solid.  The  ingoing  cells  carry  with  them  practically  all 
the  yolk  spheres,  with  the  result  that  the  embryo  comes  to 
consist  of  an  outer  layer  of  more  clear  cells,  the  ectoderm,  sur- 
rounding an  inner  solid  mass  of  granular  cells,  the  entoderm,  and 
in  this  way  the  diploblastic  condition  is  attained.  This  secretes 
around  itself  a  double-walled  sheath,  the  outer  layer  being  horny 
and  covered  with  spinous  projections,  and  the  inner,  membranous. 
All  these  changes  take  place  while  the  egg  is  still  attached  to  the 
parent,  but  now  it  drops  off  and  falls  to  the  bottom  of  the  pond. 
After  a  period  of  rest  the  entoderm  cells  become  arranged  to  form 
a  layer  around  a  central  cavity,  the  enteron  or  gastral  cavity,  and 
then  the  capsule  breaks,  allowing  the  animal  to  creep  out.  At  first 
it  has  no  tentacles  nor  mouth,  but  these  soon  appear  much  in  the 
same  way  as  they  do  in  a  bud,  and  so  we  have  produced  a  new 
Hydra. 

This,  then,  completes  the  life  history  of  a  very  simple  Metazoon, 
and  although  a  primitive  and,  perhaps,  even  a  degenerate  form,  it 
serves  as  a  type  of  the  Phylum  Ccelenterata,  the  majority  of  whose 
members  are,  however,  more  complex.  In  some  ways  it  marks  a 
very  important  step  in  advance  of  the  Protozoa,  the  first  stage  in  the 
direction  of  that  enormous  complexity  of  cellular  structure  that 
marks  the  higher  animal. 

A  Compound  Coelenterate — Obelia. 

We  now  pass  on  to  consider  a  more  highly-organised 
member  of  the  same  Phylum,  namely,  Obelia.  The  genus  Obelia  is 
a  very  common  one  around  our  shores,  and,  confined  to  the  sea,  it 
is  to  be  found  just  below  low- water  mark,  sometimes,  indeed, 
exposed  at  spring  tides,  growing  on  stones,  wooden  piles  and  other 
submerged  objects.  0.  geniculata,  perhaps  its  most  common 
species,  is  frequently  encountered  as  a  pale  greyish-brown  moss-like 
growth  forming  large  conspicuous  patches  on  the  long  fronds  of  the 
oar  weed  (Laminaria).  Its  plant-like  appearance  is  common  among 
the  Hydrozoa,  the  class  of  the  Ccelenterata  to  which  it  belongs,  and, 
in  consequence,  these  animals  have  long  been  known  to  naturalists 


160  AN    INTRODUCTION   TO  ZOOLOGY 

as  plant-animals,  Zoophytes.  Quite  small,  as  a  general  rule,  these 
organisms  nevertheless  when  viewed  alive  under  even  a  low 
magnification  form  some  of  the  most  beautiful  objects  to  be  met 
with  in  the  animal  kingdom.  Closely  adherent  to  the  seaweed  frond 
will  be  found  a  network  of  fine  threads  which  constitute  a  kind  of 
root,  the  hydrorhiza,  serving  for  attachment.  From  this  arise  a 
number  of  thread-like  stems,  the  hydrocauli,  which  on  closer  examina- 
tion are  seen  to  give  off  a  number  of  small  lateral  branches,  each  one 
terminating  in  a  tiny  knob.  Even  a  low  magnification  will  reveal 
the  fact  that  each  of  the  small  lumps  is  in  reality  a  tiny  flower-like 
hydriform  being,  closing  in  response  to  the  slightest  touch  when 
living,  and  variously  termed  a  hydranth,  a  polyp  or  a  zooid.  Thus 
we  have  a  compound  organism  composed  of  a  large  number  of 
hydroid  individuals  united  together  by  common  stems  and  roots,  and 
such  an  assemblage  we  call  a  colony  or  stock.  We  saw  that  in  Hydra 
sometimes  the  individual  could  bud  off  daughters,  and  that  before 
these  were  shed  they  could  in  themselves  give  rise  to  buds  producing 
a  sort  of  temporary  colony,  and  the  stock  in  Obelia  is  formed  much 
in  the  same  way.  A  single  hydriform  being  gives  rise  to  a  bud 
which  grows  upwards,  turning  the  head  of  the  parent  to  one  side 
and  also  produces  a  bud,  and  this  in  its  turn  another  bud,  and  so  on. 
In  this  way  we  have  produced  a  long  stem,  the  end  of  which  is  always 
growing,  and  in  Obelia,  as  the  buds  are  produced  on  alternate  sides, 
we  have  formed  a  characteristic  zigzag  stem  with  zooids  borne 
on  short  stalks  at  its  angles.  From  their  mode  of  origin,  too,  it  will 
be  seen  that  the  gastral  cavities  of  all  the  hydroids  will  remain  in 
continuity  with  one  another  by  means  of  the  hollow  stem.  The 
hydrorhiza  is  also  a  series  of  tubes,  so  that  the  whole  colony,  some- 
times consisting  of  an  enormous  number  of  individuals,  has  a  series 
of  gastral  cavities  intercommunicating  by  means  of  gastro-vascular 
canals. 

In  order  to  keep  the  stem  erect  we  find  that  the  colony  secretes 
around  itself  a  supporting  and  protective  exoskeleton  composed  of  a 
horn-like  substance,  chitin.  We  distinguish  the  skeletal  substance 
as  the  perisarc  as  opposed  to  the  ccenosarc,  i.e.  the  living  parts  which 
it  surrounds.  In  the  regions  of  the  hydroids  the  perisarc  expands 
into  cup-shaped  receptacles,  the  hydrothecae,  into  which  the  polyps 
can,  and  do,  completely  withdraw  when  disturbed.  Then,  too, 
the  perisarc  of  the  stalk  joining  the  hydranth  to  the  main  axis 
exhibits  a  series  of  ringlike  constrictions  or  annulations,  as  may  also 
the  main  stem  itself  at  these  points.  The  number  and  arrangement 
of  these  differs  in  various  species. 

The  particular  type  of  individual  noticed  above  is  almost 
exclusively  concerned  with  the  procuring,  digestion  and  distribution 


THE   CCELENTERATA  161 

of  food,  and  so  is  spoken  of  as  the  nutritive  zooid,  but  two  other  kinds 
of  person  are  to  be  found.  In  the  first  place  sexual  reproduction  is 
brought  about  by  a  special  set  of  individuals  in  the  form  of  small 
saucer-shaped  zooids  produced  as  buds  by  the  colony,  but  afterwards 
cut  off  as  minute  free-swimming  jelly-fish  or  medusae,  and  these  are 
the  reproductive  individuals.  Secondly,  these  reproductive  forms  are 
not  budded  from  any  part  of  the  colony  indiscriminately,  but  only 
from  specially  modified  hydroids,  the  blastostyles,  which  have  no 
mouth  or  tentacles,  and  consist  of  a  long  slender  body.  Here,  then, 
as  a  result  of  the  division  of  labour  among  its  constituent  persons,  we 
encounter  the  phenomenon  of  polymorphism,  or  the  production  of 
different  forms  of  individual  in  one  and  the  same  species.  It  is  even 
more  marked  in  the  case  of  some  of  the  allies  of  Obelia,  and  is  met 
with  in  various  groups  of  the  animal  kingdom,  as,  for  example,  in  the 
social  insects,  ants  and  bees.  The  particular  type  of  polymorphism 
in  the  present  case,  since  it  involves  three  varieties  of  individual,  the 
hydranth,  the  blastostyle  and  the  medusa,  may  be  called  trimorphism. 

The  blastostyles  are  always  to  be  found  in  a  constant  position. 
Usually  they  are  absent  from  the  upper  or  younger  end  of  the 
hydrocaulus,  but  almost  always  present  lower  down.  They  occur 
in  the  angle  between  the  polyp  stalk  and  the  main  stem  (in  the  axil, 
as  we  should  say,  of  leaves)  toward  the  upper  side.  Each  is  enclosed 
in  a  special  urn-shaped  investment  of  the  perisarc,  the  gonoiheca, 
which  is  almost  sessile  and  has  not  a  distinct  stalk  like  that  of  the 
hydrotheca. 

The  structural  plan  and  histology  of  the  whole  colony  is 
very  similar  to  that  of  Hydra,  but  perhaps,  on  the  whole,  more 
simple.  The  ccenosarc  of  the  hydrocaulus  and  hydrorhiza  consists 
of  a  series  of  tubes  which  are  only  loosely  connected  with  the 
perisarc  save  at  the  growing  ends  of  the  branches.  It  is  composed 
of  an  outer  clear  layer  of  ectoderm  which  lacks  cnidoblasts,  and  an 
inner  more  granular  layer  of  entoderm  whose  cells  are  sometimes 
ciliated,  and  bear  flagella  in  order  to  assist  in  the  circulation  of  the 
nutritive  fluid.  The  two  layers  are  separated  by  the  mesoglea,  and 
in  neither  are  the  muscular  processes  at  all  well  developed. 

The  nutritive  zooid  somewhat  resembles  Hydra,  but  has  a  circlet 
of  about  thirty  well-marked  filiform  tentacles.  They  are  fairly 
contractile  and  plentifully  supplied  with  cnidoblasts.  Their 
ectoderm  is  on  the  whole  similar  to  that  in  Hydra,  but  the  entoderm, 
which  completely  fills  them,  is  composed  of  a  single  axial  line  of 
peculiar  cells.  These  cells  are  large  and  very  vacuolated  with  a 
nucleus  near  their  centre,  and  are  tough  and  elastic.  The  hypostome 
is  enormously  developed,  forming  a  dome-like  enlargement  between 
the  bases  of  the  tentacles.  Within,  it  forms  a  sort  of  initial  gastral 


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AN   INTRODUCTION   TO  ZOOLOGY 


cavity,  almost  as  large  as  the  proper  one  whose  walls  are  lined  with 
an  entoderm  containing  a  large  number  of  glandular  cells.  The  body 
of  the  hydranth  calls  for  no  special  notice,  but  it  is  suddenly  con- 
stricted at  its  base,  which  sits  on  a  kind  of  perforated  shelf  at  the 
bottom  of  the  hydrotheca.  It  is  continued  through  this  central 
hole  as  a  very  narrow  tubular  neck-like  portion,  the  hydrocope, 


FIG.  53. — A.,  part  of  a  colony  of  Obelia  geniculata  magnified. — 
From  Bourne. 

ht.,  hydrotheca  containing  a  hydranth  ;  gt.,  a  gonotheca  enclosing  a  blastostyle  with  medusa 
buds  ;  p.,  perisarc  ;  t.,  terminal  growing  point.  B.,  a  sexually  mature  female  medusa,  seen  from 
below  ;  m.,  mouth  ;  r.c.,  radial  canal  ;  cc.,  circular  or  ring  canal ;  go.,  gonads.  C.,  diagrammatic 
longitudinal  section  through  a  medusa  ;  m.,  mouth  ;  mb.,  manubrium  ;  g.v .,  gastro-vascular  cavity  ; 
r.c.,  radial  canal ;  cc.,  ring  canal  ;  e.l.,  endoderm  lamella  ;  oc.,  ocellus  ;  ot.,  otocyst.  The  section  is 
supposed  to  pass  through  a  radial  canal  on  the  left  side  and  an  adradial  tentacle  on  the  right. 
Endoderm  black  ;  mesoglcea  shaded  ;  ectoderm  represented  by  a  broken  line.  D.,  the  bases  of 
two  tentacles  magnified,  showing  oc.,  ocelli ;  ot.,  an  otocyst  on  an  adradial  tentacle  ;  cc.,  ring  canal. 

which,  however,  soon  swells  out  and  becomes  continuous  with  the 
coenosarc.  Thus  the  gastral  cavity  of  each  polyp  is  continuous  with 
the  ccenosarcal  canal,  and  so  food  eaten  by  any  one  individual  can 
be  utilised  for  the  benefit  of  the  whole.  The  function  of  this  zooid 
is  purely  nutritive,  except  in  so  far  as  its  budding  to  form  another 
hydranth  can  be  looked  upon  as  a  kind  of  reproduction,  which  in 


THE   CCELENTERATA      ^  163 

some  senses  it  is.  By  this  means,  of  course,  more  individuals  are 
added  to  the  colony,  but  new  colonies  are  apparently  only  formed 
by  the  process  of  sexual  reproduction.  When  living,  the  hydranths 
are  in  a  constant  state  of  motion,  waving  their  tentacles  about  in 
search  of  prey,  expanding  and  contracting,  so  that  some  of  the  more 
brightly-coloured  allies  of  Obelia  are  most  beautiful  objects  and, 
looking  as  they  do  like  hundreds  of  animated  flowers,  well  deserve 
their  name  of  zoophytes,  plant  animals. 

The  blastostyle  is  a  hollow  outgrowth  of  the  coenosarc  with  an 
internal    cavity    continuous    with    the    gastro- vascular    canal.     It 
corresponds  with  a  drawn-out  body  of  a  hydranth,  and  has  a  knob- 
like  enlargement  at  its  distal  extremity  in  place  of  the  hypostome. 
No  indication  of  mouth  or  tentacles  is  to  be  found,  and  it  is  quite 
incapable  of  catching  prey,  so  that  it  is  dependent  for  its  food  on  the 
nutritive  polyps.     The  relation  between  it  and  an  ordinary  zooid 
is  not  so  obvious  in  Obelia  as  it  is  in  certain  other  members  of  the 
same  class,  irr  which  the  medusoid  person  is  actually  budded  off  from 
an  ordinary  hydranth.      We  therefore  regard  the  blastostyle  as  a 
highly  modified  and  degenerate  polyp,  specialised  for  the  production 
of  medusoids.     As  has  been  noted,  the  blastostyle  is  contained  in  an 
urn-shaped   expansion   of   perisarc,   the   gonotheca,   and   the   two 
together   are   sometimes   termed   a   gonangium.     A   succession   of 
hollow  buds  are  produced  by  the  blasostyle,  and  these  gradually  grow 
into  rudimentary  medusoid  persons  which  after  a  while  become 
separated  off.     They  undergo  a  little  further  development  in  the 
gonotheca,  but  ultimately  the  top  of  this  ruptures  and  allows  them 
to  escape. 

The  free-swimming  reproductive  individual  of  Obelia,  the  medusa, 
differs  very  considerably  in  appearance  from  either  the  other  zooids 
or  from  Hydra.  It  is  a  small  transparent  organism  about  2  or  3  mm. 
in  diameter,  and  shaped  like  a  saucer,  with  a  short  stout  handle  in 
the  middle.  From  its  likeness  to  an  open  umbrella  the  outer, 
convex,  or  aboral  surface  is  termed  the  exumbrella,  and  it  was  by  the 
centre  of  this  that  it  was  attached  to  the  blastostyle.  The  under, 
concave,  or  oral  surface  is  spoken  of  as  the  sub-umbrella,  and  bears 
in  the  middle  the  short  more  or  less  rectangular  handle,  the  nianu- 
brium,  on  the  distal  extremity  of  which  opens  a  cross-shaped  mouth. 
The  rim  of  the  umbrella  is  fringed  with  a  row  of  delicate  tentacles, 
which  may  be  as  few  as  sixteen  when  the  animal  is  first  liberated,  but 
increase  to  more  than  a  hundred  with  age.  The  tentacles  are  well 
provided  with  batteries  of  nematocysts,  which,  in  the  case  of  some  of 
the  larger  allies  of  Obelia,  are  capable  of  stinging  human  beings  so  as 
to  cause  considerable  pain.  At  the  base  of  each  tentacle,  where  it 
is  fixed  into  the  umbrella,  it  enlarges  slightly,  and  is  covered  by  a 


164  AN   INTRODUCTION   TO  ZOOLOGY 

patch  of  pigment ed  epithelium,  in  which  are  a  number  of  sense  cells. 
This  area  is  sensitive  to  light,  and  so  forms  a  very  rudimentary  sort 
of  eye,  the  ocellus. 

The  mouth  leads  straight  into  the  enteron,  which  continues 
up  the  manubrium,  and  at  its  base,  within  the  thickness  of  the 
umbrella,  enlarges  to  form  a  rounded  rectangular  cavity,  the  stomach. 
From  the  corners  of  the  stomach  four  delicate  tubes,  the  radial 
canals,  pass  out  to  the  periphery,  there  to  open  into  another  tube, 
the  circular  canal,  which  runs  round  the  edge  of  the  umbrella. 
About  half-way  to  the  circumference  each  radial  canal  has  a  small 
downwardly  directed  sac,  around  which  the  germ  cells  are  congre- 
gated, forming  an  outbulging  in  the  sub-umbrella  surface,  the  gonad. 
In  this  way  we  have  what  is,  compared  with  a  polyp,  a  complex 
gastro-vascular  system,  which  is  throughout  lined  by  entoderm. 
The  radial  canals  are  not  isolated,  but  connected  together  and  with 
the  circular  canal  by  a  very  delicate  double  layer  of  cells,  forming 
a  thin  sheet  known  as  the  entodermal  lamella.  All  the  remaining 
part  of  the  thickness  of  the  umbrella  is  made  up  by  the  jelly-like 
mesoglea,  which  fills  up  all  the  spaces  between  the  external  covering 
ectoderm  and  the  internal  entoderm.  It  corresponds  with  the 
mesoglea  of  the  polyp  or  Hydra,  and  is  secreted  by  the  other  layers, 
but  is  very  much  thicker. 

Like  the  polyp,  the  medusa  is  radially  symmetrical  about  a  main 
axis,  constituted  by  a  line  passing  through  the  centre  of  the  ex- 
umbrella  and  the  mouth.  Four  secondary  axes  are  marked  out  by 
lines  drawn  through  the  radial  canals,  which  also  pass  through  the 
arms  of  the  cruciform  mouth.  These  are  distinguished  as  the 
perradii,  and  the  canals  in  consequence  are  sometimes  termed  the 
perradial  canals.  The  axes  formed  by  bisecting  the  angles  between 
these  perradii  are  termed  the  interradii,  and  the  eight  axes  half-way 
between  these  and  the  perradii  are  "the  adradii.  The  tentacles  at 
the  end  of  all  these  different  radii,  sixteen  in  number,  are  the  first 
to  be  formed,  and  the  medusa  may  be  set  free  at  this  stage,  but  not 
before  and  often  not  until  other  tentacles  have  appeared.  Each 
adradial  tentacle  has  not  only  an  ocellus,  but  in  addition  another 
sense  organ  in  the  form  of  a  minute  spherical  sac.  This  is  lined  with 
ectoderm,  and  the  cells  at  its  lowest  part  possess  sensory  hairs,  while 
small  calcareous  particles  are  present  inside.  These  bodies  are  the 
lithocysts  or  statocysts.  From  a  somewhat  superficial  resemblance 
to  the  sensory  parts  of  the  membranous  labyrinth  in  the  ear  of 
higher  animals  they  were  formerly  termed  otocysts,  but  this  is  not 
a  good  term.  There  is  little  doubt  that  their  function  is  the  per- 
ception of  the  position  of  the  animal  in  space,  particularly  its 
orientation  with  regard  to  the  action  of  gravity,  and  hence  they  are 


THE  CGELENTERATA  165 

organs  of  balance  or  equilibration.  In  these  structures  and  in  the 
ocelli  we  have  a  decided  advance  upon  the  conditions  obtaining  in 
Hydra  or  even  the  polyps  of  Obelia,  for  here  we  have  definite  parts 
specialised  for  a  particular  function,  in  other  words  sense  organs. 
In  correlation  with  this  sensory  system  we  find  that  the  nervous 
system  also  is  more  highly  organised,  for  in  addition  to  the  ordinary 
scattered  nerve  cells  in  the  mesoglea  there  are  also  developed  two 
much  denser  rings  of  these  cells  around  the  margin  of  the  umbrella, 
although  even  yet  we  have  no  definite  aggregations  to  form 
ganglia.  By  means  of  the  marginal  nerve  rings  the  animal  is  able 
to  co-ordinate  the  action  of  all  the  umbrella  muscles  and  perform 
definite  swimming  movements.  If  they  are  cut  away  the  umbrella 
seems  incapable  of  co-ordinate  movement.  Even  more  than  this, 
it  has  lost  all  its  automatism,  that  is  to  say,  the  power  of  originating 
the  stimuli  that  bring  about  muscular  contraction,  so  that  it  cannot 
move  of  its  own  accord.  Thus,  although  we  have  an  increased 
development  of  the  nervous  system,  it  is  accompanied  by  the 
circumscription  of  the  power,  for  if  Hydra  be  cut  into  pieces  each  is 
capable  of  separate  contraction  and  expansion.  In  other  words, 
the  automatism  is  diffuse  in  Hydra.  Throughout  the  animal 
kingdom  in  general  we  find  that  differentiation  of  structure  is 
accompanied  by  localisation  of  function. 

Hydranth  and  Polyp. 

Superficially  there  is  little  resemblance  between  the  hydroid 
and  medusoid  type  of  person,  and  yet  a  closer  examination  shows 
that  the  fundamental  structure  is  the  same  in  both  cases.  They  are 
constructed  upon  the  same  plan,  and  are  to  be  regarded  as  homologous. 
Suppose  we  imagine  a  polyp  much  shortened  in  its  main  axis,  and 
at  the  same  time  the  transverse  diameter  much  increased  by  the 
expansion  of  the  narrow  rim  between  the  hypostome  and  the  bases 
of  the  tentacles,  then  a  slight  curvature  will  produce  a  medusa-like 
form.  The  rese'mblance  is  not  complete,  for,  although  we  should 
have  comparable  oral  and  aboral  sides,  tentacles,  and  manubrium, 
and  the  same  main  axis  of  symmetry  passing  through  mouth  and 
aboral  pole,  the  enteron  and  mesoglea  would  differ  markedly.  The 
next  stage  necessary  would  be  the  thickening  of  the  mesoglea,  which 
would  bring  about  a  reduction  in  the  large  gastral  cavity,  otherwise 
occupying  the  whole  of  the  space  between  oral  and  aboral  surfaces 
right  out  to  the  periphery.  If  the  mesoglea  thickened  much  more 
over  certain  areas  than  others  it  could  lead  to  the  obliteration  of  the 
enteron,  save  in  the  positions  occupied  by  the  stomach  and  canals 
and  the  formation  of  the  entoderm  lamellae,  and  so  we  should  have 
the  condition  of  the  adult  medusa.  Needless  to  say,  the  medusa 


i66 


AN   INTRODUCTION   TO  ZOOLOGY 


does  not  arise  in  this  way,  for  it  is  formed  directly  by  the  modification 
of  a  hollow  bud  from  the  blastostyle,  and  is  never  at  any  time  a 
polyp.  The  above  stages,  however,  are  not  purely  imaginary,  for 
we  have  indications  that  the  gastral  cavity  is  at  first  extensive,  and 


that  the  entoderm  lamella  is  formed  in  the  way  suggested.  It 
serves  to  show  how  nearly  alike  the  two  forms  are,  and  perhaps  we 
are  justified  in  regarding  them  as  modifications  of  the  one  type  each 
adapted  for  its  own  particular  mode  of  life.  The  medusa  is  a  free- 
swimming  wanderer,  and  needs  therefore  to  have  specially  developed 
muscular  and  sensory  systems,  which  in  turn  necessitate  an  increase 


THE   COELENTERATA  167 

of  nervous  control.  Mesoglea  is  excessively  developed,  not  so  much 
for  protection  as  to  lower  the  specific  gravity  of  the  animal  to  allow 
of  floating.  The  hydranth,  on  the  other  hand,  is  a  fixed  form  not 
needing  locomotor,  sensory  and  nervous  specialisation,  but  requiring 
a  longer,  stalk-like  body  to  hold  it  up  from  the  ground  and  a  thinner 
tougher  mesoglea  for  support.  The  exumbrella  of  the  medusa, 
which  is  the  part  attached  to  the  blastostyle,  corresponds  to  the  foot 
of  the  hydranth,  while  tentacles,  mouth  and  manubrium  are  similar 
structures  in  both  forms. 

When  living  the  medusa  swims  slowly  by  a  series  of  sudden 
contractions  of  the  umbrella,  that  propel  it  through  the  water  aboral 
side  foremost,  by  forcing  out  the  water  contained  in  the  sub- 
umbrella  space.  It  is  not  capable  of  active  locomotion,  being  in  the 
main  a  floating  form.  The  power  of  contraction  lies  partly  in  the 
epithelio-muscular  cells  of  the  sub-umbrella  ectoderm,  but  their 
efficiency  is  greatly  augmented  by  a  definite  ring  of  muscle  fibre  cells 
around  the  margin  of  the  umbrella.  These  cells  are  ectoderm  cells 
that  have  become  much  modified  and  then:  muscle  processes  greatly 
enlarged,  until  ultimately  they  form  almost  pure  muscle  fibres, 
which  sink  down  into  the  mesoglea,  and  so  come  to  take  up  a  sub- 
epidermal  position.  Here  then  we  have  another  advance  on  Hydra, 
the  production  of  a  specially  developed  ring  of  muscle  elements. 
The  food  is  caught  by  the  tentacles  and  conveyed  to  the  mouth  as 
in  the  polyp,  thence  it  is  passed  to  the  stomach  and  digested.  The 
substances  in  solution  are  circulated  to  all  parts  by  the  gastro- 
vascular  canals,  and  the  undigested  residue  is  expelled  through  the 
mouth,  no  anus  being  developed. 

The  sexual  reproduction  in  Obelia  is  brought  about  by  these 
medusoid  persons,  which  are  the  bearers  of  the  germ  cells.  They 
are  dioecious,  that  is  to  say, the  sexes  are  separate,  and  any  individual 
medusa  is  either  male  or  female,  but  not  both,  the  four  gonads  in 
the  one  case  being  testes,  and  in  the  other  ovaries.  When  first 
liberated  the  medusae  show  no  signs  of  the  sexual  cells,  and  even 
when  the  downgrowths  of  the  radial  canals,  the  future  gonads, 
hpve  been  formed  there  is  still  no  sign  of  the  germ  cells.  It  is 
interesting  to  note  that  the  oogonia  and  spermatogonia  do  not  make 
their  first  appearance  in  these  gonadial  rudiments,  but  on  the  other 
hand  they  are  first  discernible  in  the  walls  of  the  manubrium. 
Thence  they  migrate  along  the  entoderm  of  the  gastral  cavity  and 
radial  canals  to  their  definitive  position  between  the  ectoderm  and 
mesoglea,  where  they  undergo  maturation.  It  is  not  until  this 
migration  has  taken  place  that  we  can  strictly  term  the  down- 
growths  gonads.  In  other  allied  forms  the  primordial  germ  cells 
first  appear  in  the  hydrocaulus,  and  then  wander  to  the  place  where 


i68  AN   INTRODUCTION  TO  ZOOLOGY 

a  medusa  is  being  formed,  and  then  on  into  the  medusa.     Such  a 
migration  of  the  germ  cells  has  been  observed  in  a  number  of  other 
invertebrates,  and  also  in  certain  vertebrates,  in  all  of  which  cases 
the  primordial  germ  cells  are  first  recognisable  at  some  distance 
from  the  points  at  which  the  gonads  will  later  be  formed.     Further- 
more, in  some  cases  a  mother  sexual  cell  is  to  be  distinguished  at  a 
very  early  stage  in  the  cleavage  when  only  a  few  cells  are  present. 
From  these  facts  we  may  draw  a  line  of  demarcation  between  the 
ordinary  cells  that  go  to  form  the  body,  the  somatic  cells,  and  the 
cells  destined  to  give  rise  to  all  the  germ  cells,  and  we  also  see  that 
the  latter  are  to  a  certain  degree  independent  of  the  former,  a  con- 
sideration that  is  often  referred  to  in  discussing  theories  of  heredity. 
The  medusae  float  about  in  shoals  of  enormous  numbers, 
and  when  the  germ  cells  are  ripe  the  gonads  rupture,  setting  the  ova 
and  spermatozoa  free  in  the  sea  where  fertilisation  occurs.     Thus 
the  medusa  serves  not  only   for  multiplication  but  also  for  the 
dissemination  of  the  species.     The  zygote  undergoes  complete  and 
regular  segmentation,  leading,  as  in  Hydra,  to  the  formation  of  a 
hollow  sphere  of  cells,  the  blast ula.    The  cavity  within  it,  the  blasto- 
ccel,  is  later  filled  in  a  similar  way  by  cells  derived  from  the  periphery, 
but  these  cells  are  only  produced  at  one  part  of  the  circumference, 
and  consequently  the  process  is  described  as  unipolar  delamination 
and  immigration.     Thus  arises  a  solid  embryo  with  an  outer  layer 
of  columnar  cells,  the  ectoderm,  enclosing  a  solid  mass  of  entoderm 
cells.     This  elongates,  develops  a  coating  of  cilia,  and  leads  an 
independent  life,  floating  near  the  surface  of  the  sea.     When,  in  an 
early  stage  of  its  development,  before  it  resembles  the  adult,  an 
animal  lives  freely  and  independently  we  term  it  a  larva,  and  in  the 
particular  case  of  Obelia  the  larval  form  is  known  as  a  Planula.     It 
is  quite  a  common  stage  in  the  class  Hydrozoa,  although  missing  in 
Hydra  itself.     Sooner  or  later  the  entoderm  cells  become  arranged 
around  an  internal  cavity,  the  enteron,  which  makes  its  appearance 
as  a  slit.     The  larva  then  settles  down  and  attaches  itself  by  an 
expanded  basal  disc  to  some  object  in  the  sea,  often,  in  the  case  of 
0.  geniculata,  a  frond  of  Laminaria.     It  sheds  its  coat  of  cilia  and 
becomes  transformed  into  a  hydroid  individual  by  the  enlargement 
of  the  distal  extremity  and  the  appearance  of  tentacles,  hypostome 
and  mouth.     The  ectoderm  now  secretes  a  tubular  investment  of 
perisarc,  and  in  this  condition,  when  it  is  a  simple  hydra-like  animal 
with  a  marked  basal  or  attachment  disc,  it  is  termed  a  hydrula. 
It  starts  now  to  grow  into  a  colony  :  a  bud  appears  some  way  up  the 
stalk,  and  this  turns  into  a  second  hydranth,  soon  to  give  off  a  third 
polyp,  and  so  we  have  the  hydrocaulus  formed.    Simultaneously  with 
this,  small  processes  arise  from  the  attachment  disc  that  grow  out  and 


THE   CCELENTERATA  169 

branch  and  anastomose  in  a  network,  the  hydrorhiza,  which  in  its  turn 
serves  for  the  origin  of  a  number  of  new  hydrocauli.  In  this  way 
there  is  produced  the  complex  colony,  or  stock,  with  which  we  started. 

Before  leaving  Obelia  there  is  one  further  phenomenon 
concerning  it  that  calls  for  notice.  Starting  with  the  hydrula  we 
find  that  by  a  process  of  budding,  similar  to  that  which  in  Hydra 
produces  a  number  of  separate  individuals,  there  is  formed  a  large 
colony  of  hydranths  joined  by  the  coenosarcal  canals.  It  is  a  matter 
of  opinion  whether  we  regard  the  whole  stock  as  one  individual 
with  the  hydranths,  blastostyles  and  medusae,  as  separate  parts  or 
organs,  or  look  on  each  of  them  as  an  individual  in  itself,  in  organic 
unity  with  its  fellows.  The  majority  of  zoologists  adopt  the  latter 
point  of  view,  and  regard  the  colony  as  the  result  of  the  asexual 
reproduction  of  the  one  person.  Further  asexual  multiplication 
leads  to  the  formation  of  the  blastostyles,  and  yet  again  to  the 
medusae.  The  whole  colony,  together  coming  from  the  one  hydrula 
and  producing  medusae,  forms  the  asexual  generation  or  agamobium. 
The  medusae,  however,  do  not  reproduce  by  budding,  but  by  the 
formation  of  sexually  differentiated  gametes,  and  so  they  constitute 
the  sexual  generation  or  gamobium.  Thus  the  life  history  of  Obelia 
includes  an  asexual  generation  alternating  with  a  sexual  one  ;  it  is 
an  instance  of  metagenesis  or  the  alternation  of  generations.  It 
should  be  noted  in  passing,  however,  that  such  an  alternation  is  not 
identical  with  that  with  which  the  botanist  has  to  deal-  in  certain 
plants,  e.g.  the  fern.  In  these  plants  both  the  generations  produce 
reproductive  cells.  The  form  known  as  the  sporophyte  produces 
single  cells,  the  spores,  which  are  capable  of  giving  rise  to  an  entirely 
new  and  different  individual  without  first  undergoing  any  kind  of 
fertilisation.  This  second  form,  the  gametophyte,  cannot  produce 
spores,  but  only  sexual  gametes,  which  must  unite  to  form  a  zygote 
before  development  can  proceed  any  further.  Thus  we  have  an 
alternation  of  generations  often  very  different  in  appearance,  each 
capable  of  giving  rise  to  reproductive  cells,  a  condition  which  is  not 
paralleled  in  the  animal  kingdom.  Moreover,  among  animals,  one 
generation  or  the  other,  only  rarely  both,  is  capable  of  vegetative 
reproduction  by  budding  in  a  very  similar  way  to  Hydra. 

We  have  now  completed  our  review  of  the  diploblastic 
Ccelenterate  types  Hydra  and  Obelia,  and  have  noted  in  them  not 
only  a  considerable  advance  over  the  Protozoa,  but  also  a  certain 
progressive  series  of  specialisations  within  the  phylum  itself,  leading 
to  the  production  of  definite  localised  organs  for  the  performance  of 
certain  functions.  Thus  in  these  lowly  members  of  the  animal 
kingdom  we  see  indications  of  the  beginning  of  that  complexity  of 
organisation  brought  about  by  the  physiological  division  of  labour 
that  becomes  more  and  more  marked  in  the  higher  groups. 


CHAPTER  VII 
THE  CCELOMATA  INVERTEBRATA 

The  Earthworm,  Lumbricus  sp.,  a  Free-living  Annelid — Tcenia  solium, 
a  parasitic  flat  worm. 

THE  next  grade  of  animal  organisation  above  the  Ccelenterata  is 
the  Coelomata,  and  it  includes  all  the  remaining  animals  with  a  few 
possible  exceptions.  The  main  point  of  difference  between  them  is 
that  in  the  latter  not  only  do  we  find  a  gut  cavity  or  enteron  lined 
by  entoderm,  but  in  addition,  between  it  and  the  outside  layer,  the 
ectoderm,  is  another  cavity,  the  coelom,  whose  walls  are  lined  by  a 
third  cellular  layer,  namely,  the  mesoderm  or  middle  layer.  Thus 
there  are  two  independent  cavities,  enteron  and  coelom,  and  the  main 
structural  plan  of  such  an  animal  reduced  to  its  simplest  form  is  that 
of  a  narrow  tube,  the  gut,  within  a  wider  one,  the  body  wall,  both 
being  joined  together  at  the  two  ends.  In  addition  to  this  funda- 
mental difference  there  are  a  number  of  smaller  ones  which  can  be 
better  dealt  with  after  we  have  examined  the  structure  of  a  primitive 
coelomate. 

The  Earthworm — Lumbricus  sp.,  a  Free-living  Annelid. 

Earthworms  are  widely  distributed  over  the  face  of  the  earth, 
and  are  found  in  almost  all  places  where  there  is  a  certain  amount  of 
moisture.  Of  the  twenty  or  more  species  common  in  the  damp  soil 
of  this  country  the  two  largest  and  commonest  belong  to  the  Genera 
Lumbricus  and  Allolobophora.  They  differ  from  one  another  only  in 
unimportant  points,  and  for  convenience  in  the  laboratory  we  use 
the  largest  of  all  the  British  species,  namely,  L.  herculeus.  They 
burrow  into  the  ground  by  literally  swallowing  the  soil  in  front  of 
them,  from  which,  as  it  passes  through  their  alimentary  canal,  they 
obtain  their  food,  consisting  of  decaying  animal  and  vegetable 
matter.  It  is  this  earth  which  is  constantly  being  passed  through 
their  bodies,  even  when  upon  the  surface,  that  produces  the  character- 
istic "  worm  casts  "  which  spoil  the  appearance  of  a  lawn  or  the 
surface  of  a  putting  green.  The  worm  plays  a  far  more  important 
part  in  the  biology  of  the  soil  than  appears  at  first  sight,  and  is 

170 


THE   CCELOMATA  INVERTEBRATA  171 

invaluable  to  the  plant  life.  Its  burrows  allow  of  the  percolation 
of  the  rain  to  the  deeper  layers,  and  also  of  the  air.  Darwin,  in  his 
masterly  book  on  "  The  Formation  of  Vegetable  Mould  through  the 
Action  of  Earthworms/'  estimates  that  by  their  castings  they  bring 
to  the  surface  more  than  ten  tons  of  the  deeper  soil  per  acre  per  year, 
and  so  gradually  cover  up  stones,  etc.,  that  may  be  lying  upon  it. 
They  also  pull  in  to  line  their  burrows  leaves,  which,  of  course, 
disintegrate,  forming  a  rich  mould. 

The  earthworm's  body  is  long  and  cylindrical,  running  off  into 
a  bluntly  pointed  anterior  end  and  a  somewhat  flattened  obtusely 
truncated  posterior  end,  and  its  greatest  diameter  is  about  one- 
third  of  the  way  from  the  anterior  end.  It  reaches  a  maximum 
length  of  about  seven  inches.  The  general  colour  is  a  pinkish-brown, 
but  it  is  much  darker  above  than  below.  As  it  always  travels  with 
the  same  surface  to  the  ground,  we  can  distinguish  a  ventral  and  a 
dorsal  surface,  the  ventral  being  slightly  flatter  than  the  dorsal. 
Thus  it  exhibits  a  definite  orientation  not  only  with  front  and  hinder 
ends,  dorsal  and  ventral  surfaces,  but  also  with  right  and  left  sides, 
which  are  similar  to  one  another,  so  that  it  is  a  bilaterally  symmetrical 
animal. 

A  series  of  transverse  ring-like  grooves  divides  the  body  off  into 
about  150  segments,  somites  or  metameres,  which  are  larger  in  front 
than  at  the  hinder  end.  We  also  find  on  dissection  that  this 
segmentation  is  not  superficial,  but  that  the  inside  of  the  animal 
exhibits  clearly  the  repetition  of  the  essential  organs  of  the  body  in 
compartments  of  the  crelom,  separated  off  from  one  another  by 
transverse  partitions  or  septa,  which  coincide  with  the  grooves  on 
the  outside.  In  its  general  characters  Lumbricus  agrees  with  a  large 
number  of  other  worm-like  forms,  which  from  their  ringed  bodies 
are  classed  in  the  Phylum  Annelida  or  Annulata.  This  repetition 
of  a  number  of  parts  in  a  series,  so  very  strongly  marked  in  the 
Annelids,  is  a  morphological  feature  of  considerable  importance,  and 
an  animal  so  built  up  is  said  to  be  metamerically  segmented,  while 
the  repeated  parts  are  described  as  serially  homologous.  It  is  a 
feature  marked  even  in  the  higher  animals  like  the  dogfish,  and 
distinct  traces  of  it  are  still  to  be  found  in  the  frog,  rabbit,  and 
ourselves. 

At  the  extreme  anterior  end  of  the  earthworm  is  a  blunt  lobe, 
the  prostomium,  not  homologous  with  the  somites.  Behind  it,  the 
next  part  if  the  worm,  the  first  true  somite,  surrounds  the  ventrally 
situated  mouth,  and  so  is  termed  the  peristomium.  On  the  dorsal 
surface  a  backward  prolongation  of  the  prostomium  is  dovetailed 
into  the  peristomium  ;  in  Allolobophora  the  projection  extends 
completely  across  the  peristomium,  but  in  Lumbricus  it  only  goes 


172  AN   INTRODUCTION  TO   ZOOLOGY 

about  half-way.     In  the  worm  the  ventral  surfaces  of  segments 
8-12  are  swollen  by  the  so-called  capsulogen  glands,  and  on  the 
dorso-lateral  aspect  of  segments  32-37  inclusive  in  the  adult  there 
is  a   saddle-shaped   thickening,    the   clitellum   or   cingulum.    The 
position  and  appearance   of   this    structure  is  a  useful   character 
for  determining  the  species  of  the  worm,   for  it   varies  consider- 
ably  in   different   forms,  but    is  constant  in  the  one  species.     It 
is   functional  in   secreting   a  cocoon  in  which  the  eggs  are  laid, 
and  also   in   furnishing  them  with    a   nutritive  fluid.      The  end 
somite  is  small  and  bears  terminally  the  anus,   and  so   is   called 
the   anal   segment.     The  outermost  layer  of   cells,  the  epidermis, 
does    not    remain    exposed    freely,   but    secretes  a  very  delicate 
membrane,  the   cuticle,  which   adheres   closely  to  it.     If   a   dead 
worm   be   left   some   time   in  water   this   cuticle   can   be   readily 
stripped  off,  and  appears  as  a  thin  transparent  irridiscent  sheath. 
In  spite  of  this  covering  of  smooth  cuticle  a  certain  roughness  is 
felt  if  the  ventral  surface  of  the  worm  is  drawn  across  a  finger. 
Closer  inspection  reveals  the  fact  that  this  is  due  to  the  presence  in 
the  skin  of  a  number  of  very  tiny  bristles,  the  setae  or  chsetse. 
These  are  not  irregularly  distributed,  but  arranged  in  four  pairs  in 
every  segment,  save  the  first  and  last  ;   two  pairs  lie  on  the  ventral 
surface,  and  the  other  two  are  lateral  to  these.     Each  seta  has  the 
form  of  an  f-shaped  rod  of  chitin  pointed  at  its  ends  and  thickened  in 
the  middle,  and  is  embedded  in  an  invagination  of  the  epidermis, 
known  as  the  setigerous  sac,  to  which  are  attached  certain  muscles. 
They  are  generally  directed  backwards,  acting  as  a  series  of  points 
that  help  the  worm  in  its  crawling,  but,  when  pointed  forward,  as  they 
can  be,  make  it  very  hard  to  remove  the  worm  from  its  burrow. 
In  the  fifteenth  segment  the  two  pairs  of  ventral  setae  lying  close  to 
the  male  external  aperture  are  modified  to  form  the  penial  setse. 

Not  only  do  we  find  a  mouth  and  anus,  but  a  number  of  other 
apertures  opening  to  the  outside,  some  unpaired  and  the  others 
paired.  The  unpaired  openings  consist  of  a  number  of  minute 
holes  in  the  mid-dorsal  line,  the  dorsal  pores,  lying  in  the  grooves 
between  the  somites  and  leading  into  the  ccelem  of  the  segment  in 
front.  They  are  present  in  all  the  grooves  save  the  first  seven  or 
eight.  The  remaining  apertures  are  paired  and  connected  with  the 
excretory  or  reproductive  systems.  On  every  segment  of  the  body, 
except  the  first  three  and  the  last,  are  a  pair  of  apertures,  the 
nephridiopores,  the  openings  of  the  excretory  organs.  They  are 
situated  just  in  front  of  the  outermost  seta  of  the  ventral  pairs. 
On  the  ventral  surface  of  segment  15  are  a  pair  of  conspicuous 
slits  with  swollen  glandular  lips  ;  these  are  the  male  apertures,  the 
openings  of  the  vasa  deferentia  or  the  spermiducal  pores.  In  a 


THE  CCELOMATA   INVERTEBRATA  173 

similar  position  on  the  segment  in  front  are  two  small  holes,  the 
openings  of  the  female  ducts,  the  oviducal  pores.  Lastly,  there  are 
two  pairs  of  minute  openings,  the  spermathecal  pores,  lying  in  the 
Central  part  of  the  grooves  between  segments  9  and  10  and  10  and  n, 
slightly  to  each  side  of  the  middle  line.  They  open  into  the  sper- 
mathecae,  whose  function  we  shall  deal  with  later. 

Two  of  the  main  features  that  will  at  once  be  seen  on 
opening  up  an  earthworm  have  already  been  alluded  to,  namely, 
the  fact  that  the  alimentary  canal  is  a  tube  lying  in  the  coelom,  and 
that  the  coelom  itself  is  cut  up  by  a  number  of  septa  into  separate 
compartments.  The  thin  muscular  septa  run  from  the  intersomitic 
groove  to  the  gut  wall,  into  which  they  are  inserted,  causing  as  a  rule 
a  constriction.  They  are  perforated  by  small  holes,  so  that  the 
various  compartments  of  the  coelom  are  not  absolutely  isolated  from 
one  another.  Septa  are  absent  in  at  any  rate  the  first  two  or  three 
somites. 

The  alimentary  canal  is  a  straight  tube  running  from  mouth  to 
anus  and  over  the  greater  part  of  its  course  is  alike,  but  at  its  anterior 
end  it  is  much  modified.  As  we  have  seen,  the  mouth  is  situated 
on  the  ventral  side  of  the  first  somite,  and  it  leads  into  a  wide  thin- 
walled  receptacle,  the  buccal  cavity,  filling  the  first  two  somites. 
Following  this  is  a  large  thick- walled,  muscular  portion  of  the  canal, 
the  pharynx,  which  extends  back  into  the  sixth  or  seventh  segment, 
but  is  really  in  front  of  the  septum  between  5  and  6.  From  it 
radiate  out  muscular  strands,  some  even  piercing  the  septa  behind, 
by  means  of  which  its  cavity  can  be  enlarged,  and  in  this  way  it 
can  act  as  a  suction  pump,  drawing  in  anything  to  which  the  mouth 
has  been  attached.  Buccal  cavity  and  pharynx  together  constitute 
the  first  part  of  the  alimentary  canal,  the  fore  gut  or  stomodceum, 
and  are  marked  off  from  the  next  portion  by  the  fact  that  they  are 
formed  by  an  ingrowth  of  ectoderm,  whereas  the  succeeding  parts 
of  the  gut  are  derived  from  the  entoderm,  an  important  distinction. 
The  oesophagus  is  a  straight  narrow  tube  running  back  from  the 
end  of  the  pharynx  to  the  fourteenth  somite.  In  the  eleventh  and 
twelfth  segments  three  small  sac-like  swellings,  the  cesophageal  glands, 
are  to  be  found  on  the  sides  of  the  oesophagus.  The  first  pair  are 
actually  pouches,  cesophageal  pouches,  opening  directly  into  the 
oesophagus,  and  the  last  two  pairs  are  thickenings  of  its  walls,  only 
communicating  with  it  via  the  pouches,  \\ithin,  they  are  divided 
by  partitions  Into  small  chambers,  and  they  produce  a  whitish  fluid 
containing  a  multitude  of  small  calcareous  particles  excreted  by  the 
large  cells  which  line  them,  and  hence  their  name  calcareous  glands. 
In  segments  15  and  16  the  alimentary  canal  is  dilated  to  form 
a  large  thin-walled  distensible  sac,  the  crop  or  proventriculus. 


174 


AN   INTRODUCTION  TO  ZOOLOGY 


This  in  its  turn  is  followed  by  a  strong  muscular  expansion,  the 
gizzard,  occupying  segments  17-19  or  20  ;  but  it  may  project 
beyond  this,  carrying  the  septum  with  it.  It  has  very  strong  walls 
and  a  chitinous  lining,  in  order  to  grind  up  the  food  before  handing 
it  on  to  the  intestine. 

The  intestine  itself    is  a    long  straight  tube  extending  from 


lc 


fh. 


oe 


FIG.  55. — Lumbricus  herculeus. — From  Bourne. 

A,  A  view  of  the  organs  contained  in  the  first  twenty-two  somites,  as  seen  when  the  animal  is 
opened  by  a  longitudinal  dorsal  incision,  and  the  body  walls  are  pinned  out  without  cutting  the 
septa.  The  pins  are  placed  in  the  srd,  gth,  and  i8th  somites.  B,  View  of  the  first  sixteen  somites 
of  the  same  worm  after  removal  of  the  alimentary  tract,  to  show  the  nervous  system  and  reproductive 
organs. 

b.c.,  buccal  cavity,  cut  across  ;  e.g.,  cerebral  ganglia  ;  g.,  gizzard  ;  int.,  intestine  ;  nph., 
nephrida  ;  od.,  oviduct ;  oe.,  oesophagus  ;  ov.,  ovary  in  somite  13  ;  ph.,  pharynx  with  radiating 
muscular  strands  ;  prv.,  proventiculus  ;  s.,  septa  ;  s.d.,  sperm  duct ;  s.f.,  seminal  funnels  ;  spth. 
spermatheccB  in  somites  9  and  10  ;  sp.s.,  sperm  sacs  ;  t.,  testis. 

segment  21  to  the  anus.  In  each  somite  it  swells  up  into  a  saccu- 
lation,  which  is  well  marked  at  the  anterior  end,  but  diminishes 
as  we  pass  backwards  until  it  practically  disappears  at  the  posterior 
end,  and  intersomitically  the  intestine  is  contricted  where  the  septa 
are  inserted  into  it.  The  sacculations  considerably  increase  the 
area  of  the  internal  surface,  an  end  also  served  by  the  dorsal  wall 


THE  CCELOMATA   INVERTEBRATA  175 

being  folded  into  the  cavity  to  make  a  very  large  ridge-like  swelling, 
the  typhlosole,  running  the  whole  way  down  the  intestine.  It  is 
lined  by  entoderm,  most  of  whose  cells  are  ciliated,  but  there  are 
also  scattered  in  it,  particularly  on  the  typhlosole,  glandular  cells 
which  secrete  the  digestive  fluids.  The  intestine  corresponds 
functionally  to  both  the  stomach  and  intestine  in  the  higher  animals. 
The  outside  of  it  is  covered  and  the  interior  of  the  typhlosole  is 
filled  with  large  granular  cells  of  a  bright  golden  yellow  colour, 
causing  the  intestine  to  stand  out  in  a  striking  manner.  These 
chloragogen  cells,  glandular  though  they  are,  are  not  concerned  with 
digestion,  but  with  the  excretion  of  waste  nitrogenous  matter,  as 
we  shah1  see  later. 

The  earth  in  front  of  the  worm  containing  decaying  organic 
matter,  its  food,  is  sucked  in  through  the  mouth  by  the  action  of  the 
pharynx,  and  is  passed  on  by  a  series  of  waves  of  contraction  which 
travel  slowly  down  the  gut  one  after  another,  a  movement  termed 
peristalsis.  The  earth  is  thus  passed  through  the  oesophagus,  where 
the  acids  in  it  are  neutralised  by  the  calcium  carbonate  from  the 
calcareous  glands,  which  may  also  serve  for  ridding  the  body  of 
excess  of  lime  salts.  In  the  gizzard  it  is  ground  up,  and  in  the 
intestine  it  undergoes  most  of  its  digestion  and  absorption.  The 
proteids  are  turned  into  amino-acids,  the  starches  to  sugars,  and  so 
on,  and  the  soluble  products  pass  by  dialysis  through  the  intestinal 
wall  into  the  adjacent  blood-vessels  to  be  distributed  to  the  various 
parts  of  the  body.  The  insoluble  matter  from  the  food  and  all  the 
earth  is  passed  out  of  the  anus,  forming  the  "  casts  "  already 
referred  to. 

Between  the  gut  wall  and  body  wall  the  ccelom  is  filled  with  a 
colourless  watery  matter,  the  ccelomic  fluid,  which,  as  the  animal 
contracts  and  twists  about,  is  driven  backwards  and  forwards 
through  the  perforations  in  the  septa.  Thus  is  set  up  a  very 
rudimentary  circulation  which  may  also  help  in  the  conveyance  of 
food,  and  the  fluid  may  at  times  exude  in  tiny  drops  through  the 
dorsal  pores.  The  plasma  of  the  ccelomic  fluid  contains  two 
varieties  of  corpuscles  ;  in  the  first  place  large  immobile  spherical 
cells  containing  many  large  granules,  and  in  the  second  typical 
active  amcebocytes  with  a  finely  granular  protoplasm. 

For  the  first  time  in  the  animal  series  we  encounter  an 
animal  that  has  a  definite  circulatory  system  and  blood  ;  for  such  is 
present  in  Lumbricus,  in  addition  to  the  moving  ccelomic  fluid.  The 
blood-vessels  are  a  series  of  tubes  rendered  conspicuous  by  the  bright 
red  blood  within  them.  The  blood  owes  its  colour  to  Hcemoglobin, 
which,  however,  is  not  found  confined  to  red  corpuscles,  as  in  Rana, 
but  is  in  solution  in  the  plasma.  Corpuscles  are  indeed  present, 


176  AN   INTRODUCTION   TO  ZOOLOGY 

but  they  are  small  flat  colourless  cells  of  an  elongated  oval,  almost 
spindle  shape,  and  comparatively  few  in  number.  Circulation  is 
effected  by  peristaltic  contractions  of  the  main  trunks,  but  this  is 
greatly  augmented  by  the  action  of  five,  or  more  rarely  six,  pairs  of 
enlarged  muscular  vessels,  the  so-called  hearts.  As  in  vertebrates, 
the  circulation  is  constant,  for  the  main  trunks  give  off  branches 
which  ultimately  lead  to  a  capillary  network  whence  the  blood  is 
gathered  up  again  by  other  vessels  and  returned  to  the  main  trunks, 
thus  forming  a  closed  system.  In  spite  of  this,  however,  we  cannot 
employ  the  terms  "  arteries  "  and  "  veins,"  as  there  is  no  such  thing 
as  a  central  heart,  and  the  vessels  are  described  with  reference  to  the 
organs  they  supply  ;  those  bringing  blood  to  the  organs  are  afferent, 
and  those  conveying  it  away  again,  efferents.  There  are  four  main 
series  of  longitudinal  vessels  :  (i)  The  dorsal  or  supra-intestinal 
vessel  lies  in  the  mid-dorsal  line  above  the  alimentary  canal,  to 
which  it  is  more  or  less  closely  attached,  and  is  the  largest  trunk  in 
the  body.  The  contractions  of  its  walls  drive  the  blood  in  it  from 
hinder  to  front  end,  and  the  vessel  itself  breaks  up  at  both  ends  into 
an  indefinite  series  of  capillaries.  In  the  region  of  the  intestine 
this  trunk .  is,  as  it  were,  reinforced  by  another,  the  typhlosolar 
vessel,  which  runs  near  the  ventral  side  of  the  typhlosole.  It  is  not 
usually  considered  as  a  separate  vessel,  as  it  is  itself  somewhat 
indefinite,  often  duplicated,  and  connected  with  the  supra-intestinal 
by  a  large  number  of  anastomosing  trunks.  (2)  The  second  largest 
trunk  is  the  ventral  or  sub-intestinal  vessel  situated  in  the  mid- 
ventral  line  under  the  gut,  from  which  it  is  suspended  by  a  sort  of 
mesentery  formed  by  a  fold  of  tissue.  The  blood  in  it  passes  from 
before  backwards,  and  at  each  end  it  runs  into  the  same  capillary 
plexus  as  the  dorsal  vessel.  These  two  trunks,  the  sub-  and  supra- 
intestinal,  are  placed  in  direct  communication  with  one  another  by 
means  of  the  five  pairs  of  hearts  which  are  situated  in  segments  7-11, 
and  sometimes  an  additional  pair  in  somite  12.  As  the  blood 
passes  forwards  in  the  dorsal  vessel  it  must  flow  into  and  fill  the 
hearts,  which  by  their  muscular  contraction  drive  it  down  into  the 
ventral  vessel.  The  direction  of  flow  anterior  to  these  structures  is 
not  obvious.  (3)  Right  on  the  ventral  side  of  the  ccelom  lies  the 
nerve  cord,  and  attached  to  the  ventral  surface  of  this  is  the  sub-- 
neural  vessel,  in  which  the  blood  flows  from  before  backwards. 
In  the  twelfth  or  thirteenth  segment,  and  all  posterior  to  them,  this 
vessel  is  directly  connected  with  the  dorsal  vessel  by  a  commissural 
vessel  running  laterally  around  the  side  wall  of  the  coelom.  (4)  The 
last  two  longitudinal  trunks  are  a  pair  of  less  important  ones  lying 
at  the  side  of  the  nerve  cord,  and  known,  therefore,  as  the  lateral 
neural  vessels.  The  dorsal  vessel  in  segment  10  gives  off  a  pair 


178  AN  INTRODUCTION   TO  ZOOLOGY 

of  branches  which  pass  out  on  to  the  sides  of  the  oesophagus,  and  then 
run  forwards  into  the  anterior  plexus  as  the  lateral  cesophageal 
vessels,  and  each  of  these  also  sends  a  short  branch  backwards  to 
flow  into  the  first  of  the  commissural  vessels. 

The  distribution  of  the  branches  from  these  trunks  is  best  con- 
sidered in  relation  to  the  three  main  capillary  circulations  that  are 
formed,  although  the  exact  details  of  both  the  vessels  and  the 
direction  of  the  flow  of  the  blood  in  them  is  by  no  means  satisfactorily 
determined,  (i)  The  intestinal  circulation,  which  is,  of  course, 
mainly  concerned  with  digestion,  lies  in  the  intestinal  wall.  The 
supra-intestinal  trunk  gives  off  two  well-marked  branches,  the 
afferent  intestinal  vessels,  in  each  somite  which  run  down  on  the 
outside  of  the  gut  wall.  In  it  they  form  a  rich  capillary  network, 
from  which  the  blood  is  collected  up  by  two  efferent  intestinal  vessels 
in  each  segment.  These  convey  the  food-laden  blood  along  the 
inner  gut  wall  into  the  typhlosolar  vessel,  and  this  in  turn  is,  as  we 
have  seen,  closely  connected  with  the  dorsal  vessel  itself.  (2  and  3) 
The  next  two  systems,  the  dermaj^or  body  wall  circulation  and  the 
nephridial  circulation,  are  closely~connectecT  together.  From  the 
sub-intestinal  trunk  in  every  segment  arises  a  pair  of  vessels  which 
pass  laterally  for  a  short  distance  and  then  divide  into  two.  One 
branch,  the  afferent  dermal,  goes  to  the  skin,  breaking  up  in  the  sub- 
epidermal  layers,  and  the  other,  the  afferent  nephridial,  goes  to  the 
nephridium  or  excretory  organ,  and  both  form  a  very  close  capillary 
network.  The  skin  serves  as  a  respiratory  organ,  neither  gills  nor 
lungs  being  present,  and  the  oxygenated  blood  is  gathered  up  and 
returned  to  the  commissural  vessel  by  a  series  of  small  dermal 
efferents.  In  the  nephridium  the  waste  nitrogenous  matter,  and 
probably  also  excess  of  water,  is  removed  from  the  blood,  and  thus 
purified  it  is  collected  by  one  fairly  large  trunk,  the  efferent  nephridial 
vessel,  and  this  also  flows  into  the  commissural  vessel. 


As  has  just  been  indicated,  the  nitrogenous  excretion  in 
Lumbricus  is  carried  out  by  the  nephridia  or  segmental  organs. 
These  consist  of  a  series  of  complex  convoluted  tubes  arranged 
metamerically,  one  pair  in  each  segment  of  the  body  save  the  first 
three  and  the  last.  They  lie  in  the  ventro-lateral  portions  of  the 
coelom  and  are  closely  related  to  the  septa.  A  small  part  of  each, 
consisting  of  a  funnel-like  opening,  the  nephrostome,  projects  into  a 
somite  from  the  front  of  its  hinder  septum,  and  hence  is  known  as 
the  preseptal  portion.  This  communicates  through  the  septum 
with  the  much  larger  postseptal  portion,  which  is  a  long  convoluted 
tube  loosely  bound  together  by  connective  tissue  into  three  laterally 


THE   CCELOMATA   INVERTEBRATA 


179 


directed  main  loops.  The  nephrostome  is  situated  near  the  mid- 
ventral  line,  from  2-3  mm.  from  the  nerve  cord,  and  the  end  of  the 
tube  conies  back  to  near  the  middle  line,  and,  running  through  the 
body  wall,  reaches  the  exterior  as  the  tiny  hole,  the  nephridiopore, 
whose  position  has  already  been  pointed  out.  Thus  a  nephridium 


FIG.  57. — Stereogram  showing  the  relations  of  organs  in  the  posterior 
part  of  the  earthworm.  (Worked  out  by  Professors  McGregor  and 
Calkins,  and  drawn  by  Miss  Hedge.) 

opens  into  the  coelom  of  one  segment,  but  to  the  outside  through  the 
wall  of  the  following  one. 

The  preseptal  part  consists  of  the  nephrostome  proper  and  a 
neck-like  portion.  The  nephrostome  is  a  flat  oval  kidney-shaped 
structure,  the  main  part  of  which  is  composed  of  a  single  large 
crescent-shaped  cell,  the  central  cell.  Around  the  outer  margin 
of  this  is  a  row  of  slightly  wedge-shaped  columnar  cells,  the  marginal 


i8o 


AN   INTRODUCTION   TO  ZOOLOGY 


cells,  covered  on  their  dorsal  surface  with  long  fine  cilia,  they  are 
longer  in  the  middle  and  reduced  as  they  pass  towards  the  side  of  the 
crescent.  The  actual  opening  is  also  crescentic,  being  limited  by 
the  central  cell  vent  rally  and  a  projecting  lip  dorsally.  It  leads 
into  a  narrow  ciliated  tube,  the  neck,  which  after  a  short  distance 
perforates  the  septum  and  passes  on  into  the  next  part  of  the 
nephridium.  This  tube  is  surrounded  by  a  sort  of  collar  composed  of 
a  syncytium,  that  is  to  say,  a  mass  of  protoplasm  not  divided  up  into 
discrete  cells,  but  containing  a  large  number  of  nuclei. 

The  postseptal,  or  the  main  portion  of  the  nephridium,  is  a  very 
long  coiled  and  looped  tube,  in  which  we  can  easily  distinguish  four 
distinct  regions,  (i)  The  narrow  tube  is  the  direct  continuation  of 


A. 

FIG.  58. — A,  Diagram  of  a  nephridium  of  the  earthworm. 

ns.,  nephrostome  ;  s.,  septum  ;  ct.,  narrow  ciliated  tube  ;  m.t.,  middle  tube  ;  gl.,  wide  grandular 
non-ciliated  tube  ;  m.d.,  muscular  duct.  B,  a  portion  of  A  enlarged  to  show  the  passage  from  the 
middle  tube  to  the  glandular  tube.  C,  a  nephrostome  enlarged  ;  m.,  marginal  cells  ;  c.s.,  central 
cell ;  g.c.,  grooved  cells.  (After  Benham.) — From  Bourne. 

the  preseptal  tube,  and  it  runs  the  complete  length  of  the  first  and 
second  loops  returning  to  its  starting-point.  It  remains  narrow 
throughout,  and  is  much  twisted  and  coiled  upon  itself,  and  is 
ciliated  at  any  rate  in  parts  of  its  course.  At  its  end  it  finally  leaves 
the  first  loop  and  passes  to  the  inner  point  of  the  second  loop, 
becoming  transformed  as  it  does  so  into  the  next  portion.  (2)  The 
ciliated  or  middle  tube  is  much  shorter,  starting  at  the  base  of  the 
second  loop  and  only  running  out  to  its  lateral  extremity.  It  is 
wider  than  the  former,  and  owes  its  name  to  the  fact  that  it  is 
particularly  well  supplied  with  long  cilia,  which  remain  active  and 
can  be  seen  working  vigorously  under  the  microscope  for  a  consider- 
able time  after  the  nephridium  has  been  removed  from  the  body. 
The  walls  of  this  tube  are  very  granular  and  of  a  brownish  colour. 


THE   CCELOMATA   INVERTEBRATA  181 

(3)  The  succeeding  part,  the  wide  tube,  is  still  wider  than  the  middle 
tube,  and  starts  at  the  distal  extremity  of  the  second  loop  in  a 
distinct  vesicular  enlargement,  the  ampulla.     This   tube   also  has 
coloured  and  very  granular  walls,  for  which  reason  it  is  sometimes 
distinguished  as  the  glandular  tube.     It  runs  to  the  base  of  the 
second  loop  and  then  back  through  the  whole  of  the  first  loop  to 
near  the  point  of  entry  of  the  narrow  tube,  where  it  finally  leaves  the 
first  loop  and  passes  over  into  the  last  part  of  the  nephridium. 

(4)  The  terminal  portion,  constituting  the  entire  third  loop,  is  the 
muscular  duct  or  bladder.     It  opens  to  the  exterior  by  the  nephridio- 
pore,  and,  unlike  the  other  tubes,  has  a  muscular  wall  forming  a  sac, 
in  which  the  excretory  fluid  can  be  stored  and  ejected  to  the  exterior 
from  time  to  time.     The  three  loops  are  bound  together  by  con- 
nective tissue,  with  which  is  associated  a  large  number  of  chloragogen 
cells,  and  in  which  is  found  a  capillary  plexus  that  is  particularly 
marked  in  the  portions  occupied  by  the  ciliated  and  glandular  tubes. 

An  important  difference  is  to  be  noted  between  the  muscular 
duct  and  the  remaining  parts  of  the  nephridial  tubes.  It  is  lined  by 
flattened  cells,  so  that  its  cavity  is  intercellular,  whereas  the  lumen 
of  all  the  other  portions  is  intracellular.  They  are  actually  com- 
posed of  a  series  of  hollow  cylindrical  cells  joined  end  to  end,  which, 
from  their  appearance,  are  known  as  drain-pipe  cells.  This  difference 
is  reflected  in  their  development  also,  the  muscular  duct  arising  as 
a  hollow  invagination  of  the  ectoderm,  while  the  remaining  portion 
is  laid  down  as  a  solid  strand  of  ectodermal  cells,  in  which  the  lumen 
is  formed  later. 

Excretion  in  these  organs  is  of  a  twofold  nature.  The 
ciliated  and  wide  tubes  are  granular  in  appearance,  owing  to  the 
presence  in  them  of  excretory  particles  that  they  have  formed  by 
secretion  from  the  blood.  These  are  passed  into  the  lumen  and 
washed  down  into  the  bladder  by  the  current  set  up  by  the  cilia, 
and  so  in  this  way  nitrogenous  waste  and  water  are  eliminated. 
The  other  kind  of  excretion  is  possible,  owing  to  the  fact  that  the 
nephrostome  opens  directly  into  the  coelomic  fluid,  and  the  cilia 
covering  the  marginal  cells  work  in  such  a  way  as  to  convey  any 
small  particles  to  its  opening,  which  is,  however,  too  small  to  allow 
of  the  passage  of  amcebocytes.  Any  foreign  bodies,  bacteria,  etc., 
that  may  have  entered  through  the  dorsal  pores  can  be  removed  in 
this  way.  The  main  bulk  of  the  matter  removed  by  this  channel, 
however,  is  derived  from  the  chloragogen  cells.  These  gradually 
secrete  within  themselves  granules  of  excrement,  and  when  full  come 
loose  from  the  gut  wall  into  the  coelomic  fluid.  Here  they  break 
down  into  tiny  fragments,  which  are  gradually  swept  down  the 
nephrostome  by  the  action  of  the  cilia.  The  amcebocytes  also 


182  AN   INTRODUCTION  TO  ZOOLOGY 

appear  to  play  an  active  part  in  this  process,  by  collecting  up  the 
various  granules  and  conveying  them  to  the  nephrostome,  where  they 
yield  them  up  again.  It  is  considered  by  some  authorities  that  this 
excretory  function  on  the  part  of  the  peritoneal  cells  is  a  primitive 
one,  and  was  the  sole  one  before  the  evolution  of  the  special  and 
complex  segmental  organs,  the  nephridia. 


The  muscular  system  in  the  worm  is  a  very  simple  one, 
consisting  in  the  main  of  two  double  sheaths  of  muscle  fibres.  The 
first  is  the  somatic  or  body  wall  musculature,  and  it  consists  of  a 
series  of  muscle  fibres  arranged  circularly  all  along  the  animal 
underneath  the  epidermis,  save  where  it  is  interrupted  interso- 
mitically  at  the  grooves,  and  a  much  thicker  layer  of  longitudinal 
muscles.  The  latter  are  bounded  internally  by  the  lining  of  the 
body  cavity,  the  peritoneum,  and  are  arranged  in  bands.  Two 
dorso-lateral  bands  extend  from  the  mid-dorsal  line,  where  they  are 
just  separated  from  one  another  in  the  line  of  the  dorsal  pores,  to 
the  line  of  the  lateral  setae,  whose  enveloping  sacs  again  form  a  break. 
The  two  latero- ventral  bands  occupy  the  spaces  between  the  lines 
of  lateral  and  ventro-lateral  setae,  and,  lastly,  a  ventral  band 
occupies  the  mid-ventral  line.  The  tiny  longitudinal  muscle  fibres 
are  arranged  on  each  side  of  a  large  number  of  radially-running 
partitions,  and  exhibit  in  transverse  section  a  very  characteristic 
feather-like  appearance.  The  second  sheath  is  the  splanchnic  or 
gut  wall  musculature,  and  it  consists  of  an  inner  very  thin  layer  of 
circularly  arranged  fibres  lying  beneath  the  entoderm  and  an  outer 
even  thinner  layer  of  longitudinal  fibres.  These  two  layers  are 
considerably  developed  in  the  pharynx  and  gizzard.  They  are 
bounded  by  the  splanchnic  or  gut  layer  of  peritoneum,  whose  cells 
are  highly  modified  in  the  intestinal  region  to  form  the  chloragogen 
cells.  In  addition  to  these  sheaths  there  are  the  special  muscle 
strands  already  mentioned  as  radiating  from  the  pharynx.  All  the 
muscles  seem  to  be  composed  of  cellular  non-striated  fibres. 


The  normal  method  of  reproduction  takes  place  sexually 
by  the  production  of  gametes,  but  if  a  worm  is  cut  up  into  a  number 
of  pieces,  each,  as  in  Hydra,  possesses  the  power  of  regeneration, 
enabling  it  to  regrow  the  lost  parts.  Thus  the  agriculturist  who 
viciously  cuts  a  worm  in  halves  with  his  spade  is  doing  his  land  more 
good  than  he  knows,  not  by  removing  a  worm,  but  by  adding  to 
their  numbers.  Lwnhricus  is  monoecious  or  hermaphrodite,  the 
two  essential  sexual  organs,  ovaries  and  testes,  occurring  in  the 


THE   CCELOMATA   INVERTEBRATA  183 

same  individual.  In  spite  of  this,  however,  it  is  unable  to  fertilise 
its  own  ova,  and  cross- fertilisation  by  another  worn  is  brought  about 
in  a  manner  we  shall  discuss  shortly.  The  reproductive  organs  lie 
towards  the  anterior  end,  and  are  included  in  segments  9-15. 

The  most  conspicuous  parts  of  the  reproductive  organs  are  large 
white  sac-like  structures,  lying  ventrally  and  laterally  to  the  oeso- 
phagus, apparently  in  segments  9-12  or  13.  These  are  the  sperm 
reservoirs,  or  vesiculse  seminales,  in  which  are  to  be  found  sperma- 
tozoa in  all  stages  of  development,  and  although  they  vary  in  size 
at  different  periods  of  the  year  they  are  always  readily  made  out. 
They  consist  of  two  mesial  sacs,  the  anterior  and  posterior  median 
sperm  sacs,  situated  in  segments  10  and  n  respectively.  From  the 
front  corners  of  the  former  arise  a  pair  of  antero-lateral  sperm  sacs, 
which  push  forward  in  front  of  them  the  septum  between  segments 
9  and  10,  and  so  appear  to  lie  in  somite  9.  The  hinder  corners  in  a 
similar  way  give  off  the  median  lateral  sperm  sacs,  which  push  back 
the  septum  between  10  and  n,  and  so  seem  to  lie  in  segment  n. 
The  postero-lateral  sperm  sacs  come  off  from  the  caudal  corners  of 
the  posterior  median  reservoir,  and  although  really  lying  in  somite 
ii  project  backwards  into  segment  12  and  often  13,  carrying  the 
septum  between  n  and  12  with  them.  They  are  very  large  struc- 
tures. In  almost  any  part  of  the  vesiculae  seminales  there  may  be 
living  Monocystis  agilis,  the  protozoon  parasite  already  studied,  which 
may  be  found  by  examining  a  smear  of  their  contents  under  the 
microscope.  The  actual  male  reproductive  organs  are  difficult  to 
see,  as  they  lie  within  the  sperm  sacs,  and  the  dorsal  wall  of  these 
structures  must  first  be  removed.  There  are  two  pairs  of  testes, 
one  pair  in  each  of  segments  10  and  n.  They  are  small  flattened 
digitate  bodies,  attached  to  the  hinder  surfaces  of  the  septa  between 
somites  9  and  10,  10  and  n,  and  projecting  freely  into  the  cavity 
of  the  sperm  sacs.  Each  is  formed  by  a  localised  thickening  of  the 
ccelomic  epithelium  covering  the  septum,  and  is  composed  of  a  mass 
of  cells,  the  spermatogonia  or  sperm  mother  cells.  The  spermatozoa 
are  not  actually  produced  in  the  testes,  which  only  shed  into  the  sperm 
sac  small  groups  of  from  eight  to  sixteen  spermatogonia,  and  there 
these  are  transformed  into  sperms.  On  the  anterior  faces  of  the  other 
septa  in  the  same  somites  and  opposite  to  the  testes  are  four  fairly 
large  funnel-shaped  apertures,  which  from  the  complexly  folded  nature 
of  their  margins  are  known  as  the  "  ciliated  rosettes."  It  is  to 
these  structures  that  the  large  parasite  Monocystis  magna  is  to  be 
found  attached  ;  it  does  not  float  freely,  as  does  its  smaller  relative 
M.  agilis.  These  funnels  lead  through  the  septa  into  very  fine 
convoluted  tubes,  the  vasa  efferentia,  which  unite  towards  the  posterior 
end  of  the  twelfth  segment  to  form  the  single  vas  deferens  on  each 


184  AN   INTRODUCTION  TO  ZOOLOGY 

side  of  the  body.  This  fine  duct  passes  straight  back  on  the  ventral 
floor  of  the  coelom  to  somite  15,  where  it  passes  through  the  body  wall 
to  the  external  male  aperture. 

The  female  part  of  the  reproductive  system  is  much  less  con- 
spicuous than  the  male.  Only  one  pair  of  ovaries  is  present,  situated 
on  the  hinder  surface  of  the  septum  between  segments  12  and  13, 
and  projecting  freely  into  the  cavity  of  the  latter.  The  ovary 
is  again  a  localised  thickening  of  the  ccelomic  epithelium,  forming 
a  pear-shaped  body  about  2  mm.  in  length.  At  the  anterior  wider 
end  is  a  mass  of  small  cells,  the  primordial  ova  and  the  oogonia,  and 
the  narrower  end  is  formed  by  a  single  row  of  large  nearly  ripe 
ova,  each  surrounded  by  an  envelope  of  thin  cells  constituting  a 
follicle.  Two  short  ciliated  funnel-shaped  openings,  the  oviducal 
funnels,  are  situated  on  the  septum  at  the  opposite  end  of  the  somite 
vis-a-vis  to  the  ovaries.  They  perforate  the  septum  as  oviducts, 
and  immediately  on  the  posterior  side  of  it  each  undergoes  an 
enlargement  to  form  a  small  sac,  the  receptaculum  ovorum  or  ovisac. 
After  a  very  short  straight  course  the  oviducts  open  to  the  outside 
in  segment  14. 

In  addition  to  these  sets  of  male  and  female  organs  we  also  find 
four  accessory  structures,  the  spermathecse,  whose  function  we  shall 
discuss  shortly.  These  are  two  pairs  of  ovoid  whitish  or  light 
yellow  sacs,  lying  ventro-laterally  in  the  posterior  halves  of  somites 
9  and  10. 

The  testis,  as  has  been  noted,  sheds  masses  of  protoplasm  with 
from  eight  to  sixteen  nuclei,  but  only  indistinctly  divided  into  cells, 
the  spermatogonia.  In  the  vesiculae  seminales  these  apparently 
undergo  two  successive  divisions,  producing  32  to  64  much  smaller 
cells,  thespermatids,  around  a  central  mass  of  protoplasm  known  as  the 
cytophore.  These  cells  gradually  alter  until  they  form  very  typical 
spermatozoa  with  small  heads  and  long  filiform  tails,  which  still 
retain  a  spherical  arrangement  around  the  central  mass.  These 
groups  of  cells  exhibit  a  very  characteristic  mulberry-like  appear- 
ance during  these  changes,  and  are  in  consequence  known  as  the 
sperm  morulae.  Finally,  the  ripe  sperms  are  set  free  into  the  fluid 
content  of  the  sperm  sacs.  It  is  into  the  cytophore  of  the  spermato- 
zonia  that  the  young  sporozoite  of  M.  agilis  bores  its  way  and 
gradually  consumes  it  while  the  surrounding  spermatozoa  are  under- 
going their  maturation  divisions.  The  tail  of  the  ovary,  as  we  have 
seen,  contains  a  string  of  large  nearly  ripe  ova  each  enclosed  in  a 
follicle,  which  presently  bursts  releasing  the  ovum,  which  is  taken  up 
by  the  oviducal  funnel  and  stored  in  the  receptaculum.  Here  it 
undergoes  further  ripening  until  it  becomes  mature. 

During  the  act  of  copulation  two  worms  lie  together  in  head 


THE   CGELOMATA   INVERTEBRATA  185 

to  tail  directions  in  such  a  way  that  the  opening  of  the  vasa  deferentia 
of  the  one  come  to  lie  opposite  to  the  openings  of  the  spermathecae 
in  the  other  and  vice  versa.  The  sperms  are  then  transferred  from 
each  worm  to  the  spermathecae  of  the  other,  where  they  can  live 
for  some  time  until  required,  after  this  the  worms  separate.  It  is 
these  spermatozoa  that  fertilise  the  ova  of  the  worm  in  which  they 
are  lodged,  and  consequently  the  ova  of  one  worm  are  cross-fertilised 
by  the  sperms  obtained  from  another.  Some  time  after  copulation 
the  clitellum  secretes  freely  a  substance  which  as  it  hardens  forms 
an  elastic  band,  the  cocoon,  round  the  worm  and  at  the  same  time 
it  also  secretes  into  it  a  nutrient  fluid.  The  worm  then  wriggles 
backwards  in  this,  and  when  it  comes  to  lie  opposite  to  the  oviducal 
pore  the  eggs  from  the  receptaculum  are  laid  in  it,  so  that  it  can  also 
be  spoken  of  as  an  egg  capsule.  Further  wriggling  brings  it  over 
the  spermathecal  pores,  and  here  the  sperms  are  discharged  and 
fertilisation  takes  place,  one  sperm  entering  each  ovum.  A  supply 
of  albuminous  fluid  is  added  to  the  contents  of  the  cocoon  to  serve 
as  food,  and  finally  the  worm  completely  escapes  from  it  and  it 
automatically  closes  at  its  two  ends  by  its  own  elasticity.  The 
cocoon  when  left  behind  in  the  earth  is  a  toughish  oval  sac  of  yellow 
or  yellowish-brown  colour  about  5  or  6  mm.  in  length.  Although 
a  large  number  of  eggs  are  laid  and  even  fertilised,  only  a  few  or 
perhaps  even  one  alone  undergoes  complete  development.  The 
nutritive  fluid  serves  as  food  for  the  growing  embryo,  which  when  it 
leaves  the  cocoon  is  already  a  minute  little  worm  but  quite  complete, 
and  it  only  needs  to  grow  in  order  to  become  a  mature  adult.  Thus 
we  have  no  free  living  larval  stage  to  correspond  with  the  planula 
of  Hydra,  and  the  development  is  direct  from  an  egg  through  an 
embryo  to  the  definitive  adult  form. 

As  in  the  frog,  the  nervous  system  of  Lumbricus-  may  be 
divided  for  convenience  of  description  into  a  central  and  a  peripheral 
portion,  and  so  represents  a  condition  considerably  in  advance  of 
that  in  Hydra,  for  it  is  a  concentrated  and  not  a  diffuse  system.  On 
the  internal  side  of  the  ventral  body  wall  in  the  middle  line  the'nerve 
cord  is  to  be  seen  as  a  long  whitish  cord  exhibiting  an  enlargement  or 
ganglionic  swelling  in  each  segment  of  the  body.  In  the  fourth 
somite  the  swellings  are  quite  marked,  forming  the  sub-pharyngeal 
ganglia,  which  diverge  from  one  another  and  are  continued  forward 
around  the  pharynx  as  two  moderately  fine  nervous  strands,  the 
circum-pharyngeal  connectives.  These  pass  on  to  the  dorsal  side  of 
the  alimentary  canal  and  terminate  in  the  third  somite  in  two 
conspicuous  enlargements  touching  one  another  in  the  middle  line, 
the  so-called  cerebral  or  supra-pharyngeal  ganglia.  Although 
appearing  as  a  single  strand  the  ventral  cord  is,  morphologically 


i86  AN   INTRODUCTION   TO  ZOOLOGY 

speaking,  two  cords  running  side  by  side  and  joined  by  transverse 
bands,  the  commissures,  and  this  structure  becomes  more  apparent 
when  viewed  in  transverse  section.  It  arises  also  as  two  distinct 
cords  on  the  ectoderm  of  the  embryo,  which  become  approximated 
as  they  sink  in  from  the  outside,  and  it  is  to  be  regarded  as  a  modifica- 
tion of  a  type  of  nervous  system  quite  common  in  invertebrate 
animals.  This  sort  of  nervous  system  consists  typically  of  two 
parallel  ventrally  situated  lines  of  ganglia  linked  on  each  side  by 
longitudinally  running  connectives  and  joined  by  a  transverse 
commissure  joining  the  two  ganglia  in  each  segment.  From  its 
characteristic  appearance  this  sort  of  nervous  system  is  termed  the 
rope-ladder  type.  It  is  masked  in  the  worm  by  the  apposition  of 
the  two  cords  and  the  shortening  of  the  commissures  transversely, 
but  extension  of  them  longitudinally.  Running  in  the  groove  on  the 
dorsal  side  of  the  cord  are  three  long  fibres,  the  giant  fibres,  contain- 
ing the  axons  of  certain  large  ganglion  cells,  and  these,  together  with 
the  cord  itself,  the  ventral  neural  and  lateral  neural  vessels,  are  all 
bound  together  by  one  fairly  tough  fibrous  connective  tissue  sheath. 
The  peripheral  nervous  system  is  composed  of  the  nerves  and  the 
sensory  cells  situated  in  the  epidermis.  A  series  of  paired  nerves 
come  off  from  the  central  nervous  system  and  are  distributed  gene- 
rally to  the  viscera  and  body  wall.  The  first  pair  arise  from  the  front 
end  of  the  cerebral  ganglia  and  pass  forwards  to  supply  the  pro- 
stomium,  being  apparently  mainly  sensory  in  function.  A  second 
pair  start  from  the  circum-pharyngeal  connectives  and  supply  the 
peristomium.  The  third  pair  are  large  and  run  forward  from  the 
sub-pharyngeal  ganglia  to  be  distributed  partly  to  the  peristome, 
but  mainly  to  the  second  and  third  somites.  In  each  of  the  remain- 
ing segments  there  are  typically  three  pairs  of  nerves  ;  two  pairs 
come  off  close  together  from  the  ganglion  in  the  mid  somitic  region, 
and  the  last  pair,  originating  in  the  connectives  at  the  front  end  of 
each  somite,  are  distributed  mainly  to  the  septum  just  behind  which 
they  arise.  The  sensory  cells  are  present  in  the  epidermis,  occurring 
singly  or  in  groups,  forming  the  so-called  sense-buds.  A  large  number 
of  these  are  present,  particularly  at  the  anterior  end  of  the  animal. 
They  lie  among  the  ordinary  epidermal  cells  and  are  arranged  in 
rings,  of  which  there  may  be  three  in  each  somite,  the  middle  one 
containing  as  many  as  sixty  buds. 

The  mass  of  the  central  nervous  system  is  composed  of 
ganglion  cells  and  their  processes,  and  they  are  in  the  main  of  two 
sorts,  although  a  third  variety,  a  sensory  ganglion  cell,  may  also  be 
present,  i.  The  motor  neurons  are  typical  nerve  cells,  mainly 
pear-shaped  and  bipolar  but  sometimes  multipolar,  whose  dendrons 
are  situated  within  the  ventral  cord.  They  are  more  numerous  in 


THE   CCELOMATA   INVERTEBRATA  187 

the  ganglionic  enlargements  than  in  the  connectives,  although  they 
are  also  present  in  these.  Their  axon  leaves  the  cord  as  a  non- 
medullated  afferent  or  motor  nerve  fibre,  andruns  outwards,  frequently 
terminating  in  a  muscle.  They  may,  however  be  acceleratory, 
inhibitory,  or  excitatory  in  function  when  they  are  distributed  to 
other  parts  than  the  actual  muscle  elements.  2.  The  commissural 
or  association  neurons  are  only  to  be  found  in  the  cord  itself  and  are 
bipolar.  The  axon  and  single  dendron  are  very  similar  to  look  at, 
and  they  function  in  placing  the  various  other  nerve  elements  in 
communication  with  one  another.  A  sense-bud,  which  causes  a 
.slight  projection  on  the  surface  of  the  epidermis,  is  composed  of  a 
number  of  elongated  rod-like  sensory  cells.  Each  cell  bears  at  its 


,s.c. 


M.F 


N.C.  . 

FIG.  59. — Diagram  to  show  the  relations  of  the  nerve  cells  in  Lumbricus, 
adapted  and  modified  from  Lenhossek  and  Retzius. 

A.N.,  association  neuron  ;  C.,  cuticle  ;  E.,  epidermis  ;  M.F.,  motor  fibre  ;  M.N.,  motor  neuron  ; 
N.,  nerve  ;  N.C.,  nerve  cord  ;  S.C.,  sensory  cell  or  sensory  neuron  ;  S.F.,  sensory  fibre  ;  S.P., 
sensory  process. 

external  end  a  short  stiff  sensory  or  receptive  process,  and  the  cuticle 
overlying  it  is  pierced  with  a  small  hole  to  allow  it  to  project  freely 
to  the  exterior.  At  its  inner  end  the  sensory  cell  is  continued  on  as 
an  afferent  or  sensory  fibre,  whose  terminal  dendrite  arborises  in 
connection  with  the  association  neurons.  The  whole  structure  is  a 
typical  neuro-epithelial  cell,  whose  receptive  process  corresponds  with 
the  dendron  and  the  fibre  with  the  axon,  and  is  comparable  with  the 
olfactory  cell  in  the  frog.  It  is  also  probable  that  certain  sensory 
neurons  are  to  be  found  in  the  cord,  and  in  this  case  their  dendrons, 
which  terminate  by  branching  round  a  modified  epidermal  cell, 
would  form  the  afferent  fibre,  but  such  cells  are  apparently  not  very 
numerous.  Through  these  various  sensory  cells  the  earthworm  is 
put  in  communication  with  the  outer  world,  and  so  can  appreciate 


188  AN   INTRODUCTION  TO  ZOOLOGY 

sensations  of  touch,  taste,  smell  and  light,  but  so  far  as  we  can 
ascertain  it  is  incapable  of  receiving  sound  impressions.  The  sense 
of  touch  is  the  most  widely  distributed  of  them,  and  is  spread  all  over 
the  body.  Next  to  that  is  the  sense  of  light,  which  is  present  all  over 
the  anterior  end  but  is  absent  from  the  posterior  half  of  the  worm. 
The  senses  of  taste  and  smell  are  more  limited  in  their  extent,  being 
confined  almost  entirely  to  the  prostomium,  the  peristomium  near 
the  mouth,  buccal  cavity  and  pharynx. 

Let  us  now  consider  the.  mode  of  action  of  the  nervous  system 
as  a  whole.  Four  elements  are  involved  in  the  reflex  arc  in  the  case 
of  the  worm,  three  of  them  are  nervous  and  the  other  a  muscle. 
The  sensory  receptor  projecting  from  the  epidermis  with  its  afferent 
process  arborising  in  the  cord  is  in  connection  with  the  dendron  of  an 
association  neuron,  and  the  axon  of  this  cell  is  in  turn  related  to  the 
dendron  of  a  motor  cell.  Thus  the  stimulus  is  transferred  from  the 
skin  to  a  motor  neuron,  and  thence  sent  down,  via  its  axon,  to  the 
muscle  connected  with  the  epidermis,  bringing  about  a  movement  of 
the  latter.  This  then  is  a  very  simple  reflex  arc,  and  although  quite 
a  small  stimulus  is  bound  to  affect  a  large  number  of  sensory  cells 
and  an  even  larger  number  of  motor  neurons  the  process  is  essentially 
the  same.  The  cerebral  ganglia  certainly  play  a  part  in  co-ordinating 
the  movements  of  the  animal  as  a  whole,  but  not  to  nearly  such  a 
large  extent  as  does  the  brain  of  the  frog,  and  the  main  activities  of 
the  worm  are  the  result  of  these  reflex  actions.  This  is  most  clearly 
seen  when  a  worm  is  cut  to  pieces,  for  each  part,  although  entirely 
removed  from  any  connection  with  the  cerebral  ganglia,  still  ex- 
hibit normal  movements,  which  must  of  necessity  be  reflexes,  and  the 
ganglion  pair  in  each  somite  is  the  centre  of  such  actions  for  its  own 
segment.  The  presence  of  the  commissural  neurons  with  their  well- 
branched  axon  and  dendron  makes  it  possible  for  a  sensory  impulse, 
even  coming  from  a  limited  area,  if  sufficiently  strong,  to  be  dis- 
tributed to  a  fairly  large  number  of  motor  cells,  and  consequently 
to  bring  about  adjusting  movements  on  the  part  of  larger  portions  of 
the  body  or  even  of  the  animal  as  a  whole.  By  these  means,  then, 
the  movements  of  the  worm  in  response  to  the  stimuli  produced  by 
changes  in  its  environment  are  brought  about. 

An  examination  of  a  transverse  section  of  Lumbricus 
when  compared  with  one  of  Hydra  will  reveal  certain  of  the  impor- 
tant differences,  not  merely  between  the  animals  themselves,  but 
between  the  Ccelomata  and  Coelenterata.  In  Hydra,  as  we  have  seen, 
the  body  wall  is  simple,  consisting  of  an  ectoderm  and  an  entoderm 
with  the  intermediate  mesoglea,  and  it  bounds  a  single  internal 
cavity,  the  coelenteron.  In  Lumbricus  the  body  wall  is  composed  of 
five  distinct  layers,  i.  The  cuticle  is  the  outermost,  and  consists 


THE   CCELOMATA   INVERTEBRATA 


189 


of  a  thin  transparent  membrane,  composed  of  two  sets  of  inter- 
crossing fibres  perforated  by  fine  pores.  2.  The  ectoderm  or  epi- 
dermis below  the  cuticle  is  composed  of  short  columnar  cells,  forming 
a  simple  epithelium,  and  containing  a  large  number  of  mucous 
secreting  gland  cells,  and  also  the  sensory  cells  arranged  singly  or  in 
groups.  In  the  region  of  the  clitellum  the  epidermal  cells  are  greatly 
enlarged  and  mostly  glandular.  The  blood  capillaries  actually 
penetrate  this  modified  part  of  the  epithelium — a  very  unusual 


m 


m 


FIG.  60. — Transverse  section  of  the  earthworm  behind  the  clitellum. 

a.c.,  cavity  of  the  digestive  tract ;  c.,  cuticle  ;  cos.,  ccelom  ;  c.m.,  circular  muscles  ;  c.v., 
circular  vessel ;  d.v.,  dorsal  vessel ;  hy.,  hypodermis  ;  l.nt.,  longitudinal  muscles  ;  n.c.,  ventral 
nerve  chain  ;  p.e.,  peritoneal  endothelium  ;  s.,  seta  ;  s.g.,  setigerous  gland  ;  s.t'.v.,  sub-intestinal 
vessel ;  s.m.,  muscle  connecting  two  groups  of  setae  on  the  same  side  ;  ty.,  typhlososle. — From 
Sedgwick  and  Wilson. 

occurrence  in  the  animal  kingdom.  Certain  insinkings  of  the 
epidermis  form  the  setigerous  sacs,  in  which  the  setae  are  lodged. 
Each  is  the  product  of  one  cell,  and  as  the  setae  are  constantly  being 
worn  out  and  replaced  there  are  always  setae  to  be  found  within  the 
sac  in  various  stages  of  development.  They  lie  in  a  group  of  active 
glandular  cells  which  surround  the  bases  of  the  two  sacs  in  each  pair, 
and  these  masses  constitute  the  setigerous  glands.  3.  Next  to  the 
epidermis  we  find  a  moderately  thin  layer  of  circular  muscle  fibres. 


igo  AN   INTRODUCTION  TO  ZOOLOGY 

in  which  is  a  rich  capillary  network.  They  are  embedded  in  a  matrix 
of  connective  tissue,  which  is  pigmented  on  the  dorsal  side.  The 
muscle  fibres  themselves  consist  of  long  narrow-pointed  cells,  each 
with  a  nucleus  situated  in  a  central  core  of  unmodified  protoplasm 
surrounded  by  a  layer  of  finely  striated  contractile  material. 
4.  These  are  followed  by  a  much  thicker  layer  of  longitudinal  muscle 
fibres.  They  are  also  embedded  in  connective  tissue  and  arranged 
in  a  very  characteristic  manner  on  both  sides  of  fine  radially  disposed 
partitions.  5.  Lastly,  we  find  a  thin  layer  of  very  flattened  cells 
forming  the  somatic  part  of  the  coelomic  or  peritoneal  epithelium. 
Within  this  complex  body  wall  is  a  large  space,  the  body  cavity  or 
ccelom,  in  which  lie  the  gut,  the  blood-vessels,  the  excretory  organs, 
etc.,  and  which  is  not  represented  in  any  way  in  Hydra. 

The  wall  of  the  gut  is  also  complex,  being  composed  of  four  layers. 
i.  Lining  the  inside  of  the  alimentary  canal  is  a  single  layer  of  long 
columnar  epithelial  cells,  most  of  which  are  ciliated,  but  quite  a 
fair  number  are  glandular.  They  are  over  the  greater  part  of  its 
length  entodermal,  and  constitute  the  enteric  epithelium.  2.  Out- 
side this  is  a  thin  layer  of  circular  muscle  fibres,  by  means  of  which 
the  peristaltic  movements  of  the  alimentary  canal  are  produced. 
They  are  also  supplied  on  their  inner  side  by  the  rich  sub-epithelial 
capillary  plexus,  which  plays  a  large  part  in  the  activities  concerned 
with  digestion.  3.  A  very  thin  layer  of  longitudinal  muscle  fibres 
surround  the  circular  ones.  4.  Lastly,  these  in  their  turn  are  covered 
by  the  visceral  or  splanchnic  part  of  the  peritoneal  epithelium. 
In  the  intestinal  region  the  cells  of  this  layer  are  transformed  into 
the  striking  chlorogogen  cells,  whose  appearance  and  structure  has 
already  been  dealt  with. 


Thus  in  Lumbricus  between  the  ectoderm  and  entoderm 
there  is  a  well-developed  third  cellular  layer,  the  mesoderm,  com- 
plexly differentiated  into  muscles  and  epithelium,  and  divided  into 
a  somatic  portion,  forming  the  main  part  of  the  body  wall,  and  a 
splanchnic  part,  helping  to  build  up  the  gut  wall,  and  between  these 
two  is  the  large  ccelom.  This  cavity  exhibits  the  three  main 
characteristics  of  any  true  ccelom  ;  it  is  lined  by  mesoderm  and 
surrounds  the  gut,  the  gonads  are  produced  by  its  epithelial  lining, 
and  the  primitive  excretory  tubules  open  into  it.  Such  a  type  of 
structure  is  quite  characteristic  of  all  the  higher  Ccelomata,  although 
in  some  groups  it  may  become  secondarily  modified. 

Further  differences  are  to  be  noted  between  Hydra  and  Lumbricus. 
The  latter  form  has  a  well- developed  muscular  system  composed  of 
muscle  fibres  and  not  of  epithelio-muscular  cells.  The  alimentary 


THE  CCELOMATA   INVERTEBRATA  191 

canal  is  far  more  highly  developed,  and  possesses  two  external 
openings,  a  mouth  and  an  anus,  and  in  addition  sac-like  glandular 
outgrowths.  It  can  be  divided  into  an  anterior  part  lined  with 
ectoderm,  the  stomodceum,  a  mid  portion  lined  with  entoderm,  the 
midgut  or  mesenteron,  and  a  very  small  posterior  portion  also 
lined  with  ectoderm,  the  proctodoeum.  Owing  to  the  presence  of 
the  ccelom  many  parts  of  the  body  are  separated  from  the  alimentary 
canal,  and  this  is  itself  removed  from  the  oxygen  of  the  air,  and  so 
we  find  a  well- developed  circulatory  system  for  the  distribution 
of  nutriment  and  for  respiration.  The  gonads  are  inside  the  ccelom 
of  the  worm,  and  consequently  require  genital  ducts  to  convey  their 
products  to  the  exterior.  In  Hydra  both  layers  form  a  free  surface 
over  which  excretion  can  take  place,  but  many  layers  in  Lumbricus 
are  not  free,  so  that  it  possesses  a  series  of  complex  excretory  organs 
for  its  nitrogenous  waste  in  the  form  of  segmentally  arranged 
nephridia.  Lastly,  the  diffuse  network  of  nerve  cells  in  Hydra 
is  replaced  by  a  compact  ventrally  situated  central  nervous  system, 
connected  on  the  one  hand  with  the  muscles,  and  on  the  other  with 
a  number  of  sensory  cells  in  the  ectoderm.  All  of  these  points  are 
also  characteristic  of  the  higher  ccelomates,  save  that  in  the  Verte- 
brata  the  concentrated  nervous  system  is  on  the  dorsal  side  of  the 
animal ;  and  although  in  the  early  development  the  excretory  tubules 
are  segmentally  placed,  they  lose  this  primitive  arrangement  in  the 
adult.  Thus,  just  as  Hydra  represented  a  considerable  advance  on 
the  Protozoa,  so  Lumbricus  in  its  turn  is  on  a  much  higher  plane  than 
the  Ccelenterata,  and,  indeed,  its  metamerism  and  general  plan, 
although  sometimes  much  modified,  ca^nd)  to  be  traced  throughout 
all  the  higher  animals. 


Turning  now  to  consider  the  development  of  Lumbricus,  it  is, 
as  might  be  expected,  much  more  complicated  than  that  of  Hydra, 
and  we  need  only  take  notice  of  its  main  features.  The  egg  is  laid, 
as  we  have  seen,  with  others  in  a  cocoon  provided  with  a  store  of 
nutrient  fluid,  and  is  spherical  and  fairly  transparent,  with  a  relatively 
small  amount  of  yolk.  The  segmentation  or  cleavage  of  the  egg  is 
complete  or  holoblastic,  that  is  to  say,  the  lines  of  division  cut  the 
original  ovum  up  completely  into  separate  cells,  and  a  hollow  sphere 
of  cells  or  blastula  is  formed.  The  cells  of  the  blastula  are  of  unequal 
size,  being  divided  into  an  upper  set  of  small  ones,  the  micromeres, 
and  a  lower  set  much  larger,  the  macromeres.  All  the  cells  continue 
to  divide,  getting  smaller  in  the  process,  save  two  neighbouring 
macromeres  near  the  equatorial  line.  These  remain  of  large  size, 
and  as  they  are  destined  to  give  origin  to  the  mesoderm  are  variously 


192 


AN   INTRODUCTION  TO  ZOOLOGY 


known  as  the  teloblasts,  pole  cells  or  primary  mesoderm  cells.    The 

blastula  now  commences  to  alter  in  shape,  and  from  being  spherical 
gradually  becomes  elongated  and  flattened,  till  it  forms  a  flattish 


RM, 


M. 


FIG.  61. — Diagrams  of  the  development  of  Lumbricus,  mainly  adapted 

from  Wilson. 

L  and  II.,  early  segmentation  stages  ;  III.,  section  through  the  blastula  showing  the  primary 
mesoderm  cell  which  has  divided  ;  IV.,  section  of  beginning  of  invagination,  macromeres  forming  a 
flat  plate,  more  mesoderm  cells  formed  ;  V.,  view  of  half  an  embryo  during  invagination ;  VI., 
section  through  gastrula  after  obliteration  of  blastocoel  and  closure  of  blastopore  ;  VII.,  ventral 
view  of  embryo  at  a  later  stage  showing  stomodceum  and  germ  bands  ;  VIII.,  transverse  section  of 
lower  part  of  an  embryo  showing  the  ccelom  arising  as  a  split  in  the  mesoderm. 

Bl.,  blastocoel ;  Bp.,  blastopore  ;  C.,  ccelom  ;  E.,  entoderm  ;  EC.,  ectoderm  ;  En.,  enteron  ; 
M.,  mesoderm  ;  Ma.,  macromeres  ;  Mi.,  Micromeres  ;  N.,  cells  giving  rise  to  nerve  bands  ;  Ne., 
cells  giving  rise  to  nephridial  bands ;  P.B.,  polar  bodies  ;  P.M.,  primary  mesoderm  cells ;  S., 
stomodceum. 

oval  cake,  the  small  cells  on  the  upper  surface  and  the  large  ones  on 
the  flat  under  face.  Simultaneously  with  this  alteration  in  shape 
the  teloblasts  have  sunk  more  and  more  into  the  embryo,  until  they 
pass  right  into  the  blastoccel,  and  at  the  same  time  they  have  each 
undergone  a  series  of  unequal  divisions.  These  divisions  result  in 


THE   CCELOMATA   INVERTEBRATA  193 

the  production  of  two  internally  situated  rows  of  cells,  the  mesoderm 
bands,  passing  forwards  from  the  posteriorly  situated  original  pole 
cells.  This  process  goes  on  for  a  long  time,  and  in  its  early  stages 
considerably  reduces  the  blastocoel  cavity.  Thus  we  have  a  hollow 
embryo  with  an  upper  layer  of  small  cells,  a  lower  layer  of  large  cells, 
and  intermediate  lines  of  cells,  the  primary  mesoderm. 

After  this,  gastritlation,  the  conversion  of  the  blastula  into  a 
gastrula  by  the  formation  of  the  entoderm  and  enteron,  takes  place, 
The  lower  larger  cells  invaginate,  causing  the  edges  of  the  cake  tc 
turn  in  and  at  the  same  time  the  upper  cells  gradully  grow  over  more 
and  more.  Thus  is  formed  an  oval  still  somewhat  flattened  structure 
covered  with  smaller  cells,  the  ectoderm,  and  having  on  the  ventral 
surface  a  large  oval  aperture,  the  blastopore,  which  leads  into  the 
archenteron  or  primitive  gut,  whose  walls  are  formed  of  the  large 
cells,  the  entoderm.  In  this  way  we  have  laid  down  the  three  layers 
characteristic  of  the  adult  ccelomate,  the  ectoderm,  entoderm,  and 
mesoderm. 

The  embryo  grows  slowly  in  length  and  simultaneously  the  blasto- 
pore gets  narrower,  ultimately  closing  up  from  the  posterior  end 
forward,  leaving  only  a  tiny  round  opening  at  the  front,  which  persists 
and  marks  the  place  where  the  ectoderm  will  grow  inwards  to  form 
the  stomodoeum.  As  soon  as  this  last  structure  has  made  its  appear- 
ance the  embryo  is  able  to  swallow  the  nutritive  fluid  contained  in 
the  cocoon,  and  this  causes  the  archenteron  to  swell  until  it  touches 
the  ectoderm  save  in  the  regions  occupied  by  the  mesodermal  bands, 
thus  obliterating  the  blastoccel. 

With  further  growth  noticeable  changes  take  place  in  these  rows 
of  cells,  which  with  the  closure  of  the  blastopore  come  to  lie  quite 
near  together  close  to  the  mid- ventral  line.  Not  only  do  these  bands 
increase  in  length  by  transverse  divisions,  but  at  the  front  end  they 
also  become  wider,  and  instead  of  consisting  of  only  one  cell  they  are 
transformed  by  longitudinal  divisions  firstly  into  bands  of  two  or 
three  cells  deep.  Subsequently  they  become  wider  and  wider,  and 
at  the  same  time  each  band  is  cut  up  at  its  anterior  end  into  a  series 
of  masses  which  form  a  series  of  paired  blocks  on  each  side  of  the 
middle  line,  the  mesodermal  somites.  They  are  at  first  small  and  solid, 
but  a  cavity,  the  ccelom,  appears  in  them,  and  they  increase  fairly 
rapidly  in  size  and  grow  upwards  between  the  ectoderm  and  entoderm 
so  as  to  surround  the  gut.  Finally,  they  meet  dorsally  to  the  archen- 
teron, and  then  their  cavities  run  into  one  another  above  and  below, 
so  that  the  mesoderm  is  divided  into  two  parts,  one  surrounding  the 
gut  and  the  other  lining  the  ectoderm.  In  this  way  we  have  estab- 
lished a  body  wall  composed  of  ectoderm  and  a  layer  of  mesoderm, 
constituting  the  somatopleure,  and  a  gut,  wall  or  splanchnopleure, 


I94  AN   INTRODUCTION   TO  ZOOLOGY 

formed  of  the  entoderm  and  inner  layer  of  mesoderm.  The  double 
transverse  walls  of  the  somites  become  approximated  and  persist 
as  the  septa,  and  a  groove  is  formed  in  the  ectoderm  outside.  This 
forms  the  intersomitic  groove,  so  that  the  animal  becomes  ringed, 
and  the  segmentation  which  was  first  laid  down  in  the  mesodermal 
bands  is  visible  externally. 

By  this  time  the  embryo  as  a  whole  has  become  considerably 
elongated  and  distinctly  wormlike  in  appearance,  and  the  subsequent 
course  of  its  development  need  not  be  noticed  in  detail.  The 
ectoderm  gives  rise  to  the  epidermis  of  the  adult,  which  in  its  turn 
secretes  the  cuticle  and  the  setae.  At  a  somewhat  earlier  period, 
even  before  the  coelom  surrounds  the  gut,  a  line  of  modified  cells 
appears  in  the  ectoderm  on  each  side  of  the  mid  ventral  line,  and  this 
later  sinks  into  the  coelom  and  produces  the  ventral  nerve  cord. 
Immediately  lateral  to  these  lines  are  two  other  longitudinal  rows 
of  special  cells,  which  are  similarly  passed  into  the  ccelom,  and  these 
are  the  primordia  or  beginnings  of  the  nephridial  tubes.  Gonads 
and  genital  ducts  appear  to  be  derived  from  "the  mesoblast.  The 
somatic  mesoderm  supplies  the  muscles  and  blood  vessels  of  the 
body  wall  and  the  outer  layei  of  coelomic  epithelium.  The  muscles, 
blood-vessels  and  covering  epithelium  of  the  gut  wall  are  derived 
from  the  splanchnic  mesoderm.  The  entoderm  forms  the  main 
part  of  the  gut  lining  of  the  adult,  but  is  supplemented  by  an  anterior 
invagination  of  ectoderm,  the  stomodceum,  and  a  much  smaller 
posterior  ectodermal  invagination,  the  proetodoeum,  which  forms  the 
anus. 

As  growth  proceeds  new  somites  are  added  at  the  posterior 
end,  just  in  front  of  the  original  teloblasts,  which  therefore  mark 
the  growing  point  of  the  worm.  The  first  of  the  mesodermal  somites 
to  be  laid  down  is  the  one  surrounding  and  partly  behind  the  mouth, 
i.e.  the  peristomium,  and  thus  we  see  that  while  the  whole  of  the 
remaining  parts  of  the  worm  is  developed  in  connection  with  the 
somites  and  is  metamerically  segmented,  there  remains  the  small 
portion  in  front  of  the  mouth,  anterior  to  the  old  blastopore,  which 
never  forms  part  of  this  series.  This  is  the  prostomium.  The 
adult  worm  furnishes  a  splendid  example  of  serial  homology  in  the 
metameric  repetition  of  its  parts. 

From  the  free-living  coelomate  Lumbricus  we  now  pass  on  to 
consider  another  quite  different  and  more  degenerate  type  of  worm 
which  is  parasitic  and  only  distantly  related  to  Lumbricus. 

Tce/i/a  solium,  a  parasitic  flat  worm. 

Tceriia  solium  is  the  tapeworm  that  is  commonly  found 
living  parasitically  in  the  intestine  of  man  in  European  countries. 


THE   CCELOMATA   INVERTEBRATA  195 

The  body  has  the  form  of  a  long  narrow  ribbon,  well  deserving  its 
name  of  "  tape  "  worm,  and  may  reach  a  length  of  many  feet  or 
yards  ;  one  is  recorded  as  attaining  the  enormous  length  of  thirty 
metres.  At  the  front  end  is  a  tiny  knob-like  swelling,  the  head,  by 
which  the  animal  attaches  itself  to  the  wall  of  the  alimentary  canal, 
and  this  is  followed  by  a  very  thin  thread-like  unsegmented  portion, 
the  neck,  and  the  head  and  neck  together  constitute  the  scolex.  This 
part  is  usually  missing  in  laboratory  specimens,  and  is  so  small  that 
it  would  be  readily  overlooked.  At  the  posterior  end  of  the  neck 
are  indistinct  transverse  grooves,  which  become  more  and  more 
distinct  as  we  pass  backwards,  soon  dividing  the  body  up  into  a  series 
of  discrete  segments  or  better  proglottides,  which  constitute  the 
genital  part  of  the  animal,  the  strobilla.  The  proglottids  have  the 
form  of  narrow  transversely  placed  rectangles,  but  farther  back  they 
are  longer,  forming  almost  square  sections  and  then  longitudinally 
running  rectangles.  The  main  part  of  the  body  is  then  composed 
of  a  series  of  segments,  and  so  presents  a  superficial  resemblance  to 
a  segmented  Annelid,  but  they  are  not  truly  typical  metameres  or 
somites.  Each  contains  little  more  than  a  complete  set  of  repro- 
ductive organs,  which  are  immature  at  the  front  end.  When  fully 
formed  the  proglottids  are  termed  mature,  and  at  the  hinder  end, 
where  they  are  almost  filled  up  with  a  much  branched  uterus  laden 
with  eggs,  they  are  said  to  be  ripe.  In  this  condition  they  are  fated 
to  drop  off  from  the  end  of  the  chain  either  singly  or  in  groups,  and 
passing  to  the  outside  with  the  faeces  of  the  host  bring  about  the 
dissemination  of  the  species.  So  it  comes  about  that  the  ripe  pro- 
glottids are  shed  from  the  end  of  the  series,  new  ones  are  formed  just 
behind  the  neck,  which  is  therefore  the  growing  point,  and  the  host 
is  not  free  of  the  pest,  however  many  proglottids  are  removed,  as 
long  as  the  scolex  remains  behind,  and  this  is  always  difficult  to 
dislodge. 

The  head  is  the  organ  of  attachment,  and  for  this  end  is  furnished 
with  clinging  organs  in  the  form  of  suckers  and  hooks.  The  suckers 
in  T.  solium  are  oval,  cup-shaped,  strongly  muscular  structures  situ- 
ated on  the  head,  to  which  they  give  a  rough  rectangular  shape.  In 
front  of  the  suckers  is  a  broad  conical  projection,  the  rostellum, 
furnished  with  a  double  row  of  strong  chitinous  spines.  This  is 
capable  of  being  protruded  and  retracted.  The  number  and  shape 
of  the  suckers  and  presence  or  absence  of  a  rostellum  or  its  spines 
form  important  characters,  by  means  of  which  the  various  species 
of  tapeworm  may  be  distinguished. 

Living  as  it  does  in  the  alimentary  canal,  it  is  surrounded  by  food 
in  a  diff usable  condition,  and  as  a  result  we  find  it  devoid  of  a  mouth, 
a  gut,  or  any  form  of  digestive  system,  feeding  taking  place  by 


196 


AN   INTRODUCTION   TO  ZOOLOGY 


diffusion.  The  nervous  system  is  quite  simple,  and  consists  of  two 
well-marked  ganglia  in  the  head  connected  together  by  a  transversely 
placed  commissure.  From  each  of  the  ganglia  a  fine  nerve  cord 
runs  backwards  along  the  lateral  border  of  the  proglottids.  The 
most  conspicuous  parts  of  the  excretory  system  are  two  longitudinal 
canals  running  just  inside  the  lateral  nerve  through  the  whole 


FIG.  62. — Tapeworm  of  the  pig,  Tcenia  solium. 

A.,  entire  specimen,  reduced  ;  c.>  head  ;  B.,  head  or  scolex,  showing  adhesive  hooks  and  suckers' 
enlarged  ;  C.,  proglottid  or  segment,  enlarged,  with  mature  reproductive  apparatus  ;  ex.,  excretory 
canal ;  ».,  longitudinal  nerves  ;  ov.,  ovary  ;  Pg.,  genital  pore  ;  sh.,  shell  gland  ;  ut.,  uterus  ;  i>a., 
vagina  ;  vd.,  vas  de-ferens  ;  vit.,  vitteline  gland.  (After  Leuckart,  from  Parker  and  Haswell.) — 
From  Lull. 

length  of  the  strobilla,  and  therefore  opening  at  the  hinder  end  of  the 
last  proglottid.  They  are  connected  at  the  posterior  end  of  each 
proglottid  by  a  transverse  canal,  and  receive  a  rich  network  of  very 
fine  vessels,  ramifying  all  through  the  tissues  of  the  tapeworm  and 
terminating  in  branches  which  end  blindly  in  highly  specialised 
cells.  These  cells  form  a  pear-shaped  hollow  and  give  off  a  few  very 
long  cilia,  which  hang  down  into  the  tubule  and  execute  constant 


THE   CCELOMATA   INVERTEBRATA  197 

waving  movements,  which  give  to  the  group  as  a  whole  a  curious 
flickering  movement,  recalling  a  candle  flame  in  a  slight  draught,  and 
hence  they  are  termed  "  flame  cells."  They  are  the  characteristic 
excretory  organs  of  the  class  of  flat  worms  to  which  Tcenia  belongs, 
and  the  lumen  of  the  tubules  at  the  end  of  which  they  occur  is 
intracellular. 

A  transverse  section  shows  that  the  proglottid  is  solid  and  covered 
by  a  cuticle,  which  is  secreted  by  a  layer  of  modified  sub-cuticular 
cells.  These  represent  a  transformed  epidermis,  which  has  come  to 
be  partly  embedded  in  the  underlying  tissue.  The  whole  of  the 
interior  not  occupied  by  definite  organs  is  filled  up  with  very  cha- 
racteristic cellular  padding  tissue,  the  parenchyma.  This  is  a  meso- 
dermal  product,  and  although  a  large  coelom  like  that  in  Lumbricus 
is  never  developed,  we  still  classify  them  as  coelomates,  mainly  on 
account  of  the  development  of  the  mesoderm.  A  few  scattered 
cells  in  the  parenchyma  secrete  small  calcareous  particles,  which  are 
of  a  spherical  or  oval  shape.  The  muscular  system  is  only  feebly 
developed,  as  there  is  but  little  need  for  movement,  and  it  consists 
of  a  thin  dermal  layer  and  a  deeper  layer  made  up  of  thin  sheets 
running  in  transverse  and  oblique  directions,  some  forming  strands 
passing  through  the  parenchyma  from  side  to  side. 

As  has  been  pointed  out,  the  proglottids  just  behind  the  neck 
are  immature  and  have  at  first  no  reproductive  organs.  These  soon 
start  to  develop,  the  male  organs  appearing  first,  followed  shcrtly 
after  by  the  female  structures.  They  are  all  found  fully  developed 
in  each  mature  proglottid,  which  is  therefore  hermaphrodite. 

The  male  gonads,  the  testes,  consist  of  a  number  of  globular 
follicles  scattered  generally  throughout  the  parenchyma,  save  in  the 
middle  of  the  posterior  end  of  the  proglottid.  They  all  give  off 
delicate  tubes,  the  vasa  efferentia,  which  unite  to  form  larger  and 
larger  trunks  all  ultimately  leading  to  one  tube,  the  vas  deferens, 
about  the  middle  of  the  segment.  This  passes  transversely  to  one 
side  or  the  other,  and  opens  with  the  female  aperture  into  a  small 
common  cavity,  the  genital  sinus,  on  the  lateral  margin  of  the  pro- 
glottid. The  sinus  opens  by  an  elongated  slit,  the  genital  pore.  The 
end  of  the  vas  deferens  is  contained  in  a  muscular  sac,  and  is  modified 
to  form  a  protrusible  organ  sometimes  called  the  penis,  but  better 
termed  the  cirrus. 

The  female  gonads,  the  ovaries,  are  two  conspicuous  plate-like 
structures  lying  near  the  posterior  end  of  the  proglottid,  and  joined 
by  a  transversely  running  canal.  From  the  middle  of  this  comes  off 
the  oviduct,  which  runs  backwards  towards  the  end  of  the  segment, 
receiving  shortly  after  its  origin,  the  vagina.  Near  the  posterior 
border  the  oviduct  turns  sharply  round,  receives  a  duct  from  the 


198 


AN   INTRODUCTION  TO  ZOOLOGY 


u. 


u. 


yolk  gland,  is  surrounded  by  the  shell  gland,  and  then  passes  forwards 
as  a  large  median  closed  sac,  the  uterus.  The  yolk  or  vitelline  gland 
is  a  conspicuous  structure,  composed  of  two  lobes  lying  across  the 
posterior  end  of  the  proglottid  and  opening  into  the  oviduct  by  a 
single  median  canal.  The  shell  gland,  situated  around  and  covering 
the  bend  of  the  oviduct,  is  a  more  diffuse  gland  without  any  special 
duct.  The  external  female  aperture  leads  into  a  fairly  wide  straight 
tube,  the  vagina,  which  passes  obliquely  inwards,  and,  as  just  noted, 
opens  into  the  oviduct  a  short  distance  from  its  commencement. 
x  The  inner  end  of  the  vagina  is  swollen  out  to 

form  an  oval  seminal  vesicle. 

Copulation  has  not  been  observed  in  T. 
solium,  so  that  it  is  not  known  for  certain 
whether  the  proglottid  is  self -fertilising  or  is 
fertilised  by  another  proglottid.  In  view  of 
the  fact  that  in  certain  allied  species  only  one 
mature  proglottid  is  present  at  a  time,  it  seems 
probable  that  the  former  is  the  case.  The 
sperms  pass  down  the  vagina  and  are  stored 
in  the  seminal  vesicle.  The  ova  shed  from  the 
ovaries  pass  along  the  oviduct  and  are  fertilised 
as  they  pass  the  entrance  of  the  vagina.  As 
they  proceed  further  they  become  surrounded 
by  a  layer  of  food-laden  cells  from  the  vitel- 
line gland,  and  then  around  each  ovum  and 
its  attendant  cells  is  secreted  a  thick  chitinous 
shell,  the  product  of  the  shell  gland.  These 
ova  then  pass  on  to  the  uterus,  -where  they 
are  stored  and  where  they  undergo  a  certain 
amount  of  development.  The  production  of 
more  and  more  eggs  leads  to  the  degeneration 
of  the  gonads  and  the  filling  up  of  the  uterus.  This  is  at  first  a 
simple  sac,  but  as  it  fills  it  sends  out  lateral  branches  until  it  comes 
to  occupy  practically  the  whole  of  the  proglottid,  which  is  now 
termed  ripe  and  is  ready  to  be  passed  to  the  exterior  with  the  faeces 
of  the  host.  The  form  assumed  by  the  branched  uterus  in  the  ripe 
proglottid  is  quite  characteristic,  and  is  often  a  valuable  guide  to  the 
species  of  tapeworm.  Sometimes  when  passed  out  in  this  way  the 
proglottid  is  capable  of  moving  about  for  a  short  time  and  may 
be  swallowed  whole,  or  more  frequently  it  disintegrates,  releasing 
the  ova.  The  intermediate  host  in  the  case  of  T.  solium  is  the  pig. 

While  still  contained  in  the  uterus  in  the  proglottid  the  egg 
undergoes  its  segmentation  and  grows  into  a  tiny  spherical  body 
covered  with  a  cuticle  and  provided  with  six  chitinous  hooks,  and 


FIG.  63. — Ripe  pro- 
glottid of  Tcenia 
solium. 

G.,  genital  aperture  ;  U., 
uterus  full  of  eggs. 


THE   CCELOMATA   INVERTEBRATA 


199 


hence  known  as  the  onchosphere  or  hexacanth  embryo.  No  further 
development  can  be  undergone  until  it  is  swallowed  by  the  pig,  whose 
digestive  fluids  dissolve  off  both  the  egg  shell  and  the  cuticle,  releasing 
the  tiny  six-hooked  embryo.  This  then  bores  its  way  through  the 
wall  of  the  gut  largely  by  the  aid  of  its  hooks  and  penetrates  a  small 
blood  vessel.  In  the  blood  stream  it  is  carried  about  until  it  reaches 
its  destination,  which  in  T.  solium  is  usually  the  voluntary  muscles. 
It  increases  in  size  fairly  rapidly,  losing  its  hooks  and  becoming 
inflated  with  a  fluid  substance  until  it  forms  a  thin-walled  hollow 
vesicle  about  5  mm.  in  diameter,  known  as  the  proscolex.  At  first 
this  is  uniform  all  round  its  periphery,  but  soon  it  thickens  at  one 


\zr 


FIG.  64. — Diagram  of  the  development  of  Tcenia,  adapted  from  Leuckart. 

I.,  hexacanth  embryo  ;  II.,  early  proscolex  ;  III.,  IV.,  and  V.,  anterior  end  of  proscolex  in  different 
stages  of  development,  more  highly  magnified  ;  V.,  beginning  of  evagination  ;  VI.,  complete 
evaginated  scolex  with  caudal  vesicle. 

point.  The  thickening  invaginates  and  is  the  primordium  of  the- 
head  of  the  future  worm.  At  its  inner  end  the  invagination  dilates 
to  form  a  hollow  vesicle,  within  which  are  produced  the  suckers, 
rostellum  and  spines,  in  fact  a  miniature  head  only  inside  out.  By 
this  time  it  has  increased  considerably  in  size,  being  oval  and  about 
12  by  8  mm.  in  diameter,  and  it  is  known  as  the  bladder  worm  or 
Cysticercus.  It  consists  of  a  large  vesicle,  the  proscolex  or  caudal 
vesicle,  as  it  may  now  be  termed,  filled  with  fluid,  and  projecting  into 
this  is  the  introverted  scolex.  This  marks  the  limit  of  its  develop- 
ment in  the  pig,  and  it  now  remains  in  a  passive  condition  embedded 
in  the  muscular  tissue  which  secretes  around  it  a  secondary  cyst  of 


200  AN   INTRODUCTION   TO  ZOOLOGY 

dense  fibrous  connective  tissue.  The  particular  cysticercus  of 
T.  solium  is  called  C.  cellulosce,  and  a  piece  of  infected  or  crysticercoid 
pork  presents  a  very  characteristic  appearance,  on  account  of  which 
it  is  commonly  spoken  of  as  "  measly  pork." 

If  pork  in  this  condition  is  eaten  by  a  human  being  in  a  raw  or 
partially  cooked  condition,  for  thorough  cooking  will  kill  the  parasite, 
the  cysticercus  becomes  set  free  in  the  stomach.  It  evaginates 
its  scolex  and  throws  off  its  caudal  vesicle,  and  on  passing  into 
the  intestine  attaches  itself  by  means  of  its  head  to  the  mucous 
membrane.  Here  the  posterior  end  of  the  neck  starts  to  grow  and 
segment,  giving  rise  to  the  long  tapeworm  with  which  we  started  and 
which  is  ready  to  begin  the  cycle  over  again.  It  is  obvious  from  a 
consideration  of  its  life  cycle  that  improved  sanitation  and  the  satis- 
factory inspection  and  cooking  of  pork  can  keep  the  parasite  down  ; 
indeed  in  this  country  it  has  been  practically  eliminated,  although 
it  is  still  found  in  central  Europe  and  may  be  occasionally  introduced 
here. 

Before  passing  on  to  discuss  certain  general  problems 
connected  with  the  tapeworm,  it  seems  as  well  briefly  to  call 
attention  to  one  or  two  allied  forms  that  may  be  encountered  and 
are  of  interest. 

Tcenia  saginata. — A  form  that  is  also  found  in  man  and  is  on  the 
whole  very  similar  to  T.  solium,  reaching  a  length  of  as  much  as 
thirty-six  metres.  It  is  easily  distinguished  from  that  animal  by 
the  absence  of  rostellum  and  hooks,  which  are  replaced  by  an  addi- 
tional sucker-like  structure,  and  also  by  the  shape  of  the  ripe  uterus. 

f.  serrata. — This  species  has  been  rarely  if  ever  found  in  man, 
but  is  the  common  tapeworm  of  the  dog.  Its  cysticercus  stage 
(C.  pisiformis),  however,  is  commonly  met  with  in  the  mesentery 
of  the  rabbit  and  hare,  and  so  encountered  in  dissecting  the  former 
in  the  laboratory. 

T.  echinococcus  is  a  form  consisting  of  but  three  proglottids  at  a 
time,  one  immature,  one  mature,  and  one  ripe,  and  it  has  a  very  long 
rostellum.  It  makes  up  for  its  small  size,  about  5  mm.,  by  being 
present  in  large  numbers.  It  is  not  found  in  man  save  in  the 
cysticercus  stage,  when  man  seems  to  act  as  an  accidental  secondary 
host.  The  cysticercus,  known  as  C.  veterinorum,  or  in  man  as 
G.  hominis,  produces  the  condition,  sometimes  a  terrible  one,  known 
as  Hydatids.  The  usual  place  for  it  is  to  be  found  in  the  liver,  where 
it  may  reach  the  size  of  a  child's  head,  and  the  one  original  pro- 
scolex  gives  rise  by  internal  budding  to  thousands  of  tiny  little 
scolices. 

Dibothriocephalus  lotus. — As  a  parasite  of  man  this  "  broad  tape- 
worm/' growing  to  a  length  of  nine  metres,  is  more  or  less  confined 


THE   CGELOMATA   INVERTEBRATA 


201 


to  Eastern  countries,  where  it  is  common.  The  head  is  oval,  and 
possesses  only  two  lateral  deep  groove-like  suckers.  It  has  two 
external  openings  on  the  proglottid,  one  the  ordinary  genital  pore, 
and  the  other  that  of  the  much  curved  uterus.  The  cysticercus  is 
passed  in  the  muscles  of  some  fresh-water  fish. 

Dipylidium  caninum  is  a  small  form,  35  c.m.  in  length,  only 
rarely  found  in  human  beings,  but  is  interesting  because  the  genital 
organs  are  duplicated  in  each  proglottid,  each  set  opening  by  a 


t-G. 


FIG.  65. — Proglottids  of  A,  Tcenia  saginata,  ripe;  B,  Dibothno- 
cephalus  latus,  nearly  ripe  ;   and  C,  Dipylidium  caninum,  ripe. 

C.S.,  cirrus  sac  ;  G.,  genital  aperture  ;  U.,  uterus  full  of  eggs  ;  V.,  vagina  ;  Vi.,  vitellarium. 


separate  pore  on  the  opposite  lateral  margins, 
host  is  a  parasitic  louse  or  flea. 


Its  intermediate 


Two  problems  call  for  attention  in   the    tapeworm,   its 
segmentation  and  the  question  of  the  alternation  of  generations. 

It  is  obvious  that  there  is  a  considerable  difference  between  the 
proglottids  and  the  somites  of  such  a  form  as  Lumbricus.  In  the 
first  place  new  segments  in  the  Earthworm  are  produced  throughout 
the  period  of  growth,  immediately  in  front  of  the  last  or  anal  somite, 


202  AN   INTRODUCTION   TO  ZOOLOGY 

whereas  in  Tcenia  they  are  formed  at  the  anterior  end  of  the  chains  of 
proglottids  by  the  unsegmented  neck — a  very  fundamental  difference. 
Then,  again,  the  only  parts  that  are  repeated  in  the  latter  animal  are 
the  reproductive  organs,  whereas  in  the  earthworm  the  metamerism 
is  impressed  on  all  the  various  parts  of  the  body.  Lastly,  as  we  have 
seen,  the  segmentation  of  the  adult  Lumbricus  is  founded  upon  a 
very  early  developed  and  primitive  division  of  the  mesoderm  into 
mesodermal  somites,  a  process  not  paralleled  in  Tcenia.  For  these 
reasons,  among  others,  we  cannot  regard  the  segmented  condition 
in  the  two  animals  as  homologous  or  essentially  similar,  but  as  only 
superficially  alike.  How  then  did  this  condition  arise  in  the  tape- 
worms ?  The  most  generally  accepted  explanation  is  briefly  as 
follows  :  We  find  the  most  simple  and  primitive  members  of  the 
group  are  unsegmented,  and  have  but  a  single  set  of  reproductive 
organs  situated  towards  the  hinder  end.  Such  an  animal  living  in 
the  gut  might  conceivably  have  had  its  posterior  end  broken  off 
by  the  passage  of  the  food  and  the  peristalsis  of  the  intestine  and 
then  proceeded  to  regrow  the  lost  parts,  as  these  lowly  animals  can 
readily  do.  This  would  be  distinctly  advantageous  to  the  species, 
for  it  would  bring  about  a  greatly  increased  power  of  reproduction. 
The  possibility  of  regeneration  we  now  imagine  became  moved 
forwards  in  the  life  history  of  the  individual,  and  the  worm,  as  it 
were,  produced  several  posterior  ends  in  anticipation  of  their  being 
broken  off,  and  these  had  a  line  of  demarcation  between  them,  so 
that  when  the  break  did  come  it  would  not  be  so  injurious.  If  this 
were  done  to  a  slight  extent  we  should  have  a  form  like  T.  echinococcus 
with  its  three  proglottids,  or,  if  carried  further,  many  proglottids,  as 
in  T.  solium.  This  gives  a  possible  mode  of  origin  of  the  tape- worm 
condition,  and  so,  if  true,  segmentation  in  these  animals  may  be 
regarded  as  a  special  adaptation  to  the  peculiar  parasitic  mode  of 
life,  and  not  an  essential  of  their  plan  of  organisation,  as  it  is  in  the 
Annelids. 

In  the  case  of  T.  solium  the  egg  produces  a  single  proscolex  in 
the  muscles  of  the  pig,  and  this  in  its  turn  grows  directly  into  one 
adult  animal,  so  that  in  this  case  there  is  obviously  only  a  single 
generation  with  a  sexual  reproduction  in  the  adult  condition. 
T.  echinococcus  and  certain  other  forms,  on  the  contrary,  have  not 
such  a  simple  life  history.  Their  eggs  produce  proscolices,  as  in 
TV  solium,  but  each  of  these  in  its  young  or  cysticercoid  stage  gives 
rise  by  a  kind  of  asexual  budding  to  a  large  number,  it  may  be 
thousands,  of  scolices.  In  these  species  then  we  have  a  true  alterna- 
tion of  generations  between  the  asexual  proscolex  and  the  sexual 
tapeworm.  It  is  an  additional  wa}'  of  securing  a  continuance  of  the 
species. 


THE   CCELOMATA   INVERTEBRATA  203 

Finally,  the  tapeworms,  like  the  sporozoa,  exhibit  many 
characteristics  which  are  typical  of  internal  parasites  in  general. 
With  the  abandonment  of  a  free  and  exposed  life  they  have  given 
up  organs  of  locomotion  and  protection  against  foes  and  evolved  a 
means  of  maintaining  themselves  securely  in  their  sheltered  position. 
Their  food  is  supplied  in  a  readily  assimilable  condition  that  does  not 
require  an  alimentary  canal  or  its  equivalent,  and  consqeuently 
we  find  that  this  and  its  related  structures  have  disappeared.  These 
two  points  naturally  carry  with  them  a  corresponding  degeneration 
of  the  nervous  system  and  absence  of  sense  organs.  Although  a 
convenient  mode  of  life,  so  far  as  food  supply  and  protection  are 
concerned,  internal  parasitism  implies  a  difficulty  of  re-infecting 
new  hosts.  The  loss  of  locomotor  powers  makes  the  dissemination 
of  the  species  largely  a  matter  of  chance,  and  as  a  result  a  vast 
number  of  eggs  (or  spores  in  the  protozoa)  are  produced,  in  order  to 
ensure  that  some  of  them  at  any  rate  shall  reach  another  host. 
As  a  further  help  in  spreading  infection  we  frequently  find  they 
adopt  the  plan  of  making  use  of  a  second  or  intermediate  host, 
related  in  some  intimate  way  to  the  primary  host,  and  this  has  led 
in  many  cases  to  the  development  of  a  complicated  life  history. 

With  Tcenia  we  close  our  study  of  the  invert  ebrate  Coelomates, 
and  pass  on  to  consider  the  much  more  highly  specialised  Chordates. 


CHAPTER  VIII 
VERTEBRATE  ANIMALS— SCYLLIUM  CANICULA 

Introduction  to  Craniata — External  features,  Scyllium — Integument — 
Muscular  system — Endoskeleton. 

A  Fish — Scyllium  canicula,  the  Dogfish. 

The  next  type  to  be  studied  is  that  of  a  fish,  and  the  species 
taken  is  that  known  as  the  lesser  spotted  dogfish,  and  to  the 
Zoologist  as  Scyllium  canicula.  It  is  practically  a  little  shark,  and 
is  the  smaller  of  two  dogfish  caught  commonly  around  the  British 
coasts.  The  larger,  more  rare  5.  catulus,  the  greater  spotted  dog- 
fish or  rough  hound,  is  very  similar,  so  much  so  indeed  that  young 
specimens  of  it  are  sometimes  mixed  in  with  the  smaller  species 
supplied  to  the  laboratory,  and  the  differences  between  them  are  so 
slight  as  to  be  negligible  for  our  present  purposes. 

Compared  with  the  earthworm  the  dogfish  represents  a  great 
advance  in  the  animal  series,  for  it  is  a  vertebrate  animal  belonging 
to  the  phylum  Chordata,  the  same  phylum  as  the  frog,  with  which  we 
commenced  our  studies,  and  so  possessing  many  features  in  common 
with  that  animal.  The  Chordata  are  often  divided  into  two  groups  : 
the  ACRANIA,  comprising  a  number  of  less  familiar  forms  such  as 
the  lancelet  Amphioxus  and  sea  squirt  Ascidia,  etc.,  whose  relation 
to  the  other  members  of  the  phylum  are  not  clear,  and  all  the  remain- 
ing forms,  the  Craniata,  marked  off  by  the  possession  of  a  head  and  a 
vertebral  column.  These  are  the  Vertebrata  in  the  strict  meaning 
of  the  term,  and  contain  the  following  classes  :  PISCES,  the  Fish  ; 
AMPHIBIA,  the  Frogs,  Newts,  etc.  ;  REPTILIA,  the  Reptiles ; 
AVES,  the  Birds ;  and  MAMMALIA,  the  Mammals.  The  dogfish 
is  important  because  it  is  a  representative  of  the  Elasmobranch 
Fishes,  one  of  the  lowest  groups,  including  sharks  and  rays,  whose 
skeleton  remains  cartilaginous  throughout  life.  This  group,  although 
specialised  in  some  respects,  retains  a  number  of  primitive  features, 
and  is  consequently  of  considerable  interest  to  the  comparative 
anatomist.  The  primitive  nature  of  the  group  is  further  demon- 
strated by  the  fact  that  the  earliest  known  remains  of  fossil  verte- 
brates belong  to  it. 

204 


VERTEBRATE  ANIMALS  205 

The  lesser  spotted  dogfish  is  found  in  shoals  round  the 
coast.  It  is  a  voracious  feeder,  living  on  other  fish,  on  cuttlefish, 
octopus,  crustaceans,  etc.,  and  being  powerful  and  swift  does 
considerable  damage  to  ordinary  edible  fish.  It  is  marketed  as 
food,  but  is  much  coarser  eating  than  the  other,  bony  fish,  and  is 
consequently  not  widely  used.  Before  considering  it  in  detail  it 
will  be  well  to  glance  at  some  of  the  main  characteristics  of  a  Verte- 
brate which  are  common  to  the  dogfish,  frog  and  rabbit,  in  the  types 
we  have  to  study,  and  also,  of  course,  to  ourselves. 

Like  the  earthworm,  all  the  Craniates  are  ccelomate 
metazoa,  and  are  primitively  segmented,  and,  although  this  meta- 
merism becomes  partially  obscured  in  the  adult,  traces  of  it  are 
always  to  be  found.  The  body  is  generally  to  be  marked  off  into,  at 
any  rate,  a  head,  a  trunk  and  a  tail.  They  usually  lay  eggs,  i.e. 
are  Oviparous,  even  though  these  may  be  hatched  inside  the  mother 
as  in  some  Reptiles,  and  an  ally  of  the  dogfish  Acanthias,  a  condition 
termed  Ovo-viviparous.  In  the  higher  groups  of  the  mammals  we 
meet  with  a  truly  Viviparous  condition  in  which  the  young  are 
intimately  attached  to  the  mother  at  a  very  early  stage,  and  are 
brought  forth  alive  in  a  fairly  advanced  stage  of  development. 

The  first  and,  perhaps,  most  important  feature,  however, 
concerns  the  skeleton  which,  in  the  embryo,  is  furnished  by  a  very 
characteristic  structure,  the  Notochord  or  Chorda  dorsalis.  This 
consists  of  an  elastic  rod  of  very  highly  modified  entodermal  cells 
that  typically  come  from  the  dorsal  side  of  the  gut,  and  it  is  the 
presence  of  this  same  striking  tissue  in  the  ACRANIA  that  leads  to 
their  being  grouped  with  the  CRANIATA  in  the  one  great  phylum 
CHORDATA.  In  most  craniates  the  notochord  is  replaced  *  almost  or 
completely  by  the  Vertebral  Column,  a  mesodermal  structure  which 
comes  to  form  the  main  supporting  axis  of  the  body.  As  we  have 
seen  in  the  frog,  this  consists  of  a  number  of  separate  pieces  or 
vertebrae  movably  articulated  one  with  another.  Not  only  is  it  a 
support  for  the  animal  as  a  whole,  but  it  also  bears  on  its  dorsal  side 
a  canal,  the  Vertebral  Canal,  for  the  protection  of  the  spinal  cord. 
Its  anterior  end  is  modified  to  form  a  skull  with  which  are  associated 
upper  and  lower  jaws.*  Also  more  or  less  closely  connected  with 
the  vertebral  column  are  two  pairs  of  limbs,*  which  in  the  fish  form 
the  fins,  and  in  higher  vertebrates  the  legs,  though  the  anterior 
pair  are  sometimes  specially  modified  to  form  arms  or  organs  of  flight, 
termed  wings. 

The  second  fundamental  characteristic  of  a  Vertebrate  is  its 
central  nervous  system,  which  is  tubular  in  form  and  situated 

*  A  somewhat  aberrant  group  of  Vertebrates,  the  Cyclostomes  (Lampreys 
and  Hagfish),  are  not  taken  into  account. 


206  AN   INTRODUCTION  TO  ZOOLOGY 

dorsally  to  the  notochord.  This  marks  them  off  most  clearly  from 
the  Invertebrates,  where  the  nerve  cord  is  always  solid,  often  double, 
as  in  Lumbricus,  and  is  almost  invariably  situated  ventrally  to  the 
alimentary  canal,  except  its  most  anterior  pair  of  ganglia,  which 
may  be  dorsal.  The  lumen  of  the  tube  in  the  region  of  the  spinal 
cord,  i.e.  the  canalis  centralis,  is  quite  small,  but  in  the  head  region 
where  the  nervous  matter  enlarges  to  form  a  brain  the  canal  also 
widens  considerably  and  forms  a  series  of  hollow  cavities,  the 
ventricles,  as  we  have  observed  in  the  frog.  The  central  nervous 
system  makes  its  appearance  in  the  embryo  as  a  thickened  band  of 
ectoderm  in  the  mid-dorsal  line  known  as  the  medullary  plate,  and 
this  runs  from  the  front  to  the  hinder  end.  The  edges  of  this  plate 
rise  up  into  medullary  folds,  leaving  a  sort  of  gutter,  the  medullary 
groove,  between  them.  Gradually  the  edges  come  closer  together 
until  they  finally  meet  in  the  middle,  thus  giving  rise  to  a  tubular 
structure,  the  anterior  end  of  which  even  at  this  early  stage  already 
shows  three  vesicular  enlargements,  the  rudiments  of  the  three  main 
divisions  of  the  adult  brain.  The  general  ectoderm  closes  over  the 
top,  allowing  the  central  nervous  system  to  sink  down.  Thus  from 
the  very  beginning  the  nervous  system  is  a  tubular  structure  com- 
posed of  ectoderm. 

The  third  great  distinguishing  point  in  the  anatomy  of  the 
Chordata  is  the  possession  in  the  embryo,  and  sometimes,  as  in 
Scyllium,  throughout  life  of  a  series  of  paired  perforations  in  the 
walls  of  the  pharynx  which  lead  directly  to  the  outside  of  the  animal. 
These  openings,  the  gill  slits  or  pharyngeal  clefts,  are  not  more  than 
seven  in  number,  and  may  be  just  narrow  clefts  all  the  way,  or  enlarge 
to  form  pouches  with  slit-like  openings  to  the  exterior  and  to  the 
pharynx.  The  larger  part  of  the  branchial  cleft  is  lined  with 
entoderm,  but  the  ectoderm  turns  in  over  the  outer  portion.  The 
slits  are  supported  by  more  or  less  complex  arrangements  of  carti- 
laginous rods  termed  the  gill  bars,  the  whole  constituting  the 
branchial  basket.  The  gill  slits  are  present  throughout  life  in  the 
fish  and  are  functional  in  connection  with  the  respiratory  exchanges, 
their  walls  being  thrown  into  a  series  of  highly  vascular  folds,  the 
gills.  The  blood,  which  is  supplied  to  the  gills  and  removed  by  a 
characteristic  series  of  vessels,  is  oxygenated  from,  and  gives  up  its 
carbonic  acid  gas  to,  the  surrounding  water  as  it  passes  through  the 
capillaries  of  these  gills. 

In  the  classes  above  the  fish  the  internal  gills  lose  their  respiratory 
function,  which  is  taken  on  by  another  set  of  externally  developed 
gills  in  some  of  the  Amphibia,  and  in  the  rest  of  the  Amphibia,  the 
Reptiles,  Birds  and  Mammals,  by  entirely  new  structures,  the  lungs. 
In  spite  of  this,  however,  the  clefts,  or  at  any  rate  some  of  them, 


VERTEBRATE  ANIMALS  207 

are  always  developed  in  the  embryo,  together  with  their  character- 
istic skeleton  and  vascular  supply.  The  slits  disappear  entirely  in 
the  adult,  with  the  exception  of  the  one  pair  which  takes  part  in  the 
formation  of  the  ear  passages.  Their  skeleton  is  modified  to  form 
the  hyoid  apparatus,  and  the  remains  of  their  blood-vessels  constitute 
the  bases  of  the  three  great  arterial  arches  of  the  adult. 

We  have  dealt  with  the  three  main  anatomical  features  of 
the  Chordata,  i.e.  the  presence  of  a  notochord,  a  dorsal  hollow  central 
nervous  system,  and  the  possession  of  gill  slits,  characters  which  are 
in  themselves  diagnostic  of  the  group,  but  the  members  of  the  group 
also  possess  a  number  of  other  points  in  common. 

Three  pairs  of  sense  organs  are  present,  namely,  the  olfactory, 
the  optic  and  the  auditory,  which  are  lodged  in  protecting  capsules 
more  or  less  closely  connected  with  the  skull.  The  essential  parts 
of  these  are  always  derived  directly  or  indirectly  from  the  ectoderm, 
e.g.  the  retina  of  the  eye  is  developed  as  an  outgrowth  of  the  brain 
which  is  itself  ectodermal.  In  the  lower  classes,  i.e.  Fish  and 
Amphibia,  ten  pairs  of  cranial  nerves  come  off  from  the  brain  with 
the  same  origin  and  similar  general  distribution,  being  obviously 
homologous.  The  higher  classes,  i.e.  Reptiles,  Birds  and  Mammals, 
have  in  addition  to  these  nerves  two  more  cranial  nerves,  making 
twelve  altogether. 

The  alimentary  canal  is  a  long  folded  tube  running  from  an 
anterior  ventral  mouth  to  an  anus  that  is  posterior  and  ventral, 
and  it  is  differentiated  into  distinct  regions.  The  portion  of  the  gut 
just  beyond  the  stomach  receives  the  ducts  of  a  well-marked  liver 
and  pancreas.  The  liver  has  not  only  an  arterial  blood  supply,  but 
a  special  series  of  veins  forming  the  hepatic  portal  system  gathers  up 
the  blood  from  the  intestine  and  conveys  it  to  the  liver. 

The  vascular  system  is  composed  of  a  closed  series  of  vessels 
carrying  the  blood  to  and  from  the  central  organ,  the  heart,  which  is 
composed  of  chambers  separated  from  one  another  by  valves.  The 
blood  is  composed  of  an  almost  colourless  fluid,  the  plasma,  in  which 
float  large  numbers  of  corpuscles,  some  white,  and  others,  far  more 
numerous,  red,  owing  to  the  contained  haemoglobin. 

The  kidneys  are  compact  bodies  composed  of  a  number  of  urinary 
tubules  aggregated  together  and  possessing  a  common  duct,  the 
ureter,  for  the  conveyance  of  the  urine  to  the  exterior.  In  addition 
to  the  glands  connected  with  the  gut  and  skin  possessing  ducts 
for  the  removal  of  their  secretion,  there  are  also  a  number  of 
ductless  glands  whose  products  are  passed  directly  into  the  blood 
stream. 

The  skin  is  composed  of  an  outer  epidermis  of  ectodermal  origin 
and  an  underlying  dermis  derived  from  the  mesoderm,  and  except 


208 


AN   INTRODUCTION  TO  ZOOLOGY 


FIG.  66. — The  rough 
hound — from  Bor- 
rodaile. 

Note  mouth,  eye,  spi- 
racle, lateral  line,  gill 
clefts,  pectoral  and  pelvic 
fins,  dorsal  fins,  caudal 
fin,  vertical  fins  between 
caudal  and  pelvic  fins. 

cf.,  upper  lobe  of  caudal 
fin  :  cf1.,  lowerlobe  of  the 
same;  pl.f.,  pelvic  fin. 

on  to  the  ventral 


in  a  number  of  living  Amphibia  it  bears  exo- 
skeletal  structures  in  the  form  of  scales,  feathers 
or  hairs. 

External  Features  and  Integument. 

The  dogfish  is  bilaterally  symmetrical 
with  a  greatly  elongated  spindle-shaped  body 
admirably  adapted  for  swimming,  and  it  may 
reach  a  length  of  about  two  feet.  5.  catulus 
is  much  larger,  and  may  grow  to  almost  double 
this  size.  The  body  is  divisible  into  three 
regions  :  the  head  extends  back  to  the  beginning 
of  the  pectoral  fin  and  contains  the  skull,  sense 
organs,  buccal  cavity  and  pharynx  ;  the  trunk 
stretches  from  that  point  to  the  cloaca,  and 
contains  the  main  circulatory  organs,  the  viscera 
and  the  ccelom  which  is  confined  to  this  region  ; 
lastly,  the  remaining  part,  which  occupies 
slightly  more  than  half  the  total  length,  is  the 
tail  forming  a  very  efficient  organ  of  pro- 
gression. 

The  fins  form  a  very  striking  series  of  thin 
flat  expansions  supported  by  skeletal  elements, 
and  are  divisible  into  two  groups,  the  median 
or  unpaired  fins,  and  the  lateral  or  paired  fins. 
Of  the  four  median  fins  two  are  placed  in  the 
mid-dorsal  line,  the  foremost  and  larger,  or 
anterior  dorsal  fin,  is  about  half-way  back,  and 
the  posterior  dorsal  fin  about  two-thirds  of  the 
way  back.  They  are  triangular  in  shape,  and 
the  anterior  of  them  in  the  sharks  projects 
from  the  water  when  the  animal  is  basking  just 
beneath  the  surface,  and  so  constitutes  for 
sailors  in  warmer  seas  a  significant  indication 
of  the  presence  of  these  voracious  fish.  The 
ventral  fin  is  a  similar  but  blunter  fin  in  the 
mid-ventral  line  situated  at  a  distance  inter- 
mediate between  the  two  dorsal  fins  from  the 
front  of  the  animal.  The  fourth  of  the  unpaired 
fins,  and  by  far  the  largest,  is  the  caudal  fin,  It 
commences  about  three-quarters  of  the  way 
back  as  a  low  ridge,  and  increasing  in  height 
passes  right  round  the  posterior  end  of  the  tail 
surface.  It  will  be  seen  on  closer  inspection 


VERTEBRATE  ANIMALS  209 

that  the  tip  of  the  vertebral  column  turns  up  at  an  angle.  The 
caudal  fin  round  this  and  on  the  ventral  side  is  expanded  into  a 
bifid  lobe  larger  than  the  dorsal  lobe,  and  with  its  anterior  part 
larger  than  the  posterior.  The  result  is  a  markedly  asymmetrical 
structure,  and  it  constitutes  what  is  termed  a  Heterocercal  tail.  This 
is  a  highly  specialised,  form  of  fin  derived  from  a  much  more  primitive 
type,  the  Diphycercal  fin,  such  as  we  find  in  the  tadpole  and  young 
dogfish.  In  these  the  vertebral  column  remains  straight,  and  the 
fin  is  evenly  balanced  around  it,  terminating  in  a  point.  In  the 
common  bony  fish  a  balanced  condition  is  also  to  be  found,  but  it  is 
secondarily  acquired.  The  tip  of  the  vertebral  column  is  turned  up 
as  in  Scyllium,  but  the  simple  ventral  lobe  is  prolonged  backwards, 
producing  the  characteristic  "  fish  tail."  This  condition  is  desig- 
nated Homocercal.  There  seems  little  doubt  that  the  median  fins 
are  to  be  regarded  as  phylogenetically  older  than  the  paired  fins. 
Not  only  do  they  develop  earlier,  i.e.  are  ontogenetically  older,  but 
there  are  forms  like  Amphioxus  and  fishes  like  the  Lampreys  and 
Hagfish  in  which  they  alone  are  present  and  no  sign  of  paired  fins 
are  to  be  found. 

The  larger  of  the  paired  fins  are  the  pectoral  fins,  triangular 
structures  arising  by  their  apices  from  just  behind  the  gills,  ventro- 
laterally  about  a  quarter  of  the  way  back.  The  pelvic  fins  are 
smaller,  lying  in  the  ventral  part  of  the  body  and  arising  not  quite 
half-way  back  just  in  front  of  the  cloaca.  They  differ  in  the  two 
sexes,  and  so  for  the  first  time  in  the  higher  animals  we  find  an 
external  character  that  enables  us  readily  to  distinguish  the  two 
sexes.  In  the  female  the  mesial  borders  of  the  pelvic  fins  pass 
backwards  behind  the  cloaca  parallel  with  one  another  and  quite 
free.  In  the  male,  however,  these  two  borders  unite  to  form  a  sort 
of  elongated  pouch  within  which  is  situated  on  each  side  of  the  middle 
line  a  rod-shaped  body,  the  Clasper.  These  claspers  or  copulatory 
organs  are  grooved  on  their  inner  surface  and  bear  at  their  extremity 
a  group  of  papillae,  on  which  the  scales  point  in  the  reverse 
direction.  They  are  used  during  copulation,  and  serve  for  the  con- 
veyance of  the  sperms  to  the  oviducts.  The  paired  fins  are  of  great 
interest,  as  they  are  homologous  with  the  fore  and  hind  limbs  of 
higher  Craniates,  and  so  with  our  own  arms  and  legs.  Their  actual 
mode  of  origin  is  not  quite  clear,  but  the  most  generally  accepted 
account  is  that  they  are  modified  local  remnants  of  a  continuous 
lateral  fold  running  forward  from  the  cloaca  to  the  branchial  region  ; 
but  we  shall  return  to  this  point  later. 

The  fins,  median  and  paired,  are  all  concerned  with  locomotion 
in  water.  The  propellant  power  is  obtained  from  vigorous  strokes 
of  the  powerful  tail  with  its  strong  fin,  steering  is  mainly  carried  out 

P 


210  AN   INTRODUCTION  TO  ZOOLOGY 

by  the  pectoral  fins,  and  the  dorsal,  ventral  and  pelvic  fins  act  as 
steadying  or  equilibrating  organs. 

The  head  is  somewhat  flattened,  and  of  a  triangular  shape, 
ending  in  a  bluntly  rounded  snout.  On  the  sides  of  the  head  are 
situated  the  eyes.  They  are  provided  with  upper  and  lower  lids, 
the  latter  are  able  to  be  pulled  right  up  over  the  eye.  The  external 
nares  are  two  approximately  circular  openings  on  the  ventral 
surface  of  the  head,  a  short  distance  from  the  front  of  the  snout. 
Each  leads  into  a  hollow  cavity,  the  nasal  sac  or  olfactory  organ, 
which  has  no  internal  narial  passage  such  as  we  find  in  the  frog. 
On  the  other  hand,  the  nasal  sac  communicates  with  the  mouth  by 
means  of  a  deep  groove,  the  naso-buccal  or  oro-nasal  groove,  which 
is  partly  covered  by  a  flap-like  extension  of  the  skin  termed  the 
fronto-nasal  process.  This  is  a  very  primitive  condition,  and  one 
that  is  met  with  in  the  embryos  of  the  higher  Craniates.  It  is  a 
failure  in  the  complete  transformation  of  this  into  an  internal  nasal 
passage  in  man  that  leads  to  the  condition  known  as  cleft-palate 
or  hare-lip.  Just  behind  the  olfactory  organs  is  the  large  mouth  of  a 
crescentic  shape,  and  provided  with  a  formidable  armament  of  many 
rows  of  strong  pointed  teeth.  A  short  distance  behind  the  eye  is  a 
small  circular  orifice,  the  Spiracle,  which  leads  into  the  pharynx. 
In  spite  of  their  anterior  position  and  appearance  these  perforations 
are  homologous  with  true  gill  clefts,  and  their  development  shows 
that  they  are  in  reality  the  foremost  of  the  series.  Indication  of  the 
fact  that  they  were  at  one  time  similar  to  the  other  clefts  is  to  be 
found  in  the  presence  of  a  tuft  of  rudimentary  gill  filaments,  the 
pseudobranch,  situated  on  their  anterior  wall.  They  appear  to  be 
mainly  concerned  with  the  intake  of  water  for  respiratory  purposes. 
It  is  the  spiracular  cleft,  lying  as  it  does  in  close  proximity  to  the 
auditory  capsule,  that  takes  part  in  the  formation  of  the  ear  passages 
of  higher  vertebrates. 

A  short  distance  further  back  on  each  side  are  a  series  of  five 
vertical  slits  decreasing  in  size  from  the  front,  these  are  the  external 
gill  clefts.  When  they  are  opened  it  will  be  seen  that  they  lead  into 
a  corresponding  series  of  branchial  pouches.  The  anterior  and 
posterior  wall  of  each  pouch,  save  the  hinder  wall  of  the  last,  is 
thrown  into  a  series  of  radiating  folds,  the  branchial  filaments  or 
gills,  which  are  delicate  and  very  vascular,  so  that  they  appear 
bright  red  in  life  or  in  a  freshly  killed  fish. 

There  remain  for  mention  two  apertures  on  the  dorsal  surface  of 
the  head,  so  small  as  to  be  practically  invisible,  although  their 
position  may  be  demonstrated  in  a  fresh  specimen  by  pressing  the 
back  of  the  head  which  causes  drops  of  a  milky  fluid  to  be  exuded. 
They  lie  in  the  mid-dorsal  line  just  about  on  a  level  with  the  front  of 


VERTEBRATE  ANIMALS  211 

the  spiracles,  and  are  the  openings  of  the  endolymphatic  ducts  leading 
down  to  the  membranous  labyrinth  of  the  ear.  ., 

Scattered  over  the  head,  particularly  in  the  snout  region,  are  a 
number  of  small  apertures  leading  down  into  tiny  tubes,  the  mucous 
canals,  which  are  filled  with  a  gelatinous  substance  that  is  extruded 
if  the  head  is  squeezed.  A  similar  series  of  apertures  leads  into  a 
regular  series  of  canals  above  and  below  the  orbits,  and  on  the  dorsal 
and  ventral  sides  of  the  hinder  end  of  the  head.  These  are  connected 
near  the  spiracle  with  a  pair  of  canals  that  run  along  the  whole 
length  of  the  fish,  one  on  each  side,  in  a  shallow  groove  of  lighter 
colour,  and  so  are  easily  discernible  ;  they  constitute  the  so-called 
lateral  line  canals  and  lodge  a  series  of  sense  organs. 

As  we  have  already  noted,  the  vent  or  cloaca  is  situated 
between  the  hinder  internal  margins  of  the  pelvic  fins.  It  is  a  short 
slit  leading  into  a  shallow  cloacal  chamber  into  which  open  the  hinder 
end  of  the  alimentary  canal,  the  urinary  and  genital  apertures. 
The  anus  proper  or  termination  of  the  gut  opens  into  the  front  end 
of  the  cloacal  chamber.  Further  back  are  two  small  cavities,  the 
cloacal  pits,  partly  overhung  by  a  flap  of  skin,  the  cloacal  papilla. 
Anteriorly  the  pit  ends  blindly,  but  it  is  continued  back  as  a  minute 
canal,  the  abdominal  pore,  leading  into  the  coelom,  which  thus  com- 
municates directly  with  the  exterior,  a  condition  not  found  in  the 
higher  craniates.  A  considerable  amount  of  variation  is  met  with 
in  this  pore  showing  that  it  is  of  small  functional  importance,  and 
probably  represents  a  vestigeal  structure.  Sometimes  it  is  present 
on  one  side  only,  sometimes  on  both  sides,  or  again  it  may  be  absent 
altogether.  The  two  sexes  have  their  urinary  and  genital  apertures 
differently  arranged.  In  the  male  there  is  a  small  projection,  the 
urinogenital  papilla,  perforated  by  a  single  pore.  It  lies  in  the  middle 
line  just  behind  the  anus  and  in  front  of  the  cloacal  pits,  and  serves 
for  the  transmission  of  excretory  and  reproductive  products.  The 
female  possesses  a  structure  similar  in  appearance,  but  serving  only 
for  the  passage  of  the  urine  and  so  constituting  a  urinary  papilla. 
The  oviducal  opening  in  the  female  is  a  fairly  large  longitudinal  cleft 
on  the  dorsal  wall  of  the  cloacal  chamber  between  the  papilla  and 
anus.  It  leads  directly  to  the  two  oviducts,  which  run  together  just 
at  this  point. 

Skin  and  Exoskeleton. 

The.  skin  of  Scyllium,  like  that  of  Rana,  is  composed  of 
two  layers,  an  outer,  the  epidermis,  derived  from  the  ectoderm,  and 
an  inner  layer,  the  dermis,  of  mesodermal  origin.  The  epidermis 
is  a  stratified  epithelium  with  a  layer  of  cubical  actively  dividing 
cells,  the  Malpighian  layer,  at  the  base.  As  they  pass  outwards  the 


212 


AN   INTRODUCTION   TO  ZOOLOGY 


cells  become  more  and  more  flattened.  Scattered  among  them  here 
and  there  are  spherical  glandular  or  mucous  cells,  which  are  not 
aggregated  to  form  glands  as  in  the  frog.  The  dermis  is  composed 
of  two  layers,  an  outer  of  connective  tissue,  and  a  lower  thicker  layer 
of  tough  fibrous  tissue.  It  contains  blood-vessels  and  nerves  and  a 
number  of  pigment  cells  arranged  in  groups  to  form  the  well-marked 
spots.  The  skin  is  rough  to  the  touch,  particularly  if  the  hand  be 
drawn  from  behind  forwards,  owing  to  the  presence  in  it  of  small 
close-set  scales  with  projecting  points  directed  backwards  and 


FIG.  67. — Denticles  of  Scyllium. 

A.,  surface  view  of  denticle ;  B.,  side  view  of  denticle :  I.— IV.,  successive  stages  in  the 
development  of  a  denticle. 

C.B.,  formative  cells  of  basal  plate  ;  D.,  dermis;  d.,  dentine  ;  D.P.,  dermal  papilla  ;  E.,  epi- 
dermis ;  e.,  enamel ;  e.e.,  enamel  epithelium  ;  M.,  Malpighian  layer  ;  P.,  pulp  ;  P.C.,  pulp  cavity  ; 
Pi.,  pigment  cell. 

forming  a  characteristic  exoskeleton.  When  dried  the  skin  is  known 
commercially  as  shagreen,  and  is  utilised  for  polishing  purposes. 
These  scales  are  of  a  peculiar  type  known  as  placoid  scales  or  dermal 
denticles,  and  in  their  production  both  epidermis  and  dermis  take 
part.  They  differ  completely  from  the  scales  of  the  ordinary  bony 
fish,  which  are  entirely  dermal  structures. 

A  denticle  consists  of  a  quadrangular  basal  plate  and  a  blade  or 
spine  set  at  an  acute  angle  to  this.     The  basal  plate  lies  firmly 


VERTEBRATE  ANIMALS  213 

embedded  in  the  upper  part  of  the  dermis,  while  the  spine,  which  is 
generally  leaf-shaped  with  a  long  point  flanked  on  each  side  by  a 
shorter  one,  projects  through  the  epidermis.  The  greater  part  of 
the  scale  consists  of  hard  calcareous  substance,  dentine,  a  dermal 
product,  covered  with  a  layer  of  still  harder  enamel  secreted  by  the 
epidermis.  The  denticle  is  hollow,  containing  within  it  a  pulp 
cavity  in  the  form  of  a  median  canal  with  side  branches  which  opens 
to  the  outside  by  means  of  a  more  or  less  circular  hole  in  the  basal 
plate.  In  life  this  cavity  is  filled  with  a  loose  connective  tissue,  the 
pulp,  containing  blood-vessels,  lymph-vessels,  and  nerves  which 
pass  in  through  the  hole  in  the  base.  The  pulp  is  surrounded  by  a 
single  layer  of  characteristic  cells  known  as  odontoblasts,  the  function 
of  which  is  to  secrete  the  dentine.  This  dentine,  unlike  bone,  contains 
no  cells  within  it,  and  therefore  grows  only  on  the  side  next  to  the 
odontoblasts.  Its  matrix  contains  a  very  high  percentage  of 
calcium  salts,  considerably  more  than  in  bone,  and  is  penetrated  by 
a  number -of  extremely  fine  processes  from  the  odontoblasts  which  lie 
in  tiny  branched  dentinal  tubules.  The  enamel  or  vitrodentine,  as 
it  is  termed,  has  a  still  higher  proportion  of  calcium  salts,  and  is  in 
consequence  harder.  Although  it  does  not  possess  the  prismatic 
structure  characteristic  of  the  enamel  of  our  own  teeth,  it  is  probably 
homologous  with  it. 

It  will  be  remembered  that  the  lining  of  the  buccal  cavity  is 
essentially  the  same  as  the  skin  covering  the  body,  and,  although 
most  of  it  is  devoid  of  exoskeletal  structures,  as  it  passes  over  the 
jaws  it  produces  a  series  of  rows  of  highly  modified  denticles,  the 
teeth.  These  have  the  same  fundamental  structure  as  the  ordinary 
scales,  which  owe  their  name  of  denticles  to  this  similarity,  and  from 
which  they  have  undoubtedly  been  derived.  The  blade,  however,  is 
broader  and  not  so  long,  and  the  median  spine  has  two  or  three 
spines  on  each  side  of  it,  all  being  of  approximately  the  same  length. 
The  enamel  layer  is  also  considerably  thicker.  Various  modifica- 
tions of  the  denticles  produce  the  characteristic  spines  and  teeth 
met  with  in  all  the  Elasmobranchs. 

The  development  of  a  denticle  calls  for  notice  owing  to  its 
similarity  with  that  of  our  own  teeth.  The  first  indication  of  the 
formation  of  a  scale  is  the  aggregation  of  a  number  of  the  cells  of  the 
superficial  layer  of  the  dermis  to  constitute  a  dermal  papilla,  and  this 
represents  the  origin  of  the  pulp.  This  papilla  enlarges  and  presses 
upwards  into  the  epidermis,  the  Malpighian  layer  of  which  becomes 
modified  to  form  a  layer  of  columnar  cells,  the  enamel  epithelium. 
While  this  is  taking  place,  the  outermost  cells  of  the  papilla  become 
transformed  into  odontoblasts  which  secrete  the  dentine,  first  in  the 
form  of  a  small  cone  capping  the  papilla.  The  base  of  the  papilla 


214 


AN   INTRODUCTION   TO  ZOOLOGY 


narrows  off  considerably,  but  always  remains  open,  and  the  whole 
structure  sinks  down  into  the  dermis,  below  the  level  of  the  epi- 
dermis, causing  the  enamel  epithelium  and  the  adjacent  cells  of  the 
Malpighian  layer  to  become  infolded  in  a  very  characterictic  manner. 
The  main  part  of  the  papilla  remains  to  form  the  pulp,  which  with 
the  odontoblasts  and  dentine  of  the  blade  and  entire  basal  plate,  by 
far  the  largest  part  of  the  denticle,  is  the  product  of  the  dermis. 
The  enamel  alone,  at  any  rate  the  greater  part  of  it,  comes  from 
the  epidermis,  and,  according  to  some  authorities,  this  also  receives 
contributions  from  the  underlying  dentine. 

The  second  variety  of  exoskeletal  element  in  the  dogfish  takes 
the  form  of  fairly  long  cartilaginous  rods  or  rays  supporting  the 
distal  parts  of  all  the  fins.  In  consequence  of  their  being 
developed  in  the  dermis  they  are  termed  dermal  fin  rays  or 
dermotrichia. 

Muscular  System. 

The  muscles  of  the  dogfish,  like  those  of  the  frog,  are  pf 
two  varieties,  striate  and  non-striate.     The  latter,  including  also  the 


Ms  M 

FIG.  68. — Sketch  to  show  arrangement  of  myotomes  in  Scy Ilium, 

E.,  epiaxial  myotomes  ;  H.,  hypaxial  myotomes  ;  L.,  lateral  line  ;  M.,  myotome ; 
Ms.,  myosepta. 

cardiac  muscles,  are  confined  to  the  various  viscera.  If  the  skin  be 
stripped  off  the  body  muscles  will  be  seen  beneath  it  arranged  in  a 
very  characteristic  manner.  Unlike  the  frog,  the  fibres  are  not 
bound  together  to  form  definite  discrete  muscles,  save  in  certain 
parts,  but  arranged  in  narrow  columnar  sheets.  These  extend  from 
the  lateral  line  dorsalwards  and  ventralwards  in  parallel  zigzag 
strips  known  as  muscle  segments  or  myotomes,  which  meet  in  the 
mid-dorsal  and  mid-ventral  line  and  alternate  with  the  vertebrae,  a 


VERTEBRATE  ANIMALS  .  215 

very  primitive  arrangement  indicating  clearly  a  metameric  segmenta- 
tion. They  are  separated  by  thin  fibrous  sheets  of  connective  tissue, 
the  myosepta  or  myocomata,  to  which  the  individual  fibres  lying  in 
the  longitudinal  direction  are  attached  at  each  end.  The  lateral 
line,  which  is  partly  embedded  in  a  thick  horizontal  myoseptum, 
separates  the  muscle  segments  into  a  dorsal  or  epiaxial  series  of 
broader  and  a  ventral  or  hypaxial  series  of  narrower  myotomes. 
In  the  region  of  the  eye,  of  the  jaws  and  of  the  fins  the  fibres  are 
bound  in  bundles  to  form  discrete  muscles  more  like  those  of  Rana. 

Endoskeleton. 

As  in  the  frog,  the  endoskeleton  may  be  divided  for  descrip- 
tion into  axial  and  appendicular  portions.  The  axial  part  is  com- 
posed of  the  vertebral  column  and  the  skull,  which  in  its  turn  is 
formed  by  the  cranium  and  a  somewhat  complex  visceral  skeleton. 
The  appendicular  skeleton  consists  of  the  median  fin  supports,  and 
the  pectoral  and  pelvic  girdles  with  their  corresponding  limbs. 
The  endoskeleton  as  a  whole  is  remarkable  in  that  it  always,  even 
in  the  adult,  remains  in  a  cartilaginous  condition,  no  true  bone  ever^ 
being  developed  although  it  is  much  strengthened  by  calcification, 
i.e.  the  deposition  in  it  of  calcium  salts  in  certain  parts  of  the  vertebral 
centra. 

The  vertebral  column  is  a  very  primitive  one,  and  composed  of  a 
long  series  of  vertebrae  whose  constituent  parts  are  not  nearly  so 
completely  fused  as  in  Rana,  and  so  can  be  recognised  as  separate 
pieces.  It  contains  quite  considerable  remains  of  the  notochord 
in  the  adult.  The  centra  are  short  stout  cylinders  with  a  deep 
conical  hollow  at  each  end,  a  condition  known  as  amphiccelous,  and 
one,  it  will  be  remembered,  that  is  retained  in  the  eighth  vertebra 
of  the  frog.  The  internal  faces  of  the  two  hollows  are  lined  with 
calcified  cartilage  and  communicate  with  one  another  by  a  small 
central  hole,  the  cavities  of  the  hollows  and  the  central  holes  being 
filled  with  the  persistent  notochord.  The  centra  are  firmly  bound 
together  by  tough  fibrous  bands  of  tissue,  the  intervertebral  ligaments, 
which  pass  externally  over  the  notochord.  Dorsally  they  bear  two 
sets  of  cartilaginous  plates  which  constitute  the  neural  arches,  and 
together  with  a  third  set  form  a  closed  vertebral  canal  in  which  lies 
the  spinal  cord.  From  the  latero-dorsal  aspect  of  the  centrum  arises  a 
pentagonal  plate,  the  vertebral  neural  plate,  which  is  only  about  half 
the  length  of  the  centrum.  Filling  in  the  interstices  between  these 
are  hexagonal  intervertebral  neural  plates,  lying  above  the  inter- 
vertebral  ligaments  and  so  completing  the  neural  arches.  Fitted 
into  the  notches  left  on  the  dorsal  edges  of  the  plates  is  a  third  series, 
a  row  of  unpaired,  somewhat  wedge-shaped  cartilages,  the  neural 


216 


AN   INTRODUCTION   TO  ZOOLOGY 


spines.  A  foramen  at  the  lower  posterior  border  of  the  vertebral 
neural  plate  and  a  similar  one  at  the  upper  posterior  border  of  the 
intervertebral  neural  plate  lead  into  the  vertebral  canal.  These 
allow  for  the  passage  of  the  ventral  and  dorsal  roots  of  the  spinal 
nerves  respectively,  and  from  their  positions  it  is  obvious  that  the 
ventral  root  arises  in  front  of  the  corresponding  dorsal  root. 


FIG.  69. — Vertebral  column  of  Scy Ilium. 

I.,  lateral  view  of  two  vertebra?  in  anterior  body  region;  LA.,  end-on  view  of  vertebra  in  same 
region ;  II.,  lateral  view  of  two  vertebrae  in  tail  region ;  II. A.,  end-on  view  of  tail  vertebra ; 
III.,  median  section  of  two  vertebrae  in  body  region ;  IV.  and  V.,  two  successive  stages  in  the 
early  development  of  the  centrum  ; — adapted  from  Hasse. 

A.,  aorta  ;  C.C.,  calcined  cartilage  ;  C.E.,  chorda  epithelium  ;  E.E.,  membrana  elastica 
externa  :  E.I.,  membrana  elastica  interna  ;  H.A.,  haemal  arch  ;  H.C.,  haemal  canal ;  H.S.,  haemal 
spine  ;  I.L.,  intervertebral  ligament  ;  I. P.,  intervertebral  neural  plate  ;  N.,  notochord  ;  N.C., 
neural  canal ;  N.S.,  neural  spine ;  P.Pi.,  apertures  for  the  exit  of  the  dorsal  and  ventral  roots 
of  a  spinal  nerve  ;  Per.,  perforations  through  centre  of  the  cartilage  of  the  centrum ;  R.,  rib  ; 
T.P.,  transverse  process  ;  V.P.,  vertebral  neural  plate. 

From  the  ventro-lateral  edges  of  the  centra  in  the  middle  region 
of  the  body  two  short  projections,  the  transverse  or  haemal  processes, 
project  outwards.  To  the  end  of  each  of  these  is  attached  a  fairly 
long  slender  rod  of  cartilage,  the  rib,  which  lies  laterally  in  the  main 
horizontal  septum  between  the  epi-  and  hyp-axial  myotomes,  and  is 
also  in  the  planes  where  this  septum  is  joined  by  the  ordinary 


VERTEBRATE  ANIMALS  217 

myosepta.  The  haemal  processes  of  the  anterior  trunk  vertebrae  are 
reduced  to  mere  ribless  stumps,  while  in  the  caudal  region  they  are 
greatly  expanded  and  pass  off  more  ventrally  as  flattened  plates  to 
curve  inwards  and  meet  in  the  middle  line.  They  are  completed  by 
a  series  of  mid- ventral  haemal  spines,  which  are  the  bases  for  the 
support  of  the  ventral  portions  of  the  caudal  fin.  In  this  way 
another  tube,  the  haemal  canal,  is  formed  beneath  the  centra, 
similar  to  the  neural  canal  above  them,  and  in  life  this  contains  the 
caudal  artery  and  caudal  vein,  the  main  vessels  of  the  tail. 

Turning  now  to  consider  the  development  of  the  vertebral 
column,  we  find  that  the  first  part  of  the  skeleton  to  make  its  appear- 
ance in  the  embryo  is  a  structure  known  as  the  notochord  or  chorda 
dorsalis,  which  arises  as  a  rod  of  cells  derived  from  the  dorsal  part 
of  the  entoderm  of  the  gut.  It  is,  therefore,  ventral  to  the  central 
nervous  system  under  which  it  stretches  from  the  posterior  end  to 
just  below  the  fore-brain,  where  it  becomes  thin  and  runs  off  into  a 
downwardly  turned  end.  The  cells,  disc-shaped  to  start  with,  soon 
secrete  around  themselves  a  clear  refractive  elastic  membrane,  the 
so-called  primary  chordal  sheath  or  membrana  elastica  externa. 
The  cells  enlarge  and  become  vacuolated  by  the  formation  within 
them  of  a  jelly-like  substance,  and  so  exhibit  in  section,  a  character- 
istic appearance  producing  the  typical  notochordal  tissue.  The 
vacuoles  fill  the  inside  of  the  cells,  and  so  reduce  the  cytoplasm 
to  an  enveloping  layer.  At  first  scattered,  the  nuclei  migrate  to 
the  periphery  of  the  chorda,  there  forming  a  layer  termed  the  chorda 
epithelium.  This  soon  secretes  a  second  thicker  and  more  fibrous 
layer  within  the  former,  and  termed  the  secondary  chordal  sheath 
or  membrana  elastica  interna.  The  mesoderm  (mesenchyme) 
surrounding  the  chorda  produces  migrant  cells  which  wander  into 
the  inner  sheath  and  gradually  transform  it  into  a  thick  cellular 
layer,  now  termed  the  tunica  skeletogena. 

From  this  a  large  part  of  the  centrum  is  formed  ;  the  neural 
arches,  etc.,  are  laid  down  in  an  extension  of  the  mesenchyme  layer 
which  encloses  the  notochord  and  passes  up  to  surround  the  spinal 
cord,  and  is  called  the  skeletogenous  sheath.  In  this  are  differ- 
entiated four  longitudinal  bands  of  deeper  staining  more  tightly 
packed  cells,  two  dorso-lateral  and  two  ventro-lateral.  Paired 
cartilages  appear  in  the  bands,  and  on  the  dorsal  side  are  the 
beginnings  of  the  vertebral  and  intervertebral  neural  plates,  and  on 
the  ventral  side  the  haemal  arches.  Vertical  rings  of  cartilage  are 
now  laid  down  in  the  tunica  skeletogena  corresponding  in  position 
with  the  arches,  and  these  constitute  the  primordia  of  the  centra. 
As  they  grow  they  become  much  thicker  in  their  central  region,  thus 
constricting  the  notochord  intracentrally,  but  they  always  leave  a 


218  AN   INTRODUCTION   TO  ZOOLOGY 

small  hole  in  the  middle.  Inter  vert  ebrally  the  chorda  is  not  con 
stricted,  but  retains  its  original  size,  so  that  in  median  longitudinal 
section  it  appears  as  a  series  of  diamonds  strung  together.  The 
centrum  is  finally  completed  by  the  growth  around  it  of  a  thin 
extension  of  the  bases  of  the  arches.  Each  vertebra  is  thus  com- 
posed of  a  number  of  parts  which  develop  separately,  but  all  come 
from  mesenchyme.  Between  the  vertebrae  the  tunica  skeletogena 
is  transformed  into  the  strong  fibrous  intervertebral  ligament. 

Skull. 

The  skull  of  the  dogfish  remains  in  a  cartilaginous  con- 
dition throughout  life,  and  furnishes  a  very  good  example  of  a 
primitive  vertebrate  skull  which  is  not  modified  as  in  the  higher 
Craniates  by  the  addition  of  bony  structures  developed  either  in 
the  cartilages  or  the  surrounding  membranes.  It  consists  of  a 
cranium  or  brain  case,  with  which  are  fused  the  olfactory  and 
auditory  capsules,  and  to  which  are  connected  a  series  of  seven 
paired  segmental  visceral  arches  which  originate  as  supporting 
elements  in  relation  to  the  perforations  of  the  pharyngeal  walls 
known  as  the  gill  clefts. 

The  chondrocranium  of  Scyllium,  so  called  to  indicate  that  it 
remains  cartilaginous  throughout  life,  is  shaped  like  a  slightly 
flattened  oblong  box  within  which  lies  the  brain.  Its  floor  and  front 
end  are  complete,  and  so  also  are  the  two  long  sides,  save  for  a  number 
of  small  perforations  through  which  nerves  or  blood-vessels  pass. 
The  hinder  end  is  wide  open,  leaving  a  large  hole,  the  foramen 
magnum,  through  which  the  brain  is  continuous  with  the  spinal  cord. 
The  cartilaginous  roof  is  incomplete  at  the  anterior  end,  where  there 
is  situated  a  large  elongated  oval  opening,  the  anterior  cranial 
fontanelle,  closed,  however,  by  a  membrane. 

The  hinder  region  of  the  cranium  is  formed  by  a  ring  of  cartilage 
surrounding  the  foramen  magnum,  and  it  is  termed  the  occipital 
region.  It  bears  ventro-laterally  of  the  foramen  two  rounded  smooth 
prominences,  the  occipital  condyles,  whereby  the  cranium  articulates 
with  the  front  end  of  the  vertebral  column.  The  cranial  floor 
between  the  condyles  is  formed  by  the  basal  plate  of  cartilage  which 
runs  forward  as  far  as  a  small  median  perforation,  the  internal 
carotid  foramen,  through  which  the  similarly  named  artery  enters 
the  brain  case.  The  portion  of  the  cranium  immediately  in  front 
of  the  occipital  is  known  as  the  otic  region,  and  it  is  greatly  expanded 
owing  to  the  fact  that  it  has  fused  with  it  the  large  auditory  or  otic 
capsules  in  which  are  lodged  the  structures  of  the  internal  ear. 
Clearly  showing  on  the  roof  of  the  auditory  capsule  are  two  ridges 
marking  the  position  of  the  anterior  and  posterior  semicircular 


VERTEBRATE  ANIMALS 


219 


canals,  and  on  its  side  is  a  similar  ridge  indicating  the  horizontal 
semicircular  canal.  The  two  ridges  of  the  vertical  canals  on  each 
side  converge  to  a  median  oval  depression  in  the  roof  of  the  cranium 
in  which  lie  the  two  apertures  of  the  endolymphatic  ducts  (aqueduct! 


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vestibuli).  In  front  of  this  region,  again,  the  side  of  the  cranium  is 
hollowed  out  slightly  for  the  lodgment  of  the  eyes  or  optic  capsules, 
so  that  it  is  termed  the  optic  region.  The  hollowing  is  accentuated 
by  the  roof  and  floor  of  the  brain  case,  being  carried  out  laterally 
into  flat  wing-like  expansions  known  as  the  supra-  and  infra-orbital 


220  AN   INTRODUCTION  TO  ZOOLOGY 

ridges  respectively.  The  former  continues  into  the  nasal  capsule 
in  front,  and  the  auditory  capsule  behind.  These  capsules,  together 
with  the  ridges,  constitute  an  efficient  orbit  within  which  lies  the 
eyeball  attached  to  the  lateral  cranial  wall  by  the  optic  nerve  and  a 
series  of  muscles. 

The  anterior  end  of  the  orbit  is  also  in  part  formed  by  a  pre- 
orbital  or  lateral  ethmoidal  process  continuous  with  the  hinder  wall 
of  the  olfactory  capsule.  This  with  the  remaining  part  of  the  skull 
in  front  of  it,  composed  almost  entirely  of  the  nasal  capsules,  is 
known  as  the  ethmoidal  region,  and  into  it  the  cranial  cavity  does 
not  enter.  The  olfactory  capsules  are  thin-walled  hollow  spherical 
structures,  each  with  a  large  opening  on  the  ventral  side.  They  are 
completely  separated  from  one  another  by  an  extension  of  the  cranial 
floor  known  as  the  mesethmoidal  plate  or  internasal  septum.  This, 
again,  is  prolonged  as  a  slender  rod,  the  rostral  cartilage,  which 
together  with  two  similar  cartilaginous  bars  arising  from  the  antero- 
dorsal  walls  of  the  capsules  constitutes  the  rostrum  or  skeletal 
support  of  the  snout. 

Let  us  turn  now  to  consider  the  perforations  in  the  side  walls 
of  the  cranium.  At  the  antero-ventral  corner  of  the  orbit,  which  is 
occupied  by  a  large  blood  sinus,  is  the  orbito-nasal  foramen,  placing 
the  orbital  sinus  in  communication  with  the  nasal  sinus.  Midway 
along  the  orbit  in  its  lower  portion  is  the  conspicuous  optic  foramen, 
through  which  the  optic  nerve  passes.  In  the  postero-ventral 
corner  is  another  large  perforation  for  the  exit  of  the  main  branches 
of  the  fifth  and  seventh  and  the  entire  sixth  cranial  nerves.  Between 
this  and  the  optic  foramen  are  three  smaller  holes  for  blood-vessels. 
The  most  anterior  of  these  is  for  the  hyoidean  artery,  the  posterior 
for  the  internal  carotid  artery,  and  the  middle  one,  slightly  more 
dorsal  than  the  others,  is  the  interorbital  canal,  which  puts  the  two 
orbital  sinuses  in  communication.  The  remaining  foramina  in  the 
orbit  are  nearer  the  dorsal  side  of  the  cranium.  In  the  postero- 
dorsal  corner  is  a  foramen  for  the  ophthalmic  branch  of  the  seventh 
cranial  nerve  which  runs  forward  in  a  shallow  groove.  Just  below, 
and  in  front  of  this,  is  a  similar  aperture  for  the  ophthalmic  branch 
of  the  fifth  cranial  nerve,  from  which  also  a  groove  passes  forward 
soon  uniting  with  the  former.  Running  forward  to  the  front  end 
of  the  orbit,  the  common  groove  passes  out  on  to  the  dorsal  side  of 
the  supra-orbital  ridge  by  a  well-marked  hole.  Antero-ventrally 
of  the  foramen  for  the  fifth  ophthalmic  nerve  lies  that  of  the  third 
cranial  nerve,  and  just  dorsal  to  and  in  front  of  this,  again,  is  another 
for  the  fourth  nerve.  From  the  posterior  border  of  the  orbit  below 
the  ridge  of  the  horizontal  semicircular  canal  runs  a  well-marked 
furrow  in  the  wall  of  the  optic  capsule,  the  post-orbital  groove,  in 


VERTEBRATE  ANIMALS 


221 


PF 


which  is  a  vessel  joining  the  orbital  to  the  anterior  cardinal  sinus. 
A  short  distance  along  this  is  a  foramen  transmitting  the  ninth 
cranial  nerve,  which  comes  from  the  floor  of  the  capsule  wherein  it 
runs,  and  at  its  posterior  end  another  perforation  just  lateral  of  the 
occipital  condyles  forms  the  exit  for  the  vagus. 

The  development  of  the  cranium  in  the  dogfish  is  interesting, 
since  it  is  similar  in  its  early  stages  and  main  outlines  to  that  of  any 
typical  Craniate.  The  first  skeletal  structure  in  all  these  forms  is, 
as  has  already  been  pointed  out,  the  notochord,  which  in  Scyllium 
runs  under  the  brain  as  far  forward 
as  the  hinder  end  of  the  fore-brain, 
i.e.  up  to  the  level  of  the  pituitary 
body.  Two  separate  regions  of  de- 
velopment of  the  cranium  are  con- 
sequently to  be  noted,  a  posterior 
chordal  region,  comprising  the  occipi- 
tal and  otic  regions  of  the  adult,  and 
an  anterior  pre-chordal  portion  later 
forming  the  orbital  and  ethmoidal 
regions.  The  first  cartilages  to  appear 
in  the  hinder  region  are  two  bars,  the 
par achor dais,  one  on  each  side  of  the 
notochord,  which  subsequently  fuse 
around  this  to  form  the  basal  plate. 
In  its  occipital  region  this  shows 
signs  of  segmentation,  suggesting  its 
derivation  from  a  modified  anterior 
part  of  the  vertebral  column,*  but 
there  are  no  such  indications  in  the 
otic  region.  To  the  sides  of  this 
latter  lie  the  beginnings  of  the 
membranous  labyrinth,  the  auditory 
vesicles,  around  which  a  cartilaginous 
capsule  is  soon  developed.  In  the 
pre-chordal  region  also  the  first  elements  to  appear  are  two  carti- 
laginous bars,  the  trabeculse  cranii,  and  these  two  fuse  to  form  a 
trabecular  plate.  Their  hinder  ends,  however,  remain  open  for  a 
time,  leaving  the  pituitary  fontanelle,  but  later  grow  together  below 
the  hypophysis  cerebri,  which  thus  comes  to  be  lodged  in  a 
depression  in  the  cranial  floor.  At  its  front  end  the  trabecular 
plate  is  continued  on  to  form  the  internasal  septum. 

The  side  walls  of  the  occipital  region  are  formed  by  lateral 

*  This  suggestion  is  also  supported  by  other  evidence  into  which  we  cannot 
enter  here. 


FIG.  71. — Development  of  cra- 
nium in  a  Craniate,  adapted 
from  Ziegler's  model. 

A.,  auditory  capsule  ;  N.,  notochord  ; 
O.,  occipital  region  :  P.,  parachordal ; 
PF.,  future  position  of  pituitary  fon- 
tanelle ;  T.,  trabecula  cranii. 


222  AN   INTRODUCTION   TO   ZOOLOGY 

upgrowths  of  the  basal  plate,  while  in  the  otic  regions  they  are  com- 
posed of  the  mesial  walls  of  the  auditory  capsules.  In  the  orbital 
regions  only  a  small  part  of  the  side  wall  is  derived  from  upgrowths 
of  the  trabecular  plate  ;  it  is  mainly  built  up  from  two  independent 
dorsal  elements,  the  orbital  cartilages.  These  meet  in  the  middle 
line  posteriorly,  forming  a  cranial  roof,  and  they  also  fuse  with  the 
otic  capsules.  At  the  front  end,  however,  they  remain  apart, 
leaving  between  them  the  anterior  cranial  fontanelle.  The  rest  of 
the  anterior  end  of  the  skull  is  formed  by  the  cartilaginous  nasal 
capsules  which  are  laid  down  around  the  olfactory  vesicles. 

Visceral  Skeleton. 

In  Scyllium  there  is  also  connected  with  the  skull  a  visceral 
skeleton  composed  of  seven  pairs  of  arches  one  behind  the  other, 
each  composed  of  two  half  hoops  of  cartilage  forming  a  support 
for  the  anterior  end  of  the  alimentary  canal.  None  of  these  are 
actually  fused  with  the  cranium,  although  the  first  two  are  firmly 
attached  thereto  by  means  of  ligaments.  The  first  arch  is  the 
mandibular,  composed  of  upper  and  lower  jaws.  The  upper  jaw  or 
palato-pterygo-Quadrate  cartilage,  is  a  stout  curved  bar  of  cartilage 
somewhat  compressed  laterally  and  situated  in  the  upper  margin 
of  the  mouth.  Its  anterior  end  curves  inwards  to  the  middle  line 
to  meet  its  fellow  in  a  ligamentous  mandibular  symphysis,  and  it  is 
attached  to  the  cranium  just  behind  the  olfactory  capsule  by  the 
ethmo-palatine  ligament.  The  posterior  end  is  markedly  flattened 
and  bears  a  strong  ridge  for  the  attachment  of  powerful  jaw  muscles, 
and  it  further  bears  at  its  postero- ventral  corner  a  surface  for  articu- 
lation with  the  lower  jaw.  The  lower  jaw  itself  is  similarly  composed 
of  two  flattened  curved  bars,  Meckel's  cartilages,  lying  in  the  lower 
margin  of  the  mouth,  and  they  also  meet  in  an  anterior  median 
ligamentous  symphysis.  The  cartilage  is  narrow  in  front,  but  much 
deeper  behind  for  the  insertion  of  muscles,  and  here  it  not  only 
articulates  with  the  upper  jaw,  but  the  two  jaws  are  connected  with 
the  hyomandibular  cartilage.  It  is  noteworthy  then  that  neither 
jaw  is  directly  joined  to  the  cranium,  but  only  indirectly  through  the 
intermediation  of  ligaments  and  the  upper  portion  of  the  second  or 
hyoid  arch.  Such  a  skull  is  termed  Hyostylic,  in  order  to  distinguish 
it  from  an  Autostylic  one  such  as  our  own,  in  which  the  jaws  are 
directly  connected  with  the  cranium.  It  will  be  seen  later  that  the 
mandibular  arch  is  much  modified  from  a  typical  arch,  and  both  parts 
of  it  are  covered  by  an  integument  bearing  numerous  rows  of  sharp 
close-set  teeth  which  are  homologous  with  dermal  denticles.  The 
folds  of  skin  at  the  side  of  the  mouth  are  supported  by  a  pair  of 


VERTEBRATE  ANIMALS  223 

elongated  labial  cartilages,  of  which  the  dorsal  in  the  upper  lip  is 
the  larger. 

The  second  or  hyoid  arch  more  nearly  approaches  in  structure 
a  typical  branchial  arch,  and  it  is  divided  into  two  portions.  The 
dorsal  end,  the  hyomandibular  cartilage,  is  a  stout  short  rod  lying 
just  behind  the  spiracle,  which  articulates  with  the  auditory  capsule 
just  below  the  post-orbital  groove.  This  bar  is  firmly  connected 
with  the  jaws  by  the  stout  symplectic  ligament,  and  so  suspends 
them  from  the  cranium,  being  for  this  reason  sometimes  termed  the 
suspensorium.  To  its  ventral  end  is  also  attached  the  lower  portion 
of  the  arch,  namely,  the  ceratohyal  cartilage.  This  is  a  long  curved 
structure  more  slender  than  the  hyomandibular,  and  it  runs  antero- 
ventrally  under  the  lower  jaw  to  become  attached  in  the  middle  line 
to  a  median  basihyal  plate,  lying  in  the  floor  of  the  pharynx  and 
buccal  cavity.  A  thick  ligament  joins  the  ceratohyal  to  the  lower 
jaw.  Two  other  ligaments  just  behind  the  spiracle  help  to  keep  the 
jaws  and  hyoid  arch  in  position :  the  first  is  the  superior  post- 
spiracular  ligament  running  from  just  above  the  anterior  end  of  the 
post-orbital  groove  to  the  end  of  the  palato-quadrate  and  hyoman- 
dibular cartilages  ;  the  second,  the  inferior  post-spiracular  ligament, 
passes  from  the  postero-lateral  border  of  the  floor  of  the  skull  below 
the  orbit  to  the  hyomandibular  and  ceratohyal.  Both  portions  of 
the  hyoid  arch  bear  a  series  of  small  cartilaginous  rods,  the  branchial 
rays,  on  their  posterior  edges  which  serve  for  the  support  of  the  front 
wall  of  the  first  gill  cleft. 

It  will  be  seen,  then,  that  the  spiracle  lies  in  front  of  the  hyoid 
arch,  and  between  it  and  the  mandibular,  although  the  latter  turns 
forward  parallel  with  the  cranial  floor,  and  we  find  the  anterior 
spiracular  wall  strengthened  by  a  small  prespiracular  cartilage, 
probably  the  remains  of  the  branchial  rays  at  one  time  carried  by 
the  mandibular  arch  in  an  ancestral  form. 

The  remaining  five  visceral  arches  are  the  branchial  arches, 
numbered  i  to  5  from  before  backwards.  Each  becomes  split 
up  into  four  distinct  segments  and,  connected  together  by  a  median 
ventral  unpaired  basibranchial  plate,  form  a  series  of  supporting 
bars  for  the  walls  of  the  branchial  pouches.  The  branchial  clefts 
receive  their  name  from  the  skeletal  element  immediately  behind 
them,  so  that  the  spiracle  is  more  properly  termed  the  hyoidean 
cleft,  and  the  rest  the  true  branchial  clefts,  although,  of  course,  they 
are  strictly  homologous.  The  four  segments  of  the  arch  on  each 
side  are  in  order  dorso-ventrally  the  pharyngeo-,  epi-,  cerato-  and 
hypo-branchials.  Thepharyngeo-branchials  are  elongated  triangular 
plates  that  run  from  just  beneath  the  vertebral  column  forwards  and 
outwards  in  the  roof  of  the  pharynx,  and  the  fourth  and  fifth  are 


224 


AN   INTRODUCTION   TO   ZOOLOGY 


joined  by  the  fusion  of  their  dorsal  extremities.  The  epibranchials 
are  short  stout  rods  lying  almost  vertically  in  the  side  walls  of  the 
pharynx  and  articulating  dorsally  with  the  pharyngeo-  and  ventrally 
with  the  cerato-branchials.  These  latter  are  more  elongated  rods 
passing  forwards  and  inwards  in  the  ventral  pharyngeal  wall.  The 
hypobranchials  are  not  so  regular  as  the  other  elements.  The  first 
pair  are  short  slender  rods  running  forward  and  joining  the  first 
cerato-branchials  to  the  basihyal.  Hypobranchials  2,  3  and  4  are 
also  small  rods,  but  running  backwards  ;  the  first  pair  join  in  the 
middle  line  and  also  connect  with  the  succeeding  pair,  while  the 
third  and  fourth  pairs  put  their  respective  arches  in  connection  with 
the  basibranchial  plate.  The  fifth  arch  lacks  a  hypo  branchial,  its 
ceratohyal,  which  directly  abuts  on  to  the  basibranchial,  is  much 
expanded  and  has  a  conspicuous  notch  at  its  postero-lateral  corner 
through  which  an  important  blood-vessel,  the  ductus  Cuvieri,  passes. 
The  basibranchial  plate  is  a  median  flattened  sheet  of  cartilage 
helping  to  form  the  roof  of  the  pericardial  cavity.  The  posterior 
borders  of  all  epi-  and  cerato-branchials,  except  the  fifth,  bear  a 
series  of  small  unbranched  branchial  rays.  In  this  manner  is  con- 
stituted a  fairly  complex  skeletal  framework,  the  branchial  basket. 
Other  series  of  curved  rod-like  elements,  the  extra  branchiate,  are 
developed  just  below  the  skin  in  relation  to  the  second,  third  and 
fourth  arches,  but  as  they  are  not  firmly  attached  to  the  other 
branchials  they  are  often  missing  in  prepared  skeletons.  The 
relation  of  the  various  parts  of  a  typical  visceral  skeleton  may  be 
represented  as  follows  : — 


/ 

Upper  jaw  (palato- 

A.  First  arch  man- 

pterygo-qu  adrate  ) 

dibular 

Lower  jaw 

(Meckel's  cartilage) 

VISCERAL     i 
SKELETON.  1 

B.  Second      arch 
hyoid 

Hyomandibular 
(suspensorium) 
Ceratohyal 

Connected  in  middle 
line  by  the  basi- 
hyal. 

C.  Five    branchial 
arches,  each  com- 
posed of  :  — 

Pharyngeo-branchial 
Epibranchial 
Cerato-branchial 
Hypobranchial 

Connected  in  middle 
line  by  the  basi- 
branchial. 

Development. — The  mandibular  arch  is  first  seen  as  two  curved 
rods  of  cartilage,  one  on  each  side,  lying  in  the  hinder  margin  of  the 
mouth  and  meeting  in  the  mid- ventral  line  ;  each  later  splits  into 
upper  and  lower  jaws.  The  hyoid  arch  arises  in  a  similar  manner 
in  the  septum  between  the  spiracle  and  the  first  gill  cleft,  but  the 
rods  do  not  meet  in  the  middle  line.  Its  upper  end  soon  acquires 
an  articulation  with  the  auditory  capsule,  and  later  it  splits  into  two. 
The  branchial  arches  also  arise  as  two  half  hoops  of  cartilage  in  the 


VERTEBRATE  ANIMALS  225 

septa  between  the  gill  slits,  which  segment  first  into  two  and  then 
into  four  pieces  on  each  side.  The  basihyal  and  basibranchial  are 
separately  developed. 

Appendicular  Skeleton. 

The  remaining  part  of  the  skeleton  consists  of  the  supports 
of  the  median  fins,  the  paired  fins  and  the  girdles  supporting  the 
latter. 

The  dorsal  fin  is  composed  of  two  morphologically  distinct  parts  ; 
firstly,  endoskeletal  structures  known  as  pterygiophores  or  somactidia , 
and  secondly,  exoskeletal  structures,  the  dermotrichia.  The  somac- 
tidia take  the  form  of  a  series  of  cartilaginous  rods  (12  in  the  first 
dorsal  fin),  attached  to  a  corresponding  number  of  neural  spines. 
Each  rod  consists  of  a  short  basal  element,  these  tend  to  fuse  to  form 
basalia,  and  a  distal  longer  piece,  the  radialium,  bearing  at  its 
extremity  two  polygonal  plates.  To  these  plates  are  attached  a 
number  of  dermotrichia,  which  are  horny  fibres  lying  in  the  dermis 
and  constituting  the  support  of  the  main  part  of  the  blade  of  the  fin. 
The  ventral  fin  is  somewhat  similar  in  build,  but  the  cartilaginous 
elements  are  reduced,  while  in  the  caudal  fin  they  disappear  alto- 
gether, leaving  the  dermotrichia  to  be  borne  directly  by  the  neural 
and  haemal  spines  which  are  greatly  enlarged  in  this  region. 

The  pectoral  girdle  consists  of  a  strong  hoop  of  cartilage, 
irregular  in  shape  and  incomplete  dorsally,  lying  embedded  in  the 
muscles  of  the  body  wall  a  short  distance  behind  the  last  gill  arch. 
The  median  part  forms  a  fairly  straight  bar  running  transversely  and 
expanded  slightly  in  the  centre  to  form  a  thinner  plate  which  passes 
forward  a  little  to  form  a  platform  supporting  the  hinder  end  of  the 
floor  of  the  pericardial  cavity.  This  may  be  termed  the  coracoid 
portion  of  the  girdle,  and  in  reality  it  consists  of  two  bars,  as  is 
clearly  shown  in  its  development,  corresponding  with  the  coracoids 
of  Rana,  but  they  have  met  in  the  middle  line  without  the  inter- 
vention of  a  sternum  and  fused  so  as  to  lose  their  individuality. 
The  laterial  extremity  of  the  bar  is  marked  on  each  side  by  three 
smooth  articular  surfaces  on  its  posterior  aspect  which  bear  the 
proximal  cartilages  of  the  pectoral  fin.  The  remaining  parts  of  the 
girdle,  the  scapular  portions,  are  horn-shaped,  curved  tapering  rods 
passing  dorso-mesiaLly  to  end  in  the  muscles  on  each  side  of  the 
vertebral  column. 

The  basalia  of  the  pectoral  fin,  as  has  been  noted,  are  three  in 
number,  representing  the  fusion  of  a  series  of  somactidia.  They 
increase  markedly  in  size  from  before  backwards,  and  are  known 
as  the  pro-,  meso-  and  meta-pterygium  respectively.  Following 
these  are  a  series  of  about  16  radiala,  which  again  terminate  in 

Q 


226 


AN   INTRODUCTION   TO  ZOOLOGY 


polygonal  plates.     To  these  are  attached  a  large  number  of  dermo- 
trichia,  giving  the  characteristic  outline  to  the  fin. 

The  pelvic  girdle  is  a  more  simple  structure,  and  consists  of  one 
more  or  less  straight  bar  of  cartilage  running  transversely  in  the 
muscles  of  the  body  wall,  immediately  in  front  of  the  cloaca.  Its 

postero-lateral  corner  is  hol- 
lowed to  form  an  articular 
facet  that  bears  the  pelvic 
fin,  while  antero-laterally  it 
is  produced  forward  in  a 
short  bluntly  pointed  process. 
The  fin  skeleton  is  also 
simpler,  the  somactidia  hav- 
ing fused  to  form  a  single 
basalium,  termed  the  basi- 
pterygium.  This  bears  a 
series  of  radialia,  terminating 
in  polygonal  plates,  and  these 
in  turn  carry  the  customary 
dermotrichia.  In  the  male 
fish  the  innermost  radialium 
is  greatly  enlarged,  grooved 
on  its  dorsal  surface  and 
constitutes  the  skeleton  of 
the  clasper. 

We  have  seen,  then, 
that  the  structure  of  all  the 
fins  is  essentially  the  same,  they  are  indeed  homologous.  Furthermore, 
it  is  fairly  clear  that  the  median  fins  have  been  evolved  from  one 
long  continuous  fin  running  along  the  back,  around  the  tail  and 
forward  ventrally  to  the  anus,  for  we  find  such  a  fin  not  only  in  such 
forms  as  Amphioxus,  and  the  lowly  Cyclostomes,  but  also  in  the 
embryos  of  some  fish  and  in  the  tadpole.  This  long  fin  has  been 
split  up  probably  in  response  to  the  needs  of  stability  and  locomotion, 
and  has  been  developed  at  the  points  of  the  greatest  mechanical 
advantage,  while  the  intervening  portions  have  been  suppressed. 
The  origin  of  the  paired  fins  is  not  so  obvious,  but  is  a  matter  of 
interest.  A  theory  known  as  "  the  lateral  fold  theory  "  is  perhaps 
the  most  generally  held.  According  to  this  the  ancestral  fish  form 
had  a  lateral  fold  of  skin  running  forward  from  the  cloaca  to  the 
region  of  the  gill  clefts,  somewhat  similar  to  that  found  in  Amphioxus. 
Like  the  median  fin,  these  lateral  folds  were  developed  at  some  points 
and  suppressed  over  the  remainder.  A  fair  amount  of  evidence 
supports  this  point  of  view.  Thus  in  Cladoselache,  a  fossil  shark 


FIG. 


72. — Pelvic  girdle  and  fin  of 
Scy Ilium. 


B.,  basipter 
dermolrichia ; 
bar  ;  P.P.,  pelvic  fin 


ium ;  C.,  skeleton  of  clasper ;  D., 
polygonal  plates;  P.B.,  pelvic 
n ;  R.,  radialia. 


VERTEBRATE  ANIMALS  227 

from  the  Devonian  rocks,  we  find  the  pectoral  and  pelvic  fins  are 
similar  in  shape  to  the  dorsal  fins  of  Scyllium,  i.e.  very  broad  at  the 
base  and  lying  parallel  with  the  long  axis  of  the  body.  The  radialia 
of  the  pelvic  fins  are  just  simple  rods  of  cartilage  with  their  bases 
embedded  in  the  muscles  of  the  body  wall,  but  those  of  the  pectoral 
show  signs  of  fusion  at  their  bases.  In  some  embryos  a  very  similar 
stage  is  passed  through,  and  the  two  fins  are  even  joined  by  a  low 
ridge  of  skin.  The  girdles  are  thought  to  have  originated  by  a 
further  fusion  of  the  somactidia,  their  sinking  lower  into  the  muscles 
of  the  body  wall  and,  finally,  in  order  to  form  an  efficient  mechanical 
support  for  the  movement  of  the  fin,  extending  markedly  in  a  direc- 
tion transverse  to  the  long  axis  of  the  fish,  with  the  consequent 
development  of  an  articulation  between  fin  and  support.  Observa- 
tions on  the  development  of  the  paired  fins,  particularly  with  regard 
to  their  muscle  supply,  also  go  to  prove  that  they  represent  the 
concentrations  of  fins  that  were  originally  joined  to  the  body  by  a 
very  much  longer  base  than  at  present. 


CHAPTER   IX 
SCYLLIUM   CANICULA— (continued) 

Alimentary  system — Respiratory  system — Circulatory  system — • 
Urogenital  system. 

Alimentary  System. 

The  digestive  system  consists  of  a  long  canal  running  from 
mouth  to  anus,  mainly  in  the  coelom,  and  it  is  about  twice  as  long 
as  the  distance  between  these  two  points.  Certain  glands  are 
connected  with  this  canal  as  in  the  frog,  and  they  play  important 
roles  in  the  digestive  processes. 

The  crescentic  mouth  leads  into  a  fairly  capacious  oral  or  buccal 
cavity  whose  walls  possess  no  salivary  glands,  and  in  the  floor  of 
which  the  basihyal  cartilage  forms  a  low  projection,  having  almost 
the  appearance  of  a  tongue,  although  it  cannot  be  considered  as  such 
an  organ.  The  whole  cavity,  being  developed  as  a  stomodoeum,  is 
lined  with  ectoderm,  and  owing  to  this  fact  is  able  to  bear  the  teeth. 
It  has  been  pointed  out  previously  that  the  teeth  are  homologous 
with  the  denticles  covering  the  skin,  and,  indeed,  in  the  embryo 
they  are  in  a  continuous  sheet  with  them,  not  being  separated  until 
later  by  the  formation  of  a  scaleless  lip  groove.  The  teeth  are  borne 
in  a  series  of  parallel  lines  over  the  jaws,  in  which  they  are  not 
embedded,  but  to  which  their  bases  are  attached  by  a  very  tough 
fibrous  tissue.  As  all  are  similar  in  structure,  and  not  different  from 
one  another  like  our  own,  the  dentition  is  said  to  be  homodont.  Also, 
since  there  are  a  number  of  rows,  indeed,  as  fast  as  the  outer  row 
wears  out  a  new  row  takes  its  place,  we  find  a  number  of  successions 
of  teeth,  a  condition  termed  polyphyodont.  This  stands  in  marked 
contrast  to  ourselves,  where  but  two  sets  are  fully  developed,  the 
milk  teeth  and  the  permanent  teeth,  that  is,  a  diphyodont  condition. 

The  hinder  end  of  the  roof  of  the  buccal  cavity  bears  a  small 
transverse  fold  of  skin,  the  oral  valve,  otherwise  the  cavity  passes 
over  imperceptibly  into  the  pharynx.  There  is  a  noteworthy 
morphological  difference  between  the  two  regions,  however,  since 
the  pharynx  is  lined  by  entoderm  and  constitutes  the  beginning  of 
the  mid-gut  or  mesenteron.  The  pharynx  is  a  fairly  short  flat  tube 
for  the  passage  of  the  food,  but  being  mainly  concerned  with  the 
function  of  respiration,  and  its  ventro-lateral  walls  are  marked  by 

228 


SCYLLIUM   CANICULA  229 

the  presence  of  the  internal  spiracular  opening  and  five  long  narrow 
slits,  the  internal  branchial  clefts.  Each  cleft  leads  into  a  branchial 
pouch,  also  lined  with  entoderm,  the  anterior  and  posterior  walls  of 
which,  with  the  exception  of  the  hinder  wall  of  the  last,  are  thrown 
into  the  branchial  folds  or  filaments.  The  tissue  forming  the 
partition  between  one  pouch  and  the  next  constitutes  the  inter- 
branchial  septum,  in  which  lie  the  cartilaginous  rods  of  the  branchial 
skeleton  and  the  blood-vessels  of  the  gills.  One  septum  with  its 
skeletal  and  vascular  elements  and  the  branchial  filaments  on  each 
side  of  it  is  termed  a  complete  gill  or  holobranch,  so  that  we  find  on 
each  side  of  the  fish  four  complete  gills.  The  filaments  on  one  side 
of  the  pouch  form  a  half  gill  or  hemibranch,  so  that  in  addition  to 
the  four  holobranchs  there  is  also  on  each  side  of  the  front  wall  of 
the  first  gih1  pouch  a  single  hemibranch.  The  pseudobranch  already 
noted  on  the  anterior  wall  of  the  spiracle  is,  therefore,  to  be  regarded 
as  a  spiracular  hemibranch  which,  however,  is  vestigial  and  without 
functional  significance.  Movements  of  the  branchial  region  take 
place  during  respiration,  whereby  the  water  is  kept  circulating 
over  the  highly  vascular  filaments,  which  are  thus  kept  supplied  with 
oxygen.  The  movements  are  brought  about  by  a  fairly  complex 
series  of  muscles  connected  with  the  branchial  basket. 

Behind  the  gill  clefts  the  pharynx  passes  over  into  a  somewhat 
narrower  tube,  the  oesophagus,  which  possesses  dark-coloured  walls 
owing  to  the  presence  in  them  of  a  rich  plexus  of  capillaries,  and  is 
quite  short.  It  is  lined  by  a  characteristic  stratified  epithelium. 

The  oesophagus  leads  into  the  stomach,  and  the  transition  from 
one  to  the  other  is  marked  on  the  outside  by  a  change  in  coloration, 
and  on  the  inside  by  an  alteration  in  the  character  of  the  mucosa, 
which  becomes  a  simple  columnar  epithelium  of  a  highly  glandular 
nature.  The  stomach  is  a  large  U-shaped  tube,  whose  proximal, 
wider  end  lying  to  the  left  is  termed  the  cardiac  portion.  The  distal, 
somewhat  narrower  end  on  the  right  is  known  as  the  pyloric  portion, 
and  it  runs  forward  again  parallel  with  the  cardiac  portion  almost 
to  the  level  of  the  oesophagus,  where  it  turns  back  upon  itself  to  form 
a  short  swelling,  the  pyloric  enlargement,  followed  by  a  marked 
constriction,  the  pylorus.  The  structure  of  the  stomach  wall  is  on 
the  whole  very  similar  to  that  in  Rana,  and  its  circular  muscle  fibres 
are  very  strongly  developed  in  the  region  of  the  pylorus  to  form  the 
pyloric  sphincter,  a  circular  constrictor  muscle.  Internally  when  the 
stomach  is  empty,  or  only  moderately  full,  the  mucosa  is  thrown 
into  a  series  of  longitudinal  folds  which  disappear  when  the  organ  is 
distended.  Inside  the  pylorus  is  a  ridge  of  the  mucosa  termed  the 
pyloric  valve  which,  when  the  sphincter  contracts,  enables  the 
stomach  to  be  shut  off  from  the  intestine  while  the  food  undergoes 


230 


AN   INTRODUCTION  TO  ZOOLOGY 


the  initial  changes  of  the  digestive  processes.  The  dogfish  is  a 
voracious  feeder,  so  that  the  stomach  is  often  full  or  even  much 
distended  with  food,  and  it  frequently  contains  in  addition  an 
enormous  number  of  parasitic  worms. 


v.o. 


H.P. 


7    , 


FIG.  73. — Sketch  of  the  arrangement  of  the  hepatic  portal  factors  in  Scy Ilium, 
seen  from  the  dorsal  side.  The  vessels  were  injected  and  the  gut 
hardened  and  removed  whole  from  the  body-cavity. — After  O'Donoghue. 

A. I.,  anterior  intestinal  vein;  A.L.G.,  anterior  lieno-gastric  vein;  D.A.G.,  dorsal  anterior 
gastric  vein ;  D.G.,  dorsal  gastric  vein ;  D.O.,  dorsal  cesophageal  vein  ;  G.I.,  gastro-intestinal 
vein;  H.P.,  hepatic  portal  vein;  /./.,  intra-intestinal  vein;  M.G.,  median  gastric  vein;  P. I., 
posterior  intestinal  vein ;  P.L.G.,  posterior  lieno-gastric  vein  ;  P.S.,  posterior  splenic  vein  ; 
R.,  portion  of  posterior  intestinal  vein  on  the  rectal  gland;  V.A.G.,  ventral  anterior  gastric 
vein  ;  V.G.,  ventral  gastral  vein  ;  V.O.,  ventral  oesophageal  vein. 

The  intestine  is  composed  of  two  parts,  the  intestine  proper  and 
the  rectum.  The  intestine  lies  slightly  to  the  right  of  the  pyloric 
portion  of  the  stomach,  and  has  the  form  of  a  fairly  wide  spindle- 
shaped  tube  passing  backwards  to  a  point  just  behind  the  level  of 
the  bend  of  the  stomach.  Into  its  upper  end  open  the  pancreatic 
and  bile  ducts,  and  below  this  its  wall  is  marked  by  more  or  less 


SCYLLIUM   CANICULA  231 

circularly  running  blood-vessels.  The  intestine  is  characterised 
inside  by  the  development  of  a  very  striking  structure  known  as  the 
spiral  valve.  It  is  a  large  fold  of  the  mucosa  twisted  into  a  spiral 
form.  Although  in  lower  forms  like  the  lamprey  and  in  the  embryo 
this  is  only  a  low  fold  recalling  the  typhlosole  of  the  earthworm, 
save  that  it  is  spirally  inserted  in  the  gut  wall,  in  the  adult  Scyllium 
it  is  a  very  wide  fold.  Its  free  edges  have  united  in  the  middle  of 
the  intestinal  lumen  to  form  a  spirally  twisted  median  axis,  and  so 
the  cavity,  instead  of  being  more  or  less  straight,  is  converted  into  a 
fairly  narrow  spiral  of  about  eight  complete  turns.  If  the  wall  of 
the  intestine  be  cut,  it  presents  the  appearance  of  a  series  of  imperfect 
slightly  truncated  cones  with  their  bases  directed  backwards  and 
fitting  one  inside  the  other.  Thus,  although  the  intestine  is  relatively 
short,  the  path  actually  traversed  by  the  food  in  passing  through  it 
is  quite  long.  Like  the  typhlosole,  this  arrangement  has  two 
results  ;  firstly,  it  retains  the  food  within  the  gut  for  a  much  longer 
time  to  enable  digestion  to  be  completed,  and,  secondly,  it  provides 
a  much  larger  area  over  which  the  food  can  be  absorbed.  The  same 
ends  are  attained  in  the  higher  vertebrates  by  the  development  of  a 
long,  much-coiled  intestine.  The  beginning  of  the  rectum,  just 
behind  the  level  of  the  bend  of  the  stomach,  is  marked  by  the  presence 
of  a  small  reddish  club-shaped  structure,  the  rectal  gland,  attached 
to  its  dorsal  wall.  This  body  is  highly  glandular,  and  has  a  central 
duct  opening  into  the  beginning  of  the  dorsal  wall  of  the  rectum. 
The  function  of  this  rectal  gland  has  not  yet  been  ascertained  satis- 
factorily, but  it  is  probably  the  homologue  of  the  ccecum  of  the  higher 
animals,  so  that  the  intestine  in  front  of  it  corresponds  to  the  small 
intestine  of  those  forms,  and  ah1  there  is  to  represent  the  large 
intestine  is  the  rectum.  In  the  dogfish  the  rectum  runs  straight  on 
into  the  cloaca,  into  which  also  open  the  excretory  and  reproductive 
ducts,  as  has  been  pointed  out  previously. 

The  histology  of  the  various  parts  of  the  alimentary  canal  is  on 
the  whole  similar  to  that  of  the  frog.  In  the  connective  tissue 
underlying  the  enteric  epithelium  are  situated  a  number  of  small 
nodules  of  lymphoid  tissue,  each  enclosed  in  a  fairly  definite  capsule 
and  known  as  the  lymph  follicles. 

In  addition  to  the  rectal  gland  there  are  connected  with  the 
alimentary  canal  two  glands  that  play  important  parts  in  digestion, 
these  are  the  liver  and  the  pancreas.  The  liver  is  a  large  conspicuous 
dark  brown  gland  divided  up  into  two  main  lobes,  one  on  each  side 
of  the  body,  and  a  much  smaller  middle  lobe  lying  ventral  to  the 
stomach.  The  three  lobes  are  continuous  at  the  anterior  end  and 
bound  together  and  held  in  position  by  a  strong  membrane,  the 
suspensory  ligament  of  the  liver,  which  attaches  them  firmly  to  the 


232  AN  INTRODUCTION  TO  ZOOLOGY 

membrane  separating  the  pericardial  from  the  peritoneal  cavities.  The 
liver  is  composed  of  a  large  number  of  ramifying  branched  tubules, 
which,  as  they  originate  from  an  outgrowth  of  the  alimentary 
canal,  are  of  entodermal  origin  and  so  constitute  a  compound 
tubular  gland,  although  the  actual  structure  of  the  adult  gland  is 
masked  by  its  being  tightly  bound  together  by  mesodermal  connec- 
tive tissue.  Embedded  in  the  front  end  of  the  left  lobe  of  the  liver 
is  a  fairly  large  thin- walled  sac  of  dark  green  colour,  the  gall  bladder, 
in  which  is  stored  the  bile  secreted  by  the  liver.  From  it  comes  off 
a  small  tube,  the  cystic  duct,  which  runs  between  the  two  main  lobes 
for  a  short  distance,  receiving  from  them  several  smaller  hepatic 
ducts.  After  the  confluence  of  these,  the  main  tube,  now  distin- 
guished as  the  common  bile  duct,  runs  on  to  open  into  the  ventral 
side  of  the  intestine  a  short  distance  beyond  the  pylorus  and  behind 
the  beginning  of  the  fold  of  the  spiral  valve.  The  liver  of  Scy Ilium 
has  the  same  complex  functions  as  it  has  in  Rana. 

The  pancreas  is  a  long  thin  body  of  a  yellowish- white  colour  and 
triangular  in  cross  section,  lying  dorsally  to  the  intestine  and  pyloric 
portion  of  the  stomach.  At  the  front  end  it  expands  into  a  small 
ventral  lobe  which  lies  tucked  in  the  bend  between  the  stomach 
and  intestine  close  to  the  pylorus.  The  pancreatic  duct  runs  through 
the  substance  of  the  gland,  emerges  from  the  posterior  corner  of  the 
ventral  lobe,  enters  the  intestinal  wall  below  the  pylorus,  and  after 
running  in  the  wall  of  the  intestine  for  about  half  an  inch  opens  into 
the  inside  close  to  the  beginning  of  the  spiral  valve  near  the  aperture 
of  the  bile  duct.  Like  the  liver,  the  pancreas  arises  as  an  outgrowth 
of  the  alimentary  canal,  and  so  is  composed  of  entodermal  cells 
grouped  in  glandular  acini  and  bound  together  by  mesodermal 
tissue.  Its  function  is  similar  to  that  of  the  frog. 

Another  quite  conspicuous  very  dark  glandular-looking 
mass  is  attached  to  the  posterior  border  of  the  stomach  in  the  form 
of  a  U.  This  is  the  spleen,  and  it  is  not  connected  functionally 
with  digestion  nor  developmentally  with  the  alimentary  canal,  but 
is  mentioned  here  because  it  is  bound  to  the  stomach  by  a  strong 
fold  of  the  mesentery. 

From  the  last  part  of  the  oesophagus  to  the  end  of  the  rectum 
the  gut  lies  more  or  less  freely  in  the  coelom,  but  is  held  in  position 
by  a  reflection  of  the  peritoneum  lining  the  ccelomic  cavity.  In  the 
embryo  this  fold  is  complete  from  end  to  end  and  is  known  as  the 
dorsal  mesentery,  since  it  comes  off  from  the  mid-dorsal  line  of  the 
ccelom.  The  posterior  middle  portion  of  this  disappears  in  the  adult, 
leaving  a  large  anterior  part,  the  mesogaster,  supporting  the  stomach 
and  first  half  of  the  intestine  and  a  smaller  posterior  part,  the 
mesorectum,  attached  to  the  rectum  and  rectal  gland.  These 


SCYLLIUM   CANICULA  233 

folds  are  generally  spoken  of  as  mesenteries,  and  they  give  rise  to  a 
series  of  lateral  peritoneal  folds  which  hold  the  other  viscera  in 
position  ;  such  side  folds  are  termed  omenta  and  receive  their  name 
from  the  organs  they  surround.  Thus  we  can  recognise  a  gastro- 
hepatic  omentum  connecting  liver  and  stomach,  in  this  run  the  bile 
duct  and  portal  vein  ;  a  gastrointestinal  omentum,  binding  stomach 
and  intestine  ;  and  a  gastro-splenic  omentum,  joining  the  spleen  to 
the  stomach. 

Respiratory  System. 

The  anatomy  of  the  respiratory  system  has  already  been 
considered  in  dealing  with  the  alimentary  canal,  since  the  respiratory 
organs,  the  gills,  are  derived  from  modified  outgrowths  of  the  walls 
of  the  pharynx.  All  the  blood  leaving  the  heart  is  taken  to  the  gills 
by  a  series  of  vessels,  collected  up  by  another  set  and  then  conveyed 
all  over  the  body.  In  the  tiny  filaments  the  vessels  break  up  into 
numerous  small  capillaries,  thus  allowing  the  blood  to  be  brought 
quite  close  to  the  surrounding  water.  This  water  is  constantly 
being  changed  by  the  action  of  the  branchial  muscles,  and  so  the 
oxygen  contained  in  it  is  able  to  pass  into  the  blood,  and  at  the  same 
time  the  carbon  dioxide  in  the  latter  can  pass  out  into  the  water  by 
diffusion.  Unlike  the  frog,  respiration  is  limited  to  this  region  and 
not  carried  on  over  the  general  surface  of  the  body. 

Circulatory  System. 

The  circulatory  system  in  Scyllium  is  in  some  ways  similar 
to  that  of  Rana,  but  it  is  of  a  more  primitive  type  and  is  of  interest  in 
comparative  morphology,  as  the  main  outlines  of  its  general  plan  are 
encountered  in  the  embryos  of  all  the  higher  Craniates.  It  is  com- 
posed of  a  blood-vascular  system  and  a  lymphatic  system.  The 
blood- vascular  system  is  a  closed  one  as  in  all  Craniates,  and  consists 
of  a  heart,  arteries,  capillaries  and  veins  which  form  one  series  of 
closed  vessels  not  communicating  with  the  outside.  As  "in  the  frog, 
some  of  the  fluid  plasma  of  the  blood  oozes  through  the  walls  of 
the  capillaries  to  bathe  the  various  tissues,  and  this  is  collected  up 
by  the  lymphatic  vessels  and  returned  to  the  blood  again.  Owing 
to  the  difficulty  of  finding  the  lymphatic  vessels,  particularly  in 
Scyllium,  by  the  ordinary  methods  of  dissection  the  term  "  circulatory 
system  "  is  often  employed  to  mean  the  blood- vascular  system  only. 

Heart. 

The  heart  is  a  stout  muscular  organ  the  rhythmic 
contraction  of  whose  walls  produces  the  difference  in  pressure 
necessary  to  keep  the  blood  circulating,  and  so  it  forms  the  centre 


234 


AN  INTRODUCTION  TO  ZOOLOGY 


of  the  system.  It  lies  in  a  triangular  space,  the  pericardial  cavity, 
situated  behind  and  beneath  the  last  pair  of  gill  pouches  and  so 
occupies  a  position  underneath  the  hinder  end  of  the  pharynx. 
This  is  the  primitive  place  for  the  heart,  much  further  forward  than 

in    Rana,    and    indeed   in 
/»  what   would  be  the  neck 

region  of  the  higher  animals. 
In  the  embryo  the  peri- 
cardial and  peritoneal 
cavities  are  in  open  com- 
A.Ca  munication  with  one 
another,  but  they  become 
separated  at  quite  an 
-V  early  date  by  the  forma- 
tion of  a  septum.  This 
septum  completely  cuts  off 
the  two  cavities,  and  hence, 
as  it  forms  the  posterior 
limit  of  the  heart  chamber 
and  the  anterior  wall  of 
the  abdominal  coelom,  it 
is  termed  the  pericardio- 
peritoneal  septum.  Before 
the  adult  condition  is 
reached  a  secondary  tubular 

outgrowth,  the  pericardio-peritoneal  canal,  grows  backwards  from 
the  postero-dorsal  corner  of  the  pericardial  space,  putting  the  two 
cavities  in  communication  with  one  another  again.  The  posterior 
part  of  the  floor  of  the  pericardial  cavity  is  formed  by  the  median 
extension  of  the  coracoid  bar  and  its  roof  by  the  basibranchial 
cartilage.  The  cavity  itself  is  lined  by  peritoneum,  which  is  also 
reflected  to  form  a  smooth  visceral  layer  that  closely  invests  the 
heart  and  is  known  as  the  pericardium. 

The  heart  itself  is  composed  of  four  chambers  and  may  be  con- 
sidered as  starting  at  the  hinder  end  of  the  pericardial  cavity  by  the 
running  together  of  two  large  veins,  the  ductus  Cuvieri,  from  the 
postero-lateral  corners  of  the  cavity  to  form  the  sinus  venosus. 
This,  the  first  part  of  the  heart,  is  a  large  thin- walled  tubular  sac 
lying  transversely  to  the  long  axis  of  the  body,  continuous  laterally 
with  the  ductus  and  attached  to  the  pericardio-peritoneal  septum 
by  its  posterior  border.  It  opens  by  a  single  median  aperture  into 
the  postero-dorsal  border  of  the  single  large  auricle  or  atrium,  the 
opening  being  guarded  by  a  membranous  sinu-auricular  valve.  The 
auricle  is  a  triangular  muscular  sac  with  its  apex  directed  forward, 


PCo. 


FIG.  74. — -Lateral  view  of  the  heart  to  show 
the  arrangement  of  the  coronary  veins. 
— After  O'Donoghue. 

A.Co.,  anterior  coronary  vein ;  Au.,  auricle ;  Co., 
conus  arteries  us  ;  O.,  opening  of  ductus  Cuvieri;  P.Co., 
posterior  coronary  vein ;  S.V.,  sinus  venosus ;  V., 
ventricle. 


SCYLLIUM   CANICULA  235 

its  lateral  corners  produced  into  small  lappets,  the  auriculae,  and  it 
is  situated  in  the  dorsal  part  of  the  pericardial  cavity.  This  chamber 
communicates  with  the  ventricle  in  the  middle  of  its  postero- ventral 
border  by  a  transverse  slit  guarded  by  a  pair  of  auriculo- ventricular 
valves.  The  ventricle  is  a  very  thick-walled  muscular  flask-shaped 
structure  on  the  ventral  side  of  the  pericardial  cavity,  and  so  it 
forms  the  most  obvious  part  of  the  heart  when  dissecting  the  fish 
from  the  ventral  side.  The  neck  of  the  flask,  as  it  were,  is  continued 
forward  as  a  thick  muscular  tube,  the  conus  arteriosus,  which  passes 
up  to  the  ventral  corner  of  the  apex  of  the  pericardial  space,  outside 
which  it  is  continued  as  a  median  artery,  the  ventral  or  cardiac 
aorta.  Inside  the  conus  are  two  rows  of  semilunar  valves,  each 
composed  of  three  watch-pocket  like  flaps,  the  proximal  row  separat- 
ing it  from  the  ventricle.  The  conus  is  actually  part  of  the  heart, 
being  composed  of  the  same  cardiac  muscle  fibres  that  distinguish 
the  ventricle  and  taking  part  in  the  waves  of  contraction  that  pass 
over  the  heart.  The  object  of  all  the  valves  in  the  heart  is  to  keep 
the  blood  flowing  in  the  same  direction  by  preventing  regurgitation 
when  fhe  pressure  behind  it  is  released. 

It  will  be  seen  from  the  above  description  that  the  heart  is  in 
reality  a  single  tube,  and  its  development  shows  that  it  originates 
as  the  specialisation  of  a  part  of  a  median  ventral  vessel.  The  heart 
of  all  vertebrates  arises  similarly  as  a  single  tube,  which  later  becomes 
bent  upon  itself  in  the  form  of  an  S.  In  Scyllium  the  heart  is  inte- 
resting because  it  always  remains  in  this  primitive  condition,  and  its 
folded  nature  can  be  demonstrated  very  easily  if  it  be  removed  from 
the  pericardium  and  cut  in  median  longitudinal  section. 

Arterial  System. 

The  vessels  conveying  the  blood  from  the  heart  to  the  various 
parts  of  the  body  constitute  the  arterial  system.  All  the  blood 
leaves  the  conus  by  the  one  great  trunk,  the  ventral  aorta,  which 
runs  forward  below  the  hypobranchial  elements  giving  off  paired 
branches,  the  afferent  branchial  arteries,  to  the  gills.  The  ventral 
aorta  passes  forward  to  the  lower  ends  of  the  ceratohyal  cartilages 
to  a  spot  immediately  behind  the  thyroid  gland,  and  there  bifur- 
cates into  two  branches,  the  innominate  arteries,  which  turn  directly 
laterally.  After  a  very  short  course  each  innominate  divides  into 
two  ;  the  first  afferent  branchial  artery  which  runs  along  the  hyoid 
arch  and  supplies  the  first  hemibranch,  and  the  second  afferent 
branchial  which  runs  along  the  first  branchial  arch  and  feeds  the 
posterior  hemibranch  of  the  first  gill  pouch  and  the  anterior  hemi- 
branch of  the  second  pouch,  in  other  words  the  first  complete  gill. 
The  third  afferent  branchial  artery  arises  about  half-way  back  to  the 


236 


AN   INTRODUCTION  TO  ZOOLOGY 


conus,  and  runs  outwards  on  the  second  gill  arch  serving  the  second 
holobranch.  The  fourth  and  fifth  afferent  branchials  are  given  off 
almost  together  just  outside  the  pericardium,  and  running  on  the 
corresponding  gill  arches  supply  the  next  two  holobranchs.  The 

afferent  branchials  break 
up  into  a  rich  capillary 
plexus  in  the  gill  filaments, 
so  enabling  the  blood  to 
give  up  its  carbon  dioxide 


and  take  up  oxygen. 

The  oxygenated 
blood  from  the  gills  is 
collected  up  by  efferent 
branchial  arteries,  vessels 
running  the  complete 
length  of  each  hemibranch. 
The  first  eight  on  each 
side  are  joined  at  each 
end  so  as  to  form  complete 
loops  around  the  first  four 
gill  pouches,  and  this 
leaves  a  single  vessel  along 
the  anterior  hemibranch 
of  the  fifth  pouch,  none 
being  required  along  its 
hinder  edge  as  it  bears  no 

gill  filaments.  These  loops  are  joined  about  half-way  along  their 
length,  i.e.  laterally,  by  short  longitudinal  trunks  passing  across 
the  gill  arches,  and  by  means  of  the  last  of  these  the  hindermost  and 
incomplete  half  loop  drains  into  the  preceding  loop.  From  the  inner, 
dorsal  ends  of  the  loop  come  off  four  pairs  of  epibranchial  arteries 
which  pass  backwards  and  inwards  in  the  sub-mucosa  of  the  dorsal 
pharyngeal  wall  to  unite  in  the  middle  line  and  form  the  dorsal 
aorta,  which  is  the  main  big  visceral  artery  and  runs  back  to  the  end 
of  the  tail. 

Three  arteries  form  the  main  source  of  supply  for  the  head 
region.  A  slender  mandibular  artery  is  given  off  from  the  extreme 
ventral  corner  of  the  first  efferent  branchial  loop,  and  this  passes 
forward  to  the  muscles  of  the  lower  jaw  and  also  the  hyoidean 
region.  From  the  middle  of  the  first  efferent  branchial,  i.e.  in 
line  with  the  longitudinal  trunks,  arises  a  fair-sized  vessel  variously 
termed  the  hyoid,  the  anterior  or  ventral  carotid  artery,  which  runs 
forward  dorsal  to  the  hyomandibular  cartilage  and  in  the  anterior 
wall  of  the  spiracle.  It  goes  on  across  the  floor  of  the  orbit  and 


FIG.  75. — Ventral  aorta  and  afferent  bran- 
chial arteries  of  Scy Ilium. 

A.,  auricle  ;  A.B.,  afferent  branchial  arteries  ;  G.C., 
gill  cleft ;  H.,  hemibranch ;  H.B.,  holobranch  ;  I.,  in- 
nominate artery;  M.,  muscles ;  P.,  pericardium;  S.V., 
sinus  venosus  ;  T.,  thyroid  gland ;  V.,  ventricle  ;  V.A., 
ventral  aorta. 


SCYLLIUM  CANICULA 


237 


enters  the  cranium  by  a  foramen  whose  position  we  have  already 
seen.  The  third  vessel/  the  common  or  dorsal  carotid  artery,  takes 
its  origin  from  the  dorsal  end  of  the  first  efferent  loop  right  beside 
the  first  epibranchial  artery.  It  curves  forwards  and  inwards  in 
the  sub-mucosa  of  the  roof  of  the  pharynx  to  a  point  about  level 
with  the  spiracle.  Here  it  receives  an  anastomosing  trunk  from  the 


E.C. 


FIG.  76. — Diagram  of  efferent  branchial  arteries  of  Scy Ilium,  seen  from  the 
ventral  side  with  the  left  half  of  the  lower  jaw  and  floor  of  mouth 
stretched  out  and  the  right  half  cut  off. 

A.A.,  anterior  prolongation  of  aorta;  C.C.,  common  carotid  ;  D. A.,  dorsal  aorta  ;  E.,  epibran- 
chial artery  ;  E.B.,  efferent  branchial ;  B.C.,  external  carotid  ;  G.C.,  gill  cleft ;  H.,  hyoidean 
artery:  H.M.,  hyomandibular  cartilage;  I.C.,  internal  carotid;  M.,  mandibular  artery;  S.C., 
sub-clavian ;  Sp.,  spiracle. 

dorsal  aorta  ;  the  aorta  itself  passes  forward  from  the  union  of  the 
first  pair  of  epibranchials  as  a  small  trunk  which  bifurcates  at  the 
level  of  the  first  gill  cleft,  its  two  ends  bending  outwards  to  run  into 
the  common  carotids.  Immediately  beyond  this  the  common  carotid 
divides  into  two,  a  slender  internal  or  posterior  carotid  artery  and  a 
stouter  external  carotid.*  The  former  trunk  passes  inwards  to  the 

*  The  naming  of  these  anterior  arteries,  particularly  the  carotids,  is  not 
satisfactory,  as  their  homologies  with  the  arteries  of  the  higher  animals  have 
not  yet  been  accurately  determined. 


238  AN  INTRODUCTION  TO  ZOOLOGY 

mid- ventral  line,  where  it  penetrates  the  floor  of  the  cranium  with 
its  fellow,  through  the  carotid  foramen  just  behind  the  pituitary 
fossa.  The  external  carotid  artery  runs  forward  and  out  through  a 
small  foramen  into  the  orbit,  passing  outwards  below  the  maxillary 
branch  of  the*fifth  cranial  nerve  to  supply  the  muscles  of  the  eye, 
the  upper  jaw  and  the  snout. 

The  hinder  parts  of  the  body  are  all  supplied  by  branches  coming 
directly  from  the  dorsal  aorta.  The  first  noticeable  one  of  these, 
the  sub-clavian  artery,  arises  just  in  front  of  the  fourth  epibranchial 
artery  and  runs  outwards  and  backwards  under  the  dorsal  peritoneum 
of  the  coelom  to  the  pectoral  girdle  and  fin. 

A  short  distance  behind  this  arises  a  large  median  vessel,  the 
cceliac  artery,  which  runs  out  into  the  coelom  in  the  mesogaster  just 


7\ 


FIG.  77. — Lateral  view  of  the  arteries  of  Scylhum. 

A.,  auricle ;  A.A.,  anterior  extension  of  dorsal  aorta ;  A.B.,  afferent  branchial  artery ; 
A.M.,  anterior  mesenteric  artery;  B.,  basal  region  of  cranium;  C.,  cceliac  artery;  Ca.,  caudal 
artery;  C.C.,  common  carotid  artery  ;  C.F.,  carotid  foramen:  D.A.,  dorsal  aorta" ;  E.,  external 
carotid  artery;  E.A.,  epibranchial  artery;  E.B.,  efferent  branchial  artery;  G.C.,  gill  cleft; 
H.,  hyoidean  artery;  I.,  innominate  artery;  I.C.,  internal  carotid  artery;  II.,  iliac  artery; 
L.,  lieno-gastric  artery ;  M.,  mandibular  artery  ;  P.M.,  posterior  mesentric  artery  ;  R.,  renal 
arteries  ;  S.,  spiracle ;  S.C.,  sub-clavian  artery ;  S.V.,  sinus  venosus ;  V.,  ventricle ;  V.A., 
ventral  aorta. 

dorsal  to  the  stomach  and  divides  into  two  branches  ;  the  one 
supplies  the  anterior  end  of  the  stomach  and  the  liver,  while  the  other 
serves  the  front  end  of  the  intestine  and  the  pancreas. 

Two  further  median  trunks  take  their  origin  quite  close  together 
about  an  inch  and  a  half  further  back  ;  these  are  the  anterior  mesen- 
teric and  the  lieno-gastric  arteries.  The  former  goes  backwards  to 
supply  the  posterior  part  of  the  intestine  and  the  gonads,  and  the 
latter  runs  forward,  thus  crossing  the  anterior  mesenteric,  and  is 
distributed  to  the  posterior  bend  of  the  stomach,  the  spleen  and  also 
part  of  the  pancreas. 

The  next  median  vessel  to  come  off  from  the  aorta  is  a  smaller 
one,  the  posterior  mesenteric  artery,  which  runs  through  the  meso- 
rectum  mainly  to  the  rectal  gland.  Thus  there  are  four  large 
splanchnic  arteries  running  to  the  gut  in  the  place  of  the  large 
coeliaco-mesenteric  and  small  posterior  mesenteric  arteries  present 


SCYLLIUM   CANICULA  239 

in  the  frog,  owing  to   the  great  shortening   of   the   body  region 
that  has  taken  place  in  the  forms  ancestral  to  Rana. 

Behind  this,  again,  we  find  the  small  paired  iliac  or  pelvic  arteries 
related  to  the  pelvic  girdle  and  fins.  After  this  the  dorsal  aorta, 
now  considerably  reduced  in  size,  enters  the  haemal  arches  of  the 
caudal  vertebrae  and  runs  in  them  to  the  end  of  the  tail. 

In  addition  to  these  more  obvious  vessels  the  dorsal  aorta  also 
gives  off  a  series  of  small  paired  parietal  arteries,  one  pair  to  each 
muscle  segment  of  the  body,  and  again  in  the  region  of  the  functional 
part  of  the  kidney  a  number  of  paired  renal  arteries. 

Before  leaving  the  arterial  system  it  will  be  as  well  to  glance 
briefly  at  the  development  of  its  most  interesting  part,  namely,  the 
afferent  and  efferent  arteries.  In  the  early  embryo  with  the  gill 
slits  laid  down,  but  before  the  gill  filaments  are  developed,  we  find 
coming  off  from  the  ventral  aorta  six  pairs  of  vessels  which  pass  as 
uninterrupted  arches,  the  aortic  arches,  around  the  pharynx  in  the 
corresponding  gill  bars,  the  first  one  lying  in  the  hyoid  arch.  On 
the  dorsal  side  of  the  pharynx  the  six  arches  on  each  side  run  into  a 
separate  longitudinal  trunk,  so  that  there  are  at  this  time  two  dorsal 
aortae  running  the  length  of  the  body  which  do  not  fuse  to  form  a 
single  vessel  until  later.  This  is  a  very  important  stage  from  the 
point  of  view  of  comparative  anatomy,  since  it  persists  in  the  pharyn- 
geal  region  of  the  adult  Amphioxm  and  is  also  met  with  in  a  more  or 
less  modified  condition  in  the  embryos  of  all  the  higher  animals, 
ourselves  included.  At  a  later  stage  the  hyoid  arterial  arch  degene- 
rates, and  with  the  development  of  the  gill  filaments  the  originally 
continuous  arches  become  split,  so  giving  rise  to  the  afferent  and 
efferent  branchial  arteries. 

Venous  System. 

The  most  striking  feature  in  the  venous  system  is  the 
manner  in  which  a  number  of  the  main  vessels  have  become  dilated. 
They  have  lost  their  definite  walls  and  spread  out  to  form  very  irre- 
gular spaces  termed  sinuses,  whose  relations  to  other  veins  are  some- 
times difficult  to  make  out.  For  descriptive  purposes  it  is  convenient 
to  divide  the  veins  into  the  anterior,  those  in  front  of  the  heart, 
and  the  posterior,  those  behind. 

Anterior  Veins. 

A  very  distinct  channel  runs  from  the  snout  back  to  the 
heart  on  the  dorsal  side  of  the  fish.  This  commences  as  a  well- 
marked  crescent-shaped  nasal  sinus  which  lies  on  the  postero-mesial 
side  of  the  olfactory  organ.  It  communicates  by  means  of  a  small 
orbito-nasal  vein,  which  passes  through  the  cartilaginous  hinder 


240  AN   INTRODUCTION  TO  ZOOLOGY 

wall  of  the  capsule,  with  the  large  orbital  sinus.  The  latter  is  a 
large  expansion  occupying  the  whole  of  the  cavity  of  the  orbit  not 
taken  up  by  the  eyeball  and  its  muscles,  and  it  receives  towards  its 
antero-mesial  end  a  small  anterior  cerebral  vein  coming  through 
the  cranial  wall  from  the  front  end  of  the  brain.  The  two  orbital 
sinuses  communicate  with  one  another  by  means  of  an  interorbital 
vein  which  runs  in  the  basicranial  cartilage  just  behind  the  level  of 
the  pituitary  body  and  opens  into  the  orbit  by  a  foramen  just  in 
front  of  and  below  the  foramen  for  the  main  branches  of  the  fifth 
and  seventh  and  the  sixth  cranial  nerves.  The  orbital  sinus  is 
continued  backwards  as  the  post-orbital  sinus  lying  in  the  gutter- 
like  post- orbital  groove  which,  as  we  have  seen,  lies  on  the  side  of 
the  auditory  capsule  below  the  ridge  marking  the  horizontal  semi- 
circular canal  and  the  articular  surface  for  the  hyomandibular 
cartilage.  In  its  course  along  the  post-orbital  groove  the  sinus 
receives  the  posterior  cerebral  vein,  an  important  vessel  leaving  the 
cranial  cavity  in  company  with  the  tenth  cranial  nerve.  Behind 
the  skull  the  venous  trunk  enlarges  to  form  the  anterior  cardinal 
sinus,  a  large  irregular  sinus  situated  along  the  internal  dorsal  ends 
of  the  gill  clefts.  The  pharyngeo-branchial  cartilages  project  into 
its  floor,  and  the  branchial  branches  of  the  vagus  nerve  pass  freely 
through  its  cavity.  At  its  posterior  end  behind  the  last  gill  cleft 
this  sinus  narrows  considerably,  and  opens  into  the  posterior  cardinal 
sinus  through  an  opening  guarded  by  a  valve. 

A  small  vessel,  the  inferior  jugular  sinus,  commences  just  behind 
the  symphysis  of  the  lower  jaws  and  runs  backwards  to  the  level 
of  the  thyroid  gland,  where  it  communicates  with  its  fellow  by  an 
irregular  anastomosis  that  surrounds  the  gland.  From  this  point 
it  enlarges  and  becomes  irregular,  passing  along  the  inner  ventral 
ends  of  the  gill  clefts  and  then  along  the  pericardium,  finally  opening 
into  the  ductus  Cuvieri  by  a  common  opening  with  the  sub-clavian 
vein  just  outside  the  limits  of  the  pericardial  space. 

At  the  level  of  the  thyroid  gland  a  well-marked  hyoidean  sinus 
leaves  the  side  of  the  inferior  jugular  and  passes  out  laterally  in  a 
shallow  groove  in  the  external  side  of  the  hyomandibular  cartilage. 
It  runs  round  parallel  with  the  first  gill  cleft  to  open  into  the  hinder 
end  of  the  post- orbital  sinus,  thus  putting  the  dorsal  and  ventral 
vessels  in  communication. 

Posterior  Veins. 

The  blood  is  brought  back  from  the  tail  by  the  caudal  vein, 
which  runs  in  the  haemal  canal  accompanying  and  ventral  to  the 
caudal  artery.  It  leaves  the  canal  and  divides  into  two  renal 
portal  veins  that  pass  up  the  dorso-lateral  edges  of  the  kidneys,  to 


SCYLLIUM   CANICULA 


241 


oc 


A.C. 


c. 


FIG.  78. — Venous  system  of  Scyllium  canicula. — 'After  O'Donoghue. 
Diagram  showing  the  general  disposition  of  the  main  venous  trunks  in  Scyllium  canicula. 
The  more  dorsally  situated  vessels  are  stippled  and  the  more  ventral  ones  black.  For  the  sake 
of  clearness,  the  ventral  cutaneous  vein  has  been  omitted.  A.C.,  anterior  cerebral  vein  ;  A.Ca., 
anterior  cardinal  sinus;  Au.,  auricle;  B.,  brachial  vein;  Ba.,  basipterygium  ;  C.,  caudal  vein; 
Cl.,  cloacal  vein  ;  D.C.,  ductus  Cuvieri  ;  F.,  femoral  vein  ;  GSz.,  2nd  gill-cleft  ;  H.,  hyoidean 
sinus;  U.S.,  hepatic  sinus  ;  /./.,  inferior  jugular  sinus  ;  //.,  iliac  vein  ;  I.M.,  intestino-mesenteric 
vein  ;  I.O.,  inter-orbital  vein  ;  K.,  caudal  mesonephros  ;  L.A.,  lateral  abdominal  vein  ;  L.C., 
lateral  cutaneous  vein;  N.S.,  nasal  sinus  ;  O.C.,  olfactory  capsule  ;  O.N.,  orbito-nasal  vein;  Or., 
orbital  sinus ;  P.  An.,  pelvic  anastomosis  between  the  two  L.A. ;  P.C.,  posterior  cerebral  vein; 
P.Ca.,  posterior  cardinal  sinus  ;  Pe.,  pectoral  fin  ;  PL,  pelvic  fin  ;  Pl.C.,  pelvic  cartilage  ;  P.O., 
post-orbital  sinus  ;  Re.,  rectal  vein  ;  R.P.,  renal  portal  vein  ;  R.S.,  right  spermatic  vein  ;  S.C., 
sub-clavian  vein ;  S.S.,  sub-scapular  vein;  S.V.,  sinus  venosus  ;  T.G.,  thyroid  gland  ;  V .,  ventricle. 

K 


242  AN   INTRODUCTION  TO  ZOOLOGY 

which  they  give  off  numerous  afferent  renal  branches  and  getting 
smaller  and  smaller  disappear  at  the  anterior  end  of  these  bodies, 
into  which  all  their  blood  is  poured.  Between  the  kidneys  the  posterior 
cardinal  sinuses  take  origin  as  a  median  unpaired  trunk  formed  by 
the  running  together  of  a  number  of  efferent  renal  veins.  The 
median  trunk,  however,  is  completely  separated  into  two  by  a 
membranous  septum  as  it  passes  forward  from  the  front  end  of 
those  glands  under  the  peritoneum  on  the  dorsal  side  of  the  coelom. 
About  half-way  forward  to  the  pericardio-peritoneal  septum  their 
partition  disappears,  and  thereafter  is  only  represented  by  a  few 
bands  of  tissue.  At  the  same  point  they  receive  a  median  genital 
sinus  coming  from  the  gonads,  and  in  its  turn  receiving  a  vein  from 
the  right  side  of  the  intestine.  The  post-cardinal  sinuses  as  they 
pass  forward  broaden  out  considerably  until  a  little  further  on  they 
occupy  the  whole  of  the  dorsal  width  of  the  ccelom,  and  they  reach  as 
far  as  the  pericardio-peritoneal  septum.  The  blood  from  the  muscles 
and  skin  on  the  dorso-lateral  side  of  the  fish  is  collected  up  by  a 
lateral  cutaneous  vein  which  runs  forward  from  the  end  of  the  tail 
under  the  lateral  line  in  the  septum  between  the,  epi-  and  hypaxial 
myotomes.  At  the  level  of  the  hinder  end  of  the  pectoral  fin  it 
dives  down  into  the  muscles  and  swells  out  to  form  a  sub-scapular 
sinus  lying  on  the  internal  lateral  side  of  the  scapular  end  of  the 
pectoral  girdle.  This  sinus  opens  into  the  antero-lateral  edge  of  the 
post- cardinal  sinus  by  an  opening  that  is  somewhat  difficult  to  make 
out. 

The  blood  from  the  pelvic  fin  and  cloacal  region  is  collected  by  a 
small  iliac  vein  on  each  side  and  conveyed  to  the  dorsal  side  of  the 
pelvic  cartilage,  where  it  anastomoses  with  its  fellow  across  the 
cartilage  and  continues  forward  as  a  distinct  lateral  abdominal  vein 
in  the  ventro-lateral  wall  of  the  coelom  just  beneath  the  peritoneum. 
It  reaches  the  pericardio-peritoneal  septum  and  turns  sharply  in 
it,  dorsally  and  mesially  along  the  posterior  edge  of  the  coracoid 
cartilage.  A  short  distance  along  this  it  is  joined  by  the  large 
brachial  sinus  which  comes  in  from  the  posterior  border  of  the 
pectoral  fin,  and  the  trunk  formed  by  the  union  of  these  two  vessels 
is  termed  the  sub-clavian  vein.  This  vessel  is  a  short  trunk  entering 
the  ductus  Cuvieri,  as  we  have  noted  previously,  by  an  opening 
common  to  it  and  the  inferior  jugular  sinus. 

Thus  we  have  now  accounted  for  the  collection  of  the  blood  from 
all  regions  of  the  body  save  the  alimentary  canal,  and  this  is  dealt 
with,  as  in  the  frog,  by  a  special  hepatic  portal  system  composed  of 
a  number  of  factors.  It  starts  as  a  small  vessel,  the  rectal  vein,  on 
the  ventral  side  of  the  rectal  gland,  and  this  continues  as  the  posterior 
intestinal  vein  up  the  dorso-lateral  wall  of  the  intestine  to  the  level 


SCYLLIUM   CANICULA  243 

of  the  hinder  end  of  the  pancreas.  It  then  leaves  the  gut  wall  and, 
passing  through  the  mesentery,  enters  the  posterior  corner  of  the 
gland,  uniting  as  it  does  so  with  the  posterior  lieno-gastric  vein,  a 
trunk  coming  from  the  bend  of  the  stomach  and  the  spleen.  In 
this  way  is  constituted  the  main  hepatic  portal  vein,  which  then  runs 
up  embedded  in  the  dorsal  edge  of  the  pancreas  to  its  anterior  end, 
where  it  receives  two  large  tributaries.  The  first  of  these  is  the 
gastro-intestinal  vein  formed  by  factors  coming  from  the  pyloric  end 
of  the  stomach  and  the  spleen  adjacent  thereto  and  the  front  end 
of  the  intestine  and  a  fairly  large  ultra-intestinal  vein  which  lies  right 
in  the  central  axis  of  the  spiral  valve,  and  so  is  not  readily  seen  save 
in  a  section.  The  second  vessel  is  the  dorsal  anterior  gastric  vein 
formed  in  the  main  by  the  union  of  a  branch  from  the  dorsal  side 
of  the  front  end  of  the  stomach  and  one  from  the  oesophagus.  The 
main  hepatic  portal  vein  now  leaves  the  pancreas  and  runs  through 
the  gastro-hepatic  omentum,  where  it  is  joined  by  a  ventral  anterior 
gastric  vein  of  similar  constitution  to  the  dorsal  vessel,  but  coming 
from  the  ventral  side  of  the  oesophagus.  Finally  it  divides  into  right 
and  left  branches  feeding  the  corresponding  lobes  of  the  liver. 

The  blood  from  the  liver  is  taken  up  by  two  short  efferent  hepatic 
veins,  one  coming  from  the  anterior  end  of  each  lobe  of  the  liver. 
They  almost  immediately  unite  to  form  a  large  single  hepatic  sinus, 
which  communicates  with  the  ventro-posterior  wall  of  the  sinus 
venosus  through  the  pericardio-peritoneal  septum  by  two  separate 
openings  indicative  of  its  double  origin. 

The  ductus  Cuvieri  in  the  adult  appears  as  a  tube  projecting  a 
short  distance  into  the  post-cardinal  sinus,  wherein  it  opens  by  a 
spout-like  aperture.  It  passes  inwards  on  the  ventro-lateral  wall  of 
the  oesophagus  through  a  conspicuous  notch  in  the  posterior  border 
of  the  fifth  cerato-branchial  cartilage  and  on  into  the  pericardial 
cavity,  where  it  is  continuous  with  the  sinus  venosus.  Thus  it  will 
be  seen  it  is  the  vessel  by  which  the  whole  of  the  blood  is  returned 
to  the  heart,  save  only  that  brought  from  the  liver  by  the  hepatic 
sinus. 

In  the  embryo  the  veins  have  not  yet  swollen  up  to  form 
sinuses,  so  that  their  anatomical  relationships  are  somewhat  clearer. 
The  anterior  and  posterior  cardinal  veins  on  each  side  join  together 
at  the  level  of  the  sinus  venosus  to  form  the  ductus  Cuvieri,  and  so 
both  open  into  it.  This,  the  primitive  arrangement,  is  afterwards 
masked  by  the  enormous  enlargement  of  the  two  trunks  so  that,  as 
noticed  above,  the  anterior  sinus  opens  into  the  posterior  one  and 
the  ductus  Cuvieri  appears  as  a  tube  projecting  into  the  latter.  At 
a  very  early  stage  the  anterior  vein  is  one  continuous  vessel  only, 
but  at  a  somewhat  later  stage  the  part  of  it  in  front  of  the  point 


244 


AN   INTRODUCTION   TO   ZOOLOGY 


VC.L 


of  entrance  of  the  posterior  cerebral  vein  is  replaced  by  another 
vein,  the  lateral  cephalic  vein,  which  arises  parallel  with  the  front 
end  of  the  old  anterior  cardinal,  so  that  in  the  adult  the  vein  is 

the  result   of  the  union    of    two 
vessels. 

At  the  beginning  of  their  de- 
velopment the  posterior  cardinal 
veins  are  related  to  the  first 
embryonic  kidney  and  continuous 
with  the  caudal  vein,  but  later, 
as  in  the  frog,  the  posterior  ends 
are  split  up  by  the  growth  of 
the  mesonephroi.  The  hepatic 
portal  system  arises  in  a  some- 
what similar  manner  by  the 
splitting  of  a  primitive  sub- 
intestinal  vein. 

The  inferior  jugular  vein  when 
developed  opens  into  the  middle 
of  the  front  side  of  the  ductus 
Cuvieri  and  the  sub-clavian  vein 
riglrlf opposite  it. 

In  comparing  the  vascular 
system  of  Scyllium  with  that  of 
Rana  several  fundamental  dif- 
lerences  are  at  once  apparent. 
The  heart  in  the  fish  has  but  a 
single  atrium  or  auricle,  whereas 
in  the  frog  it  is  divided  into  two 
by  an  interauricular  septum.  Cor- 
FIG.  79. — Diagram  of  the  vessels  in  related  with  the  development  of 

a  fairly  late  embryo ,  of Scyllium     the  lungs  we  find  that  the       imi_ 

camcula.  — •  After  O  Donoghue 

from  Rabl.  tlve  arrangement  of  the  arteries, 

A.Ca.,  anterior  cardinal  sinus  ;  C.,  caudal      Although    represented    in    the    tad- 

yuiuikrSifus-hPPctic  osterfor  cerebraanS?n  ^    P°^e>  nas  undergone   considerable 

P.Ca.,  posterior  cardinal  sinus  ;  R.P.,  renai  modification.       The     blood  in  the 

portal  vein ;    S.C.,  sub-clavian   vein ;    S.I.,        ,       .-.   ,  .  ,  . 

sub-intestinal    vein  ;    S.V.,   sinus   venosus  ;  dogfish      Can     Only     travel  in  OUC 

S.S. ,  sub-scapular  vein ;  V.C.L.,  vena  capitis  j-         ..•  i          r-  ,1 

lateralis ;  V.I.,  inter-renal  vein.  direction,      namely,     first  to  the 

gills    and  thence    to    the    body, 

so  that  we  term  this  type  of  circulation,  characteristic  of  fish  in 
general,  a  branchial  circulation,  or,  since  there  is  only  one  stream 
leaving  the  heart,  a  single  circulation.  In  the  frog  the  blood  may 
take  one  of  two  courses,  it  may  go  to  the  lungs  and  skin  for  aeration 
via  the  pulmo-cutaneous  artery,  or  it  may  go  to  any  other  part  of 


V.I. 


Rp 


c. 


SCYLLIUM   CANICULA  245 

the  animal  via  the  carotid  or  systemic  arteries  and  back  to  the  heart 
again.  In  spite  of  this  double  choice,  however,  the  two  blood 
streams  returning  to  the  heart,  the  aerated  and  non-aerated  blood, 
are  not  kept  absolutely  separated  in  the  ventricle,  and  thus  we  have 
an  incomplete  double  circulation. 

Urogenital  System. 

The  excretory  and  reproductive  organs  are  conveniently 
dealt  with  together,  since  they  are  closely  related  anatomically  ; 
indeed,  the  ducts  of  the  reproductive  organs  are  mainly  derived 
from  the  primitive  excretory  ducts. 

We  may  first  consider  the  kidneys,  since  they  are  more 
nearly  alike  in  the  two  sexes,  but  in  order  to  understand  the  condition 
in  the  adult  it  is  necessary  to  glance  briefly  at  their  development. 

The  kidneys  arise,  as  in  all  Craniates,  as  tubular  organs  closely 
related  to  the  myOtomes  or  muscle  segments.  The  first  of  these  to 
develop  are  found  in  the  early  embryo  towards  the  anterior  end  of 
the  body  cavity  in  the  form  of  three  or  four  tubules  on  each  side  in 
close  proximity  to  the  post-cardinal  vein,  and  these  constitute  an 
imperfect  excretory  organ  known  as  the  pronephros.  On  the  internal 
side  each  tubule  possesses  a  ciliated  funnel-shaped  opening,  the 
nephrostome,  opening  into  the  ccelom.  This  leads  into  a  tube,  only 
slightly  coiled,  lying  in  the  thickness  of  the  ccelomic  wall,  and  a  glome- 
rulus  such  as  we  find  in  the  kidney  of  Rana  is  either  absent  or  only 
imperfectly  represented.  At  their  outer  end  these  tubules  run  into 
a  long  duct,  the  pronephric  or  segmental  duct,  which  passes  in  the 
body  wall  back  to  the  cloaca.  In  Scyllium  the  nephrostomes  fuse 
to  form  one  opening.  The  pronephros  is  apparently  never  functional, 
and  is  probably  to  be  regarded  as  the  remnants  of  a  body  functional 
in  ancestral  forms.  The  tubules  disappear  in  the  adult,  but  the  duct 
in  some  form  or  other  is  always  to  be  found. 

A  short  time  after  the  pronephros  makes  its  appearance,  a  second, 
much  longer  series  of  tubules,  about  twenty-nine  in  number,  arises 
behind  it.  These  tubules  also  have  nephrostomes,  but  become  more 
convoluted,  and  although  at  first  they  end  blindly  in  the  body  wall, 
they  soon  acquire  openings  into  the  segmental  duct.  They  constitute 
the  second  excretory  organ,  the  mesonephros  or  Wolffian  body,  which 
soon  acquires  a  duct  for  itself  by  a  tube  being  split  off  from  the 
pronephric  duct.  The  tubules  themselves  increase  in  length,  become 
highly  convoluted  and  develop  typical  Malpighian  bodies,  so  that 
they  form  active  excretory  bodies.  These  tubules  are  often  described 
as  the  primary  tubules  in  order  to  distinguish  them  from  the  secon- 
dary tubules,  of  which  two  or  three  arise,  by  budding  from  each 
primary  tubule.  The  secondary  tubules  never  develop  nephrostomes, 


246 


AN   INTRODUCTION  TO  ZOOLOGY 


and  their  appearance  causes  the  mesonephros  to  increase  markedly 
in  size  and  they  obscure  its  primitive  segment al  arrangement. 

In  the  dogfish  the  secondary  tubules  are  more  numerous  in  the 
posterior  half  of  the  body,  and  both  they  and  the  primary  tubules 
become  much  longer  and  more  coiled,  and  in  structure  they  are 
essentially  like  the  urinary  tubules  in  Rana.  Then,  too,  they  lose 
their  direct  connection  with  the  mesonephric  duct,  and  the  collecting 
parts  of  certain  tubules  enlarge  and  form  ducts  that  constitute 
secondary  ureters.  These  open  into  the  posterior  part  of  the  meso- 
nephric duct  in  the  female,  or  into  one  larger  tube  which  only  joins 


FIG.  80. — Diagram  of  development  of  kidneys  of  a  female  Scyllium. 

C.,  cloaca;  Co. M.,  caudal  mesonephros  ;  C.M.,  cranial  mesonephros  ;  M.D.,  mesonephric  duct  ; 
N.,  nephrostome  ;  P.,  pronephros  ;  P,D.,  pronephric  duct  ;  S.U.,  secondary  ureter. 

the  extreme  end  of  the  mesonephric  duct  in  the  male.  Thus  it  is 
that  the  mesonephros  in  Scyllium  becomes  divided  into  two  distinct 
parts,  a  head  kidney  or  cranial  mesophros  and  a  much  more  sub- 
stantial tail  kidney  or  caudal  mesonephros,  often,  but  quite  erro- 
neously, called  the  metanephros,  an  entirely  separate  body  developed 
only  in  the  higher  Craniata,  the  Reptiles,  the  Birds  and  the  Mammals. 
The  kidneys  of  the  adult,  as  we  have  seen,  represent  the 
persistent  functional  mesonephros  of  the  embryo,  and  are  divided 
into  anterior  and  posterior  moieties.  They  lie  between  the  dorsal 
peritoneum  and  the  muscles  of  the  back,  and  so  are  outside  the 
ccelom  as  in  Rana.  They  lie  partly  embedded  in  hollows  in  the 


SCYLLIUM   CANICULA 


247 


muscles,  and  as  the  peritoneum  covering  them  is  quite  thick  they  are 
not  seen  until  it  has  been  removed.     The  kidneys  in  the  two  sexes 
are  fairly  similar,  so  that  they  need  not  be  dealt  with  separately. 
The  front  end  of  the  kidney  in  the  female  is  very  small  and, 


B. 


FIG.  81. 


A,,  urogenlial  system  of  the  female;  B.,  urogenital  system  of  the  male  dogfish;  ab.p., 
abdominal  pores  ;  cl.,  cloaca  ;  cp.,  claspers  of  the  male  ;  /.,  rudiment  of  the  oviducal  opening 
in  the  male  ;  M.d.,  metanephric  ducts  ;  mtn.,  metanephros  ;  od.,  oviduct;  ae.,  cut  end  of  oesopha- 
gus ;  o.g.,  oviducal  gland ;  ov.,  ovary;  P./.,  pelvic  fins ;  R.,  rectum;  s.s.,  sperm  sacs  ;  T.,  testis  ; 
u.p.,  urinary  papilla  in  the  female ;  ug.p.,  urogenital  papilla  in  the  male  ;  M.S.,  urinary  sinus  ; 
v.e.,  vasa  efferentia;  v.s.,  vesicula  seminalis;  W.D.,  Wolffian  duct;  W.G.,  Wolffian  gland  or 
mesonephros. 

although  perhaps  retaining  its  excretory  function  to  a  certain 
extent,  is  so  reduced  that  it  may  be  considered  almost  vestigial.  It 
presents  the  appearance  of  a  series  of  small  isolated  patches  of  a 
brownish  colour  which  increase  slightly  in  size  towards  the  hinder 
end.  The  posterior  portion  of  the  kidney  is  much  larger,  being 
wider  and  considerably  thicker,  and  it  forms  one  discrete  body 
partially  divided  up  into  lobes. 


248  AN   INTRODUCTION  TO  ZOOLOGY 

While  the  caudal  part  of  the  mesonephros  in  the  male  is  similar 
to  that  in  the  female,  the  anterior  end  is  very  different.  It  is  larger 
than  in  the  female,  but  almost  completely  hidden  from  view  by  the 
highly  convoluted  mesonephric  duct  that  is  closely  applied  to  its 
ventral  surface.  In  the  embryo  it  is  concerned  with  excretion,  but 
in  the  adult  this  activity  is  lost  and  it  is  functional  in  secreting  a 
nutritive  fluid  for  the  sperms,  consequently  it  is  sometimes  termed  the 
epididymis. 

The  mesonephric  duct  in  the  female  is  a  straight  fairly  simple 
tube,  commencing  at  the  front  end  as  a  thin  vessel  which  runs  back- 
wards on  the  ventral  face  of  the  kidney.  In  the  hinder  part  it  en- 
larges to  form  a  urinary  sinus  into  which  open  the  secondary  ureters, 
about  six  in  number,  and  this  unites  with  its  fellow  to  constitute 
a  small  median  urinary  sinus  opening  upon  the  urinary  papilla.  The 
duct  is  concerned  with  the  conveyance  of  urine  to  the  exterior,  so 
that  it  constitutes  a  ureter. 

In  the  male  the  anterior  portion  of  the  mesonephric  duct  is  much 
enlarged  and  bent  upon  itself.  Into  it  opens  the  tubules,  enlarged, 
devoid  of  Malpighian  bodies  and  with  their  epithelium  considerably 
modified.  Near  the  line  of  demarcation  between  the  cranial  and 
caudal  mesonephros  the  bends  become  fewer,  and  it  passes  on  swelling 
out  to  form  a  seminal  vesicle  or  vesicula  seminalis.  After  receiving 
the  urinary  vessels  at  its  lower  end  it  unites  with  that  of  the  opposite 
side  to  form  a  small  median  urogenital  sinus  whose  opening  is  situated 
upon  the  urogenital  papilla.  The  sinus  on  each  side  is  produced 
into  a  forwardly  directed  long  sac-like  diverticulum,  the  sperm  sac, 
which  lies  closely  attached  to  the  ventral  surface  of  the  seminal 
vesicle.  The  first  four  or  five  of  the  secondary  ureters  do  not  open 
into  the  mesonephric  duct  directly,  as  in  the  female,  but  into  an  acces- 
sory tube  often  termed  "  the  ureter  "  which  joins  the  seminal  vesicle 
near  its  hinder  end.  Behind  this,  again,  five  or  six  secondary  ureters 
draining  the  hinder  extremity  of  the  kidney  open  separately.  These 
secondary  ureters,  the  sperm  sacs  and  the  seminal  vesicle,  cover  a 
large  part  of  the  ventral  surface  of  the  kidney.  Thus  it  will  be 
seen  that  in  the  male  the  mesonephric  duct  serves  for  the  greater 
part  of  its  length  for  the  conveyance  of  the  spermatic  fluid,  and  so 
constitutes  a  vas  deferens,  and  only  the  last  part  of  it  is  concerned 
with  the  transference  of  urine. 

In  the  female  embryo  an  ovary  starts  to  develop  on  each 
side,  but  that  on  the  left  soon  disappears,  so  that  in  the  adult  we 
find  only  one  ovary  present,  that  of  the  right  side.  It  is  a  con- 
spicuous structure  lying  slightly  to  the  right  of  the  middle  line  along 
the  median  half  of  the  coelom,  to  the  dorsal  wall  of  which  it  is  attached 
by  a  fold  of  peritoneum,  the  mesovarium.  Its  surface  is  marked 


SCYLLIUM   CANICULA  249 

by  numerous  rounded  projections,  the  ova,  which  are  of  different 
sizes  from  quite  tiny  knobs  like  pin-heads  up  to  yellow  spheres 
14  mm.  in  diameter,  according  to  their  state  of  development.  The 
pronephric  tubules  disappear,  as  has  been  pointed  out,  but  the 
funnels  of  the  two  sides  remain  and,  moving  ventrally,  unite  below 
the  oesophagus  to  form  a  single  opening,  the  oviducal  funnel.  This 
communicates  by  means  of  one  of  the  tubules  with  the  segmental 
duct.  Not  merely  does  the  pronephric  duct  persist,  but  it  enlarges 
considerably  and  constitutes  the  oviduct  of  the  adult.  The  oviduct 
is  a  thick-walled  tube  commencing  at  the  funnel  and  passing 
laterally  on  to  the  latero-dorsal  wall  of  the  coelom.  A  short  way 
along  it  swells  out  to  form  a  marked  ovoidal  enlargement,  the 
oviducal  gland,  and  then  becomes  constricted  again.  The  walls  of 
the  posterior  half  of  the  duct  are  thin,  and  the  cavity  enlarged  to  form 
a  hollow  vesicle,  the  ovisac,  in  which  the  egg  can  be  kept  until  laid. 
The  two  oviducts  open  to  the  exterior  by  a  single  median  aperture 
on  the  dorsal  wall  of  the  cloaca,  which  in  the  young  female  is  closed 
by  a  thin  membrane,  the  hymen. 

When  ripe,  the  follicle  ruptures  and  discharges  the  egg,  now  a 
spherical  mass  about  14  mm.  in  diameter  and  loaded  with  yolk,  into 
the  ccelom.  It  makes  its  way  into  the  oviducal  opening  and  passes 
into  one  oviduct — apparently  the  two  oviducts  function  alternately. 
In  the  top  end  of  the  tube  it  is  penetrated  and  fertilised  by  a  sperm 
that  has  been  introduced  previously  by  the  male  fish,  and  it  is  also 
provided  with  an  albuminous  coat.  As  it  passes  the  oviducal  gland 
and  beyond  it  is  provided  with  a  horny  shell,  the  product  of  the 
secretion  of  the  gland.  The  shell  is  an  oblong  purse-shaped  structure 
with  its  long  sides  continued  out  as  four  horns  tailing  off  as  long 
coiled  threads,  by  means  of  which  it  is  anchored  to  the  seaweed  when 
laid.  While  in  the  oviduct  the  shell  is  of  a  pale  yellow  colour,  but 
after  the  hatching  of  the  young  and  subsequent  exposure  it  turns 
black.  Such  empty  cases  belonging  to  Scyllium,  or  some  other 
Elasmobranch,  are  commonly  cast  up  on  the  seashore  and  are 
known  as  "  mermaids'  purses." 

The  segmental  duct  in  the  male  is  functionless,  but  always  remains 
as  a  vestigial  structure  in  the  form  of  a  tiny,  almost  invisible,  solid 
strand  of  tissue  in  the  position  of  the  oviduct  in  the  female.  Traces 
of  it  may  sometimes  be  seen  in  certain  specimens. 

The  testes  are  two  elongated,  soft  bodies  lying  one  on  each  side 
of  the  middle  line,  and  suspended  from  the  dorsal  wall  of  the  coelom 
by  special  folds  of  the  peritoneum  known  as  the  mesorchia.  Each 
testis  is  attached  to  the  front  end  of  the  corresponding  mesonephros 
by  a  number  of  fine  tubules,  the  vasa  efferentia,  by  whose  agency 
the  sperms  produced  in  the  seminiferous  tubules  of  the  testis  are 


250 


AN   INTRODUCTION  TO  ZOOLOGY 


conveyed  to  the  modified  urinary  tubules,  and  thence  to  the  meso- 
nepheric  duct.  As  has  been  noted  above,  this  duct,  functioning  as 
a  vas  deferens,  is  a  thick-walled,  greatly- coiled  tube  lying  on  the 
ventral  side  of  the  mesonephros.  The  cranial  mesonephric  tubules 
secrete  a  nutritive  fluid  in  which  the  sperms  live.  The  semen,  as 
this  fluid  with  its  contained  sperms  is  termed,  is  probably  stored  in 
the  vesicula  seminalis  until  required  for  use. 

For  convenience  in  reference  we  may  tabulate  the  relation 
between  the  embryonic  and  adult  excretory  ducts  in  the  following 
manner : — 

EMBRYONIC  EXCRETORY  STRUCTURES  AND  THEIR  FA.TE  IN  THE  ADULT. 


Embryo. 

Adult  female. 

Adult  male. 

Pronephros. 

Disappears  save  for  ovi- 
ducal  opening. 

Disappears. 

Pronephric  duct. 

Oviduct. 

Vestigial. 

Cranial  mesonephros. 

Of  small  functional  signifi- 
cance. 

Transformed  into  secretory 
tubules  the  "  Epididy- 
mis." 

Caudal  mesonephros. 

Functional  kidney  ;  parts 
of  the  mesonephric  tu- 
bules are  converted  into 
secondary  ureters. 

Functional  kidney  ;  parts 
of  the  mesonephric 
tubules  are  converted 
into  secondary  ureters, 
one  of  which  is  very 
large. 

Mesonephric  duct. 

Anterior  end  small,  pos- 
terior end  enlarged  to 
form  urinary  sinus, 
whole  functions  as 
ureter. 

Anterior  end  large,  forms 
the  Vas  deferens  ;  pos- 
terior end  enlarges  to 
form  Vesicula  seminalis, 
and  extreme  end  func- 
tions as  both  Vas  defer- 
ens and  ureter. 

The  histological  structure  of  the  kidneys  in  Scyllium  is 
essentially  similar  to  that  in  the  kidneys  of  Rana.  They  are  com- 
posed of  a  mass  of  urinary  tubules,  and  differentiated  into  glandular 
and  collecting  parts,  and  possessing  Malpighian  bodies.  The  primary 
uriniferous  tubules  also  open  into  the  dorsal  coelomic  space  by  means 
of  nephrostomes.  The  tubules  are  surrounded  by  a  small  amount  of 
connective  tissue  in  which  run  enlarged  blood-vessels,  the  sinusoids 
fed  by  the  renal  portal  vein  and  the  renal  arteries. 


CHAPTER  X 
SCYLLIUM   CANICULA— (continued) 

Nervous  System  and  Sense  Organs. 

Nervous  System  and  Sense  Organs. 

Again,  as  in  the  frog,  we  may  divide  the  nervous  system 
and  its  associated  sense  organs  for  convenience  in  description  into 
Central  nervous  system,  Peripheral  nervous  system  and  Sense  organs. 
The  central  nervous  system,  consisting  of  the  brain  within  the 
cartilaginous  cranium  and  the  spinal  cord  enclosed  in  the' neural 
canal,  is  enclosed  in  two  membranous  meninges,  an  outer  dura  mater 
and  an  inner  pia  mater  as  in  the  frog. 

Brain. 

The  brain  does  not  occupy  quite  the  whole  of  the  space 
within  the  cranium,  and  the  interspaces  are  filled  up  with  a  viscid 
fluid  which  becomes  semi-gelatinous  in  preserved  specimens. 
Despite  the  difference  in  external  appearance,  owing  to  the  different 
size  and  proportion  of  the  parts,  the  brain  in  Scyllium  is  composed 
of  essentially  the  same  parts  as  in  Rana. 

At  the  front  end  we  find  the  telecephalon  or  cerebrum,  which 
appears  as  a  smooth  globular  mass  at  the  extreme  end  of  which  is  a 
groove.  Although  it  does  not  appear  so  from  the  outside  it  is, 
nevertheless,  a  paired  structure,  and  within  it  there  are  two  distinct 
lateral  ventricles  separated  from  each  other  by  a  median  partition 
of  nervous  tissue.  The  two  olfactory  lobes,  stout  oval  masses,  are 
borne,  one  on  each  side  of  the  anterior  aspect  of  the  telencephalon, 
on  stout  stalks,  the  olfactory  peduncles.  They  are  closely  adherent 
to  the  cartilaginous  hinder  portions  of  the  olfactory  capsules  through 
which  numerous  fibres,  collectively  constituting  the  olfactory  nerve, 
pass.  The  latero- ventral  walls  of  the  telencephalon  are  thickened 
by  masses  of  nervous  tissue  corresponding  with  the  corpora  striata 
of  higher  forms,  although  they  are  not  so  well  defined. 

The  cerebrum  passes  over  insensibly  into  the  succeeding  narrower 
part  of  the  brain,  namely,  the  thalamencephalon,  whose  thickened 
ventro-lateral  walls  constitute  the  optic  thalami.  This  part  is 
markedly  hollow,  owing  to  the  presence  in  it  of  the  enlarged  third 

251 


252 


AN   INTRODUCTION  TO  ZOOLOGY 


ventricle,  and  it  is  roofed  by  a  thin  membrane.  This  is  composed 
of  a  part  of  the  ependyma,  or  epithelium  lining  the  ventricles,  and 
the  pia  mater.  On  each  side  at  the  front  end  this  dips  down  into 
the  third  ventricle  and  passes  forward  into  the  lateral  ventricle, 
where  it  becomes  highly  vascularised  and  constitutes  the  choroid 
plexus  of  the  lateral  ventricle.  A  short  distance  behind  this  it 
again  dips  inwards  in  the  form  of  a  noticeable  transverse  fold,  the 
velum  transversum,  which  hangs  down  into  the  cavity  and  marks 
the  actual  dorsal  line  of  demarcation  between  telencephalon  and 


o.c     O.L 


CR 


o.c 


C.R. 


I 


FIG.  82. — -Brain  of  Scyllium.     A,  dorsal  view  ;  B,  ventral  view. 

A.C.,  anterior  choroid  plexus;  A. P.,  anterior  lobe  of  pituitary  body;  C.,  cerebrum;  Cb., 
cerebellum;  C.R.,  restiform  bodies;  L.I.,  lobus  inferior;  M..  medulla  oblongata ;  O.C.,  olfactory 
capsule;  O.F.,  olfactory  peduncle  ;  O.L.,  olfactory  lobe ;  Op.,  optic  lobe;  O.X.,  optic  chiasma ; 
P.,  pineal  stalk;  P.C.,  posterior  choroid  plexus';  P.P.,  posterior  lobe  of  pituitary  body;  S.V., 
saccus  vasculosus;  T.,  lhalamencephalon ;  II.-X.,  cranial  nerves. 

thaiamencephalon.  At  its  posterior  end  the  roof  again  becomes 
nervous  and  bears  two  ganglionic  masses,  the  habenular  ganglia, 
almost  meeting  in  the  middle  line,  but  joined  by  a  band  of  transverse 
fibres,  the  superior  commissure.  Just  behind  this  in  the  middle  line 
arises  a  small  strand  which  runs  forward  as  the  pineal  stalk,  and 
terminates  in  swollen  enlargement,  the  pineal  body  or  epipnysis 
cerebri,  attached  to  the  membrane  covering  the  anterior  cranial 
fontanelle.  This  is  a  vestigial  structure,  being  the  remnant  of  a 
pair  of  eye-like  organs  present  in  ancestral  forms.  Immediately 


SCYLLIUM   CANICULA  253 

behind  the  point  of  origin  of  the  pineal  stalk  is  a  second  transverse 
band  of  fibres,  the  posterior  commissure,  marking  the  point  of 
juncture  of  thalamencephalon  and  mesencephalon. 

The  anterior  end  of  the  third  ventricle  is  limited  by  the  lamina 
terminalis,  a  strip  of  nervous  tissue  on  each  side  of  which  lies  a  well- 
marked  opening,  the  foramen  of  Munro,  through  which  the  third 
ventricle  is  continuous  with  the  lateral  ventricle.  The  lamina 
terminalis  runs  backward,  also  forming  the  floor  of  the  anterior  part 
of  the  ventricle.  It  terminates  at  a  thick  transverse  thickening, 
the  optic  chiasma,  which  sticks  up  into  the  ventricular  cavity  pro- 
ducing a  pre-optic  recess  in  front  of  it,  and  this  marks  the  ventral 
boundary  between  telencephalon  and  thalamencephalon.  The 
floor  of  the  latter  commences  with  the  chiasma,  in  which  the  fibres 
of  the  optic  nerve  cross  one  another,  those  from  the  right  eye  crossing 
over  to  the  left  side  of  the  brain  and  vice  versa.  From  the  sides  of 
the  chiasma  the  two  stout  optic  nerves  pass  sharply  outwards. 
Just  behind  this  point  the  floor  of  the  thalamencephalon  projects 
downwards  to  form  a  backwardly  running  lobe,  the  infundibulum, 
containing  a  prolongation  of  the  third  ventricle.  The  anterior  part 
of  the  infundibulum  swells  out  laterally  to  form  two  sac-like  diver- 
ticula,  the  lobi  inferiores,  whose  cavities  communicate  with  that  of 
the  infundibulum  by  oval  apertures,  while  the  postero-dorsal  portion 
enlarges  to  form  a  wide  extremely  vascular  bag,  the  saccus  vasculosus. 
Closely  adherent  to  the  ventral  wall  of  the  indundibulum  is  the 
pituitary  body  or  hypophysis  cerebri.  This  consists  of  an  anterior 
lobe,  narrow  and  stalk-like,  attached  to  the  infundibulum  between 
the  lobi  inferiores,  and  a  much  broader  posterior  lobe  which  is 
highly  glandular.  It  is  slightly  more  dorsal  than  the  former,  and 
extends  back  beyond  the  saccus. 

The  mesencephalon  or  mid-brain,  although  small,  is  easily 
distinguished.  Dorsally  it  comprises  the  two  optic  lobes  or  corpora 
bigemina  ;  these  are  small  oval  swellings  separated  in  the  middle 
line  by  a  furrow  and  overhung  to  a  large  extent  by  the  anterior  end 
of  the  cerebellum.  They  are  hollow,  their  cavities,  the  optic 
ventricles,  being  offshoots  from  the  brain  cavity,  and  their  walls 
termed  the  tectum  opticum,  are  plentifully  supplied  with  ganglion 
cells  related  to  the  terminations  of  the  fibres  from  the  optic  nerves. 
The  sides  and  floor  of  the  mesencephalon  are  constituted  by  nervous 
masses,  the  cruri  cerebri,  continuous  with  the  optic  thalami  in  front 
and  the  medulla  behind.  The  oculo-motor  nerves  arise  from  the 
ventral  surface  of  the  crura.  The  cavity  of  the  mid-brain  is  con- 
stricted laterally,  forming  the  Iter  or  Aqueduct  of  Sylvius. 

The  hind-brain  in  Scyllium  is  large,  occupying  more  than  half 
the  entire  length  of  the  whole  brain.  The  roof  of  the  front  part,  or 


254  AN   INTRODUCTION  TO  ZOOLOGY 

myelencephalon,  is  thickened  and  gives  off  a  large  hollow  outgrowth, 
the  cerebellum.  This  is  an  oval  structure  with  pointed  ends,  and 
marked  by  a  slight  median  furrow  ;  in  front  it  projects  freely  over  the 
optic  lobes  and  behind  over  the  roof  of  the  fourth  ventricle.  Its 
internal  cerebellar  ventricle  or  metaccele  opens  into  the  front  end  of 
this  ventricle.  It  is  connected  to  the  medulla  by  a  solid  tract  of 
nervous  tissue  on  each  side,  the  cerebellar  peduncle.  The  succeeding 
part  of  the  brain  is  the  myelencephalon  or  medulla  oblongata,  and, 
like  the  thalamencephalon,  its  roof  is  composed  of  ependymal 
epithelium  with  which  the  pia  mater  is  closely  associated,  and  the 
joint  membrane  so  formed  is  richly  supplied  with  blood-vessels 
forming  the  choroid  plexus  of  the  fourth  ventricle.  The  ventral 


OL 


c.a 

LL 


L.V 


SV 


FIG.  83.  —  -Longitudinal  section  of  brain  of  Scyllium  slightly  out  of  median 

plane. 

A.C.,  anterior  choroid  plexus  ;  A.  P.,  anterior  lobe  of  pituitary  body  ;  C.,  cerebrum;  CB.,  cere- 
bellum ;  Cr.,  crus  ;  F.M.,  foramen  of  Munro  ;  I.,  infundibulum  ;  It.,  iter;  L.L.,  lobus  lineae 
lateralis  ;  L.T.,  part  of  lamina  lerminalis  ;  L.V.,  lobus  visceralis  ;  M.,  medulla  oblongata  ;  O., 
opening  to  cavity  of  lobus  inferior;  O.L.,  olfactory  lobe;  Op.,  optic  lobe;  O.X.,  optic  chiasma  ; 
P.,  pineal  stalk  ;  P.B.,  pineal  body  ;  P.C.,  posterior  choroid  plexus  ;  P.  Co.,  posterior  commissure  ; 
P.O.,  pre-optic  recess;  P.P.,  posterior  lobe  of  pineal  body;  S.C.,  superior  commissure;  S.V., 
saccus  vasculosus  ;  T.,  thalamencephalon  ;  T.A.,  tuberculum  acusticum  ;  V.T.,  velum  transversum  ; 
III.  and  IV.,  third  and  fourth  ventricles. 

and  lateral  walls  of  the  ventricle  are  greatly  thickened,  and  contain 
a  large  number  of  ganglion  cells  which  are  related  to  the  5th,  7th, 
8th,  Qth  and  loth  cranial  nerves.  The  antero-lateral  walls  are 
continued  on  each  side  to  form  the  characteristic  wing-shaped 
corpora  res  tif  or  mia  orrestiform  bodies,  passing  forward  just  beneath 
the  hinder  end  of  the  cerebellum.  In  the  inside  walls  of  the  ventricle 
are  three  well-marked  projections,  the  tuber  acusticum,  containing 
centres  related  to  the  auditory  fibres  ;  above  it,  the  lobus  lineae 
lateralis,  related  to  the  nerves  of  the  lateral  line,  and  below  it  the 
lobus  visceralis.  These  two  ridges  are  readily  seen  if  the  brain  is  cut 
in  median  longitudinal  section.  The  medulla  passes  over  imper- 
ceptibly into  the  spinal  cord. 

The  cerebrum  in  Scyllium,  as  in  all  lower  Chordata,  is  mainly 


SCYLLIUM   CANICULA  255 

concerned  with  the  sense  of  smell,  and  this  is  the  sense  that  plays  a 
large  part  in  procuring  the  fish's  food.  Thus  it  is  that  it  is  relatively 
larger  than  in  Rana.  The  cerebellum,  on  the  other  hand,  is  the 
great  centre  for  the  co-ordination  of  movement,  and  is  also  closely 
related  to  the  lateral  line  sense  organs.  Hence,  in  an  active  animal 
like  the  dogfish,  this  part  of  the  brain,  too,  is  more  developed  than 
in  the  frog.  Tracts  of  nerve  fibres  run  from  the  cerebrum  to  the 
optic  thalami,  and  thence  through  the  crura  cerebri  to  the  cere- 
bellum, medulla  and  spinal  cord,  and  so  we  find  a  system  of  fibres 
linking  up  the  various  sensory  and  motor  centres  of  the  brain  and 
allowing  of  a  central  co-ordination  of  activities. 

Before  leaving  the  brain  it  may  not  be  out  of  place  to  glance 
briefly  at  its  mode  of  development,  since  it  is  fairly  simple  in 
Scyllium,  and,  just  as  in  the  adult  we  have  a  comparatively  simple 
primitive  brain  providing  a  ground  plan  upon  which  the  brains  of 
the  higher  Chordata  can  be  built,  so  we  find  its  development  pursues, 
in  general,  a  course  followed,  with  but  slight  modification,  in  other 
forms.  It  has  previously  been  pointed  out  that  the  whole  of  the 
central  nervous  system  arises  as  a  tubular  structure  by  the  closure 
of  the  medullary  folds.  From  the  very  beginning  the  anterior  end 
of  the  medullary  plate,  lying  in  the  head  region,  is  wider  than  in  the 
trunk  region,  and  as  the  concrescence  of  the  folds  is  proceeding 
this  region  dilates  to  form  at  first  two  and,  very  shortly  after,  three 
distinct  sac-like  enlargements  separated  from  one  another  by  con- 
strictions. These  are  the  three  primary  brain  vesicles,  and  are 
known  from  before  backwards  as  the  fore-brain  or  prosencephalon, 
the  mid-brain  or  mesencephalon  and  the  hind-brain  or  rhombence- 
phalon,  the  latter  passing  over  gradually  into  the  spinal  cord.  They 
lie  on  the  dorsal  side  of  the  embryo  above  the  notochord,  which 
terminates  abruptly  under  the  mid-brain.  The  dorsal  side  of  the 
fore-brain  grows  much  more  quickly  than  the  ventral  side,  with  the 
result  that  the  prosencephalon  becomes  bent  round  over  the  end  of 
the  notochord  almost  at  right  angles,  thus  producing  what  is  known 
as  the  cephalic  flexure.  This  is  characteristic  of  most  Craniates,  but 
it  disappears  again  in  the  dogfish  as  the  adult  condition  is  reached. 
The  folds  close  slowly  towards  the  front  end,  so  that  there  is  left 
for  some  time  an  anterior  opening,  the  neuropore,  which,  even  when 
it  eventually  closes,  leaves  a  small  depression,  the  neuroporic  recess. 
The  end  of  the  fore-brain  below  the  recess  is  termed  the  lamina 
terminalis. 

Very  shortly  after  the  appearance  of  the  three  primary  vesicles 
a  hollow  outgrowth  appears  on  the  lateral  wall  of  each  side  of  the 
fore-brain  towards  its  ventral  margin ;  this  is  the  primary  optic 
vesicle.  As  development  proceeds  this  differentiates  into  a  distinct 


256 


AN   INTRODUCTION   TO  ZOOLOGY 


optic  vesicle,  responsible  for  the  production  of  the  retina,  at  its  outer 
end,  and  a  tubular  portion,  the  optic  stalk,  connecting  it  with  the 
brain  and  marking  the  position  of  the  future  optic  nerve.  The  level 
where  the  stalk  joins  the  brain  is  marked  by  an  optic  groove  running 
across  the  floor.  This  persists  in  the  adult  as  the  pre-optic  recess, 
and  it  forms  an  important  landmark  in  the  brain,  since  it  marks  the 
hinder  end  of  the  lamina  terminalis,  and  also  the  ventral  posterior 
limit  of  the  telencephalon.  The  dorsal  limit  is  soon  laid  down  by 
the  appearance  of  a  fold  destined  to  form  the  velum  transversum. 
The  antero-dorsal  walls  of  the  telencephalon  now  grow  forward  on 
each  side  in  the  neighbourhood  of  the  neuroporic  recess,  to  form  the 
paired  cerebral  hemispheres,  but  the  brain  does  not  grow  actually 


FIG.  84. — Diagram  of  development  of  brain  of  Scy Ilium. 

I.-IV.,  four  successive  stages  in  development;  II.,  stage  of  cranial  flexure;  IV.,  late  stage, 
for  full  naming  compare  with  section  of  adult  brain ;  C.,  cerebrum  ;  CB.,  cerebellum  ;  F.,  fore- 
brain,  prosencephalon ;  H.,  hind-brain,  rhombencephalon ;  I.,  infundibulum ;  M.,  mid-brain, 
mesencephalon ;  Mt.,  metencephalon ;  My.,  myelencephalon ;  OO.,  optic  groove;  O.X.,  optic 
chiasma  ;  P.,  pineal  outgrowth  ;  Pi.,  pituitary  body ;  T.,  telencephalon ;  Th.,  thalamencephalon, 

in  the  middle  line.  Hence  even  in  the  higher  animals  where  the 
cerebral  development  is  great,  the  lamina  terminalis  always  remains 
to  mark  the  end  of  the  embryonic  fore-brain. 

The  next  part  of  the  brain,  the  thalamencephalon,  is  early  marked 
by  two  median  outgrowths,  while  its  roof  remains  for  the  most  part 
non-nervous.  The  dorsal  projection  is  the  beginning  of  the  epiphysis 
cerebri,  while  the  ventral  one  is  the  infundibulum.  The  hypophysis 
cerebri,  however,  is  not  an  outgrowth  from  the  brain  at  all.  It  arises 
as  a  pocket-like  diverticulum  from  the  roof  of  the  stomodoeum,  which 
then  forms  a  closed  vesicle  with  a  solid  stalk.  The  stalk  disappears 
and  the  vesicle  takes  up  its  position  beneath  the  infundibulum. 

The  roof  of  the  mid-brain  vesicle  gives  rise  to  two  enlargements, 
the  future  optic  lobes,  while  its  walls  and  floor  become  markedly 


SCYLLIUM   CANICULA 


257 


thickened  to  form  the  crura  cerebri,  and  consequently  reduce  its 
cavity  to  a  fairly  small  passage,  the  iter. 

While  these  processes  have  been  taking  place  the  hind-brain  has 
also  been  differentiating  into  its  two  subdivisions,  the  metence- 
phalon  and  the  myelencephalon.  The  roof  of  the  former  gives  rise 
to  a  large  median  dorsal  outgrowth,  the  cerebellum,  while  that  of 
the  latter  remains  thin  and  epidermal.  The  sides  and  floor  of  the 
whole  hind-brain  thicken  considerably.  At  the  hinder  end  the 
myelencephalon  thickenings  pass  over  into  those  of  the  spinal  cord. 

The  nasal  organ  and  the  auditory  organ  are  not  derived  from 
the  brain  like  the  optic  vesicle,  but  from  ingrowths  of  the  ectoderm. 

The  various  parts  of  the  brain  in  the  embryo  and  the  adult 
and  the  structures  characteristic  of  each  of  them  may  be  tabulated 
in  the  following  manner  : — 


THE  DIVISIONS  OF  THE  BRAIN  IN  SCYLLIUM  CANICULA,  AND  THE  STRUCTURES 
ASSOCIATED  WITH  EACH. 


Primary  and 
embryonic  divisions. 

Secondary  and  adult 
divisions. 

Associated  structures. 

TELENCEPHALON. 

Rhinencephalon,  i.e.  olfactory  lobes. 
Rhinoccele. 
Cerebrum  (cerebral  hemispheres). 
Lateral    ventricles.      Foramina    of 
Munro. 

(Pre-optic  recess        ) 

PROSENCE- 
PHALON 
or 
FORE-BRAIN. 

DIENCEPHALON 
or 
THALAMENCE- 
PHALON. 

\Velum  transversumj 

3rd     Ventricle.      Anterior    choroid 
plexus. 
Optic  thalami.     Habenular  ganglia. 
Superior  commissure. 
Epiphysis  cerebri  (pineal  body). 
Optic  chiasma.     Post-optic  recess. 
Infundibulum.        Lobi      inferiores. 
Saccus  vasculosus. 
Hypophysis  cerebri  (pituitary  body). 

MESENCE- 
PHALON 
or 
MID-BRAIN. 

MESENCE- 
PHALON. 

Posterior  commissure.     Optic  lobes 
(Tectum  opticum). 
Crura  cerebri. 
Aquaeductus  Sylvii  or  Iter. 

METENCE- 
PHALON. 

Cerebellum.     Metacoele. 

RHOMBENCE- 
PHALON 
or 
HIND-BRAIN. 

MYELENCE- 
PHALON. 

4th  Ventricle.      Posterior    choroid 
plexus. 
Tuber  acusticum. 

Restifonn  bodies    *^£* 

.Lobus  visceralis. 
Medulla  oblongata. 

258  AN   INTRODUCTION   TO   ZOOLOGY 

Spinal  Cord. 

The  spinal  cord  calls  for  but  brief  notice.  It  is  continuous 
in  front  with  the  brain,  and  thence  passes  backwards  in  the  neural 
canal  as  a  cylindrical  column,  slightly  flattened  dorso-ventrally 
and  gradually  diminishes  in  size  until  it  terminates  at  the  end  of  the 
caudal  vertebrae.  Above,  it  bears  a  shallow  dorsal  fissure,  and 
beneath  a  deeper  ventral  fissure.  In  transverse  section  it  exhibits 
the  two  characteristic  varieties  of  nervous  tissue,  an  inner  mass  of 
grey  matter,  but  not  so  definitely  divided  into  dorsal  and  ventral 
cornua  as  in  Rana.  Near  the  middle  the  canalis  centralis  will  be 
seen  as  an  oval  tube  lined  by  a  typical  epithelium,  which  is  ciliated. 
Running  through  both  grey  and  white  matter  are  certain  supporting 
elements  termed  neuroglia  cells,  which  are  non-nervous. 

Cranial  Nerves. 

We  find  in  Scyllium  the  same  cranial  nerves  that  have 
already  been  dealt  with  in  Rana,  and  there  is  in  addition  a  tiny 
pair  *  arising  from  the  front  end  of  the  prosencephalon,  termed  the 
nervi  terminales,  which  are  also  present,  though  very  small,  in  the 
frog.  Indeed,  the  same  ten  nerves  are  to  be  found  in  all  the 
Craniata,  arising  from  the  same  place  and  with  the  same  general 
distribution,  although  in  higher  forms,  e.g.  mammals,  there  are  added 
to  these  two  more  pairs,  making  twelve  in  all.  The  same  is  probably 
true  of  the  nervus  terminalis,  although  it  is  not  always  so  easy  to 
make  out  as  in  Scyllium,  and  it  is,  as  yet,  not  very  well  known. 
It  is  important,  therefore,  that  these  nerves  and  their  point  of  origin 
should  be  borne  in  mind,  and  for  this  reason  they  are  set  forth  in 
tabular  form  below. 

ORIGIN  OF  THE  CRANIAL  NERVES  IN  SCYLLIUM. 


Nervus  terminalis. 

Arises  from  the  front  end  of  the  prosencephalon  near 
middle  line. 

i.  Olfactorius. 

Arises  in  the  ectodermal  cells  of  the  olfactory  organ  and 
passes  as  a  series  of  small  nerves  to  the  olfactory 
bulb. 

2.  Opticus. 

Arises  from  the  optic  chiasma  on  the  floor  of  the 
diencephalon. 

3.  Oculomotorius. 

Arises  from  the  ventral  surface  of  the  crura  cerebri, 
mescencephalon. 

4.  Patheticus. 

Arises  from  the  dorsal  side  of  the  metencephalon,  in  the 
angle  between  the  optic  lobes  and  the  cerebellum. 

*  There  is  also  a  small  nervus  septalis  arising  behind  the  olfactorius,  but 
it  is  omitted  here  on  account  of  its  small  size. 


SCYLLIUM   CANICULA 


259 


5.  Trigeminal. 

Arises  from  the  latero-ventral  aspect  of  the  myelence- 
phalon  below  restiform  bodies,  by  two  roots  ;  a 
large  dorsal  sensory  root  bearing  the  Gasserian 
ganglion  and  a  smaller  ventral  motor  root. 

6.  Abducens. 

Arises  from  the  median  ventral  aspect  of  the  myelence- 
phalon. 

7.  Facialis. 

Arises  by  two  main  roots,  sensory  and  motor,  which 
unite  to  form  the  geniculate  ganglion,  from  the 
lateral  aspect  of  the  myelencephalon. 

8.  Auditorius. 

Arises  directly  behind  7. 

9.  Glosso-pharyngeal. 

Arises  by  two  or  three  roots  from  the  latero-ventral 
aspect  of  the  myelencephalon  behind  and  below  8. 

10.  Vagus. 

Arises  by  four  main  roots  from  the  lateral  aspect  of  the 
myelencephalon.  The  most  anterior,  which  is  that 
of  the  lateral  line  nerve,  is  just  behind  and  dorsal 
tog. 

The  foramina  through  which  these  nerves  leave  the 
cranium  have  already  been  dealt  with,  so  that  the  next  point  is  to 
consider  their  distribution.  The  first,  or  olfactory  nerve,  passes 
through  the  posterior  wall  of  the  olfactory  capsule  as  a  series  of  fibres 
which  are  distributed  to  the  olfactory  epithelium.  The  second,  or 
optieus,  passes  through  the  side  wall  of  the  orbit,  up  to  the  optic  stalk, 
and  is  distributed  to  the  retina.  The  third,  or  oculomotor ius,  also 
perforates  the  orbital  wall  and  supplies  the  superior,  internal  and 
inferior  oblique  muscles  of  the  eyeball.  The  fourth,  or  patheticus, 
enervates  the  superior  oblique  eye  muscle.  The  fifth,  or  trigeminus, 
after  leaving  the  ganglion,  divides  into  two  branches  inside  the 
cranium,  of  which  the  anterior,  a  sensory  branch,  passes  out  into 
the  orbit  as  the  ophthalmic  branch  of  the  fifth  nerve.  This  runs 
forward  along  the  mesial  dorsal  wall  of  the  orbit  and  out  through 
its  roof  to  the  skin  of  the  dorsal  side  of  the  front  end  of  the  head  and 
snout.  The  posterior  branch  enters  the  orbit  by  a  common  foramen 
with  the  sixth  and  part  of  the  seventh  nerves,  and  quickly  divides 
into  two,  the  maxillary  and  mandibular  nerves.  They  pass  antero- 
laterally  across  the  floor  of  the  orbit,  and  the  former  goes  to  the  skin 
and  muscles  of  the  upper  jaw  and  lip,  while  the  latter  spreads  out 
in  the  skin  and  muscles  of  the  lower  jaw.  The  sixth  nerve,  or 
abducens,  goes  to  the  external  rectus  muscle. 

So  far  the  distribution  of  the  cranial  nerves  has  been  typical, 
but  when  we  come  to  the  seventh,  or  facial  nerve,  we  find  a  striking 
difference  between  Scy ilium  and  Rana.  In  the  first  place  it  is  really 
a  mixture  consisting  of  the  branches  of  the  facialis  proper,  such  as 
we  find  in  all  Craniates  and  another  series  of  fibres  belonging  to  the 


260 


AN   INTRODUCTION  TO  ZOOLOGY 


system  of  the  lateral  line  sense  organs,  and  so  termed  lateralis  fibres. 
Such  lateralis  nerves  are  found  generally  in  fishes,  and  in  those 
members  of  the  class  Amphibia  that  permanently  inhabit  the  water, 
but  are  absent  from  all  the  air-breathing  vertebrates.  The  anterior 
or  ophthalmic  branch  of  the  seventh  is  a  lateralis  nerve,  and  runs  into 
the  orbit,  where  it  accompanies  the  ophthalmic  branch  of  the 
trigeminus  forward  to  the  snout.  There  it  serves  the  sensory 
ampullae  and  canals  of  the  supra-orbital  and  snout  regions  It  even 
arises  by  a  separate  root  from  the  brain.  The  second  branch  of  the 
seventh,  the  buccal,  is  also  composed  of  lateralis  fibres,  and  passes 
in  company  with  the  maxillary  and  mandibular  branches  of  the  fifth 


.V  *    vHt 


vil  M 


FIG.  85. — Diagram  of  distribution  of  cranial  nerves  in  Scy Ilium.     The  factors 
of  the  acustico-lateralis  system  shown  in  black. 

Au.,  auditory  capsule  ;  G.C.,  gill  cleft ;  M.,  mouth  ;  O.,  orbit ;  O.C.,  olfactory  capsule. 

I.-X.,  cranial  nerves;  V.M.,  maxillary;  V.Mn.,  mandibular;  V.O.,  ophthalmic;  VII.B., 
buccal;  VII.H.,  hyoidean;  VII. Hm.,  hyomandibular;  VII.Mn.,  external  mandibular;  VII. O., 
ophthalmic;  VII. P.,  palatine  ;  VII.Ps.,  prespiracular ;  IX.Po.,  post-trematic branch  ;  IX. Pr.,  pre- 
trematic  branch;  X.B.,  branchial  dividing  into  pre-  and  post-trematic  branches;  X.L.,  nervus 
lineae  lateralis ;  X.V.,  visceral. 

nerve  to  the  sensory  canals  in  the  infra-orbital  region  and  on  the 
ventral  surface  of  the  snout.  Immediately  on  entering  the  orbit, 
the  palatine  or  first  branch  of  the  facialis  proper  passes  forwards 
and  outwards,  at  first  behind  and  then  below  the  maxillo-mandibular 
of  five  to  the  lower  jaw.  Here  it  divides  into  two,  one  penetrating 
the  jaw  and  the  other  passing  to  the  muscles  of  the  roof  of  the  buccal 
cavity.  The  fourth  and  largest  trunk  of  the  seventh  nerve  runs 
along  the  hinder  wall  of  the  orbit,  giving  off  several  pre-spiracular 
twigs  which  supply  the  anterior  wall  of  the  spiracle,  and  then  crosses 
the  hyomandibular  cartilage,  just  beneath  the  skin,  to  the  posterior 
wall  of  the  spiracle.  Itfseparates^into  threejnain  portions  ;  two 
post-spiracular  twigs  consisting  of  true  facialis  fibres,  supplying  the 


SCYLLIUM    CANICULA  261 

mucous  membrane  and  muscles  of  the  hyoid  arch,  and  the  third,  a 
lateralis  nerve,  distributed  as  the  external  mandibular  to  the  sensory 
canals  and  ampullae  in  the  hyoid  and  mandibular  regions. 

The  eighth  nerve,  the  auditory,  passes  directly  to  the  membranous 
labyrinth  of  the  ear,  over  which  it  spreads.  The  ninth,  or  gloss  o- 
pharyngeal,  nerve  leaves  the  cranium  by  an  aperture  in  the  post- 
orbital  groove,  and  has  three  main  branches.  The  first  is  a  small 
dorsal  factor  to  the  skin  ;  the  second  is  a  palatine  branch  running 
forward  to  join  with  the  similar  branch  from  the  facial.  The  third 
and  largest  branch  soon  divides  into  two,  one  passing  in  front  of,  and 
the  other  behind,  the  first  gill  cleft  to  the  structures  surrounding 
which  they  are  related. 

The  tenth  or  vagus  nerve,  like  the  seventh,  contains  also  an 
admixture  of  lateralis  fibres.  In  the  same  way,  too,  the  lateralis 
nerve,  a  large  branch,  arises  from  the  medulla  by  its  own  root,  and, 
although  joining  the  main  trunk  for  a  short  distance,  soon  leaves  it 
again  to  pass  backwards  fairly  superficially  in  the  myoseptum 
between  the  epiaxial  and  hypaxial  portions  of  the  myomeres  just 
under  the  lateral  line  canal  whose  sense  organs  it  supplies.  The 
second  branch  of  the  tenth  is  the  visceralis  nerve,  consisting  of  true 
vagus  fibres,  passes  back  to  the  heart  and  other  viscera.  The  third 
branch  sends  a  factor  to  each  of  the  four  posterior  gill  clefts,  over 
which  they  split  into  a  pre-  and  a  post-branchial  portion.  This 
third  branch  also  is  not  found  typically  developed  in  air-breathing 
vertebrates  where,  of  course,  the  gills  are  absent,  but  is  probably 
represented  in  them  by  the  pulmonary  branch  of  the  vagus. 

When  we  consider  the  functions  of  the  cranial  nerves  we 
see  that  they  can  be  divided  into  three  groups.  The  first  is  com- 
posed of  the  oculomotorius,  the  patheticus  and  the  abducens,  all 
of  which  are  entirely  motor  in  function,  and  as  they  go  solely  to 
muscles,  in  this  case  eye  muscles,  they  are  termed  myomeric  nerves. 
The  second  group,  consisting  of  the  olfactorius,  the  opticus,  the 
auditorius,  the  lateralis  nerves  and,  perhaps,  also  the  nervus 
terminalis,  is  solely  concerned  with  conveying  impressions  to  the 
brain,  and  hence  its  constituents  are  termed  sensory  nerves.  The 
remainder  of  the  cranial  nerves  are  both  sensory  and  motor  in 
function,  and  so  spoken  of  as  Mixed  nerves  ;  viz.  the  trigeminus 
(of  which,  however,  the  ophthalmic  branch  is  entirely  sensory), 
the  glosso-pharyngeus  and  the  branches  of  both  the  facialis  and  the 
vagus  that  are  not  lateralis  nerves. 

All  the  lateralis  nerves,  namely,  the  ophthalmic,  the  buccal  and 
the  external  mandibular  of  the  facialis,  and  the  lateral  line  nerve 
of  the  vagus,  in  spite  of  the  way  in  which  they  leave  the  cranium,  all 
originate  in  the  lobus  lineae  laterals  in  the  medulla.  This  large  nerve 


262  AN   INTRODUCTION   TO  ZOOLOGY 

centre  is  closely  bound  up  with  the  tuber  acusticum,  from  which  the 
eighth  nerve  arises.  Furthermore,  other  reasons  lead  us  to  regard 
the  ear  as  a  specialised  portion  of  the  lateral  line  series  of  sense 
organs,  and  so  we  sometimes  speak  of  these  two  large  brain  ganglia 
and  all  the  nerves  directly  related  to  them  as  the  acustico  lateralis 
system.  Of  this  entire  complex  only  the  auditory  centre  and  nerve 
are  present  in  the  higher  Craniata,  i.e.  many  of  the  Amphibia,  all 
the  Reptiles,  the  Birds  and  the  Mammals. 

The  manner  in  which  the  mixed  nerves  are  distributed 
also  calls  for  notice.  It  will  be  seen  that  they  have  a  very  similar 
arrangement.  Each  consists,  when  reduced  to  its  simplest  expres- 
sion, of  three  principal  parts.  One  going  to  the  front  of  a  gill  cleft, 
and  so  termed  the  pre-branchial  or  pre-trematic  branch,  well  shown 
in  the  seventh,  ninth  and  tenth  nerves.  Another,  the  post-branchial, 
or  post-trematic  branch,  lies  behind  the  same  gill  slit.  The  third 
branch  is  situated  more  dorsaUy  and  passes  forward,  e.g.  the 
palatines  of  the  seventh  and  ninth.  It  is  clear,  then,  that  the  tri- 
geminus  is  related  to  the  mouth  in  the  same  way  that  the  others  are 
to  the  gill  clefts,  so  that  for  this  and  other  reasons  the  mouth  is 
regarded  by  some  authorities  as  representing  the  fusion  of  an  anterior 
pair  of  gill  slits.  In  order  to  indicate  this  characteristic  method  of 
distribution  of  the  mixed  nerves  they  are  sometimes  spoken  of  as 
branchiomeric  nerves. 

Spinal  Nerves. 

The  spinal  nerves  only  call  for  brief  notice.  The  typical 
nerve  arises  from  the  cord  by  two  roots,  a  dorsal  and  a  ventral,  of 
which  the  former  bears  a  ganglion  and  arises  slightly  behind  the 
latter.  A  pair  of  such  nerves  is  present  in  each  somite  of  the  body. 
The  five  anterior  spinal  nerves,  together  with  three  small  so-called 
spino-occipital  nerves  that  come  off  by  single  roots  from  the  hinder 
end  of  the  medulla  in  line  with  the  ventral  roots,  join  to  form  one 
trunk.  The  nerve  so  constituted  is  distributed  partly  to  the  ventral 
surface  of  the  pectoral  fin  and  partly  to  the  sub-pharyngeal  muscula- 
ture. Spinal  nerves  6-n  approximate  closely  to  one  another  to 
form  a  very  rudimentary  sort  of  brachial  plexus,  and  then  pass  on 
to  supply  the  pectoral  fin. 

Sympathetic  Nerves. 

These  are  quite  inconspicuous  in  Scyllium,  but  consist  of 
two  longitudinal  chains  bearing  very  small  ganglia  and  lying  on  the 
dorsal  wall  of  the  abdominal  cavity,  one  on  each  side  of  the  vertebral 
column. 


SCYLLIUM   CANICULA  263 

Sense  Organs. 

In  addition  to  the  sense  organs  dealt  with  in  the  frog,  and 
which  will  be  referred  to  again  briefly  here,  namely,  the  olfactory, 
the  gustatory,  the  tactile,  the  optic  and  the  auditory,  the  dogfish 
possesses  also  a  series  of  cutaneous  sense  organs.  These  are  widely 
distributed  over  the  body,  and  are  characteristic  of  fish  in  general. 
Of  such  organs  two  distinct  varieties  are  present,  those  termed  the 
taste  buds  or  end  buds,  and  the  sensory  organs  of  the  lateral  line,  the 
neuromas  ts. 

The  end  buds  are  very  similar  to  the  taste  corpuscles  of  the  higher 
Craniates.  They  consist  of  a  number  of  long  rod-like  sense  cells 
aggregated  together  in  a  characteristic  manner  about  a  central  cell, 
each  possessing  a  hair-like  process  projecting  above  the  general 
level  of  the  epidermis.  Closely  connected  with  their  deeper  ends 
are  arborisations  of  sensory  nerve  fibres  coming  entirely  from  the 
facial,  glosso-pharyngeal  and  vagus  nerves.  In  the  air-breathing 
Craniates  such  organs  are  confined  to  the  buccal  cavity,  whereas 
in  the  dogfish  they  are  spread  irregularly  over  the  surface  of  the 
head  and  branchial  region,  and  in  the  case  of  some  bony  fishes  they 
are  even  to  be  found  on  the  body  and  at  the  bases  of  the  fins.  They 
not  only  subserve  the  function  of  taste,  as  we  understand  it  in  the 
higher  animals,  but,  when  situated  outside  the  buccal  cavity,  also 
inform  their  possessor  when  its  proper  food  is  near  at  hand. 

The  neuromasts  are  composed  of  fewer  sensory  cells,  but  each 
individual  cell  is  larger  and  somewhat  pear-shaped.  As  in  the  end 
buds,  the  cells  have  a  hair-like  sensory  process.  These  organs  are 
supplied  exclusively  by  fibres  from  the  lateralis  nerves,  and  are 
always  situated  below  the  external  surface  of  the  body  and  covered 
by  a  fluid  or  semi-gelatinous  substance  in  which  their  processes  lie. 
The  ampullae  are  arranged  in  definite  groups  on  the  head  and 
consist  of  fairly  deep  tubes  swelling  out  into  a  chamber  at  their 
inner  end  in  which  the  sensory  cells  are  situated.  The  lateral  line 
organs  are  to  be  found  in  a  series  of  canals  of  which  the  most  con- 
spicuous is  the  lateral  line  itself.  In  addition  to  this,  which  as 
noted  previously,  runs  from  just  behind  the  spiracle  right  down  the 
tail,  there  are  also  a  canal  above  the  eye,  the  supra-orbital,  one  below 
the  eye,  the  infra-orbital,  one  in  the  hinder  dorsal  part  of  the  head, 
the  occipital,  and  one  in  the  neighbourhood  of  the  hyoid  arch  and 
lower  jaw,  the  hyomandibular  canal. 

Eye. 

The  eye  in  Scyllium  calls  only  for  brief  notice,  since  it  is  in 
the  main  similar  to  that  of  the  frog,  the  differences  being  in  detail 


264 


AN   INTRODUCTION   TO  ZOOLOGY 


SE 


only,  and  the  same  is  true,  too,  of  its  histological  structure.  The 
eyeball  itself  is  almost  hemispherical,  as  it  is  much  less  curved  on 
its  external  side  than  in  vertebrates  generally,  and  the  cornea  itself 
is  practically  flat  instead  of  being  curved.  The  lens  is  approximately 
spherical  and  not  bi-convex  as  in  the  frog.  This  fact  in  conjunction 
with  the  flattened  cornea  causes  the  anterior  chamber  to  be  relatively 
small,  and  such  a  combination  of  characters  is  found  generally  in 
fishes.  Lastly,  between  the  pigment  layer  of  the  retina  and  the 
choroid  coat  there  is  present  in  the  dogfish  a  peculiar  membranous 
layer  which,  over  the  posterior  region  of  the  eyeball,  possesses  a 
curious  bright,  silvery  metallic  lustre.  It  is  known  as  the  tapetum, 
and  presumably  causes  the  reflection  of  a  certain  amount  of  light. 
Vision  in  the  dogfish  is  apparently  much  less  keen  than  in  higher 
animals. 

Ear. 

in  Scyllium  and  fish  in  general  the  ear  is  less  complex  than 
in  the  land-dwelling  vertebrates,  and  it  consists  only  of  the  mem- 
branous labyrinth  or  internal 
ear,  and  we  find  nothing  to 
correspond  with  the  middle 
ear,  i.e.  the  tympanic  mem- 
brane and  cavity  as  it  exists 
in  Rana.  On  account  of  its 
larger  size  and  greater  acces- 
sibility the  labyrinth  is  more 
easily  studied  in  the  dogfish 
than  in  the  frog.  It  lies  em- 
bedded in  the  cartilaginous 
olfactory  capsule  on  the  outer 
walls  of  which,  as  we  have 
already  noted,  are  three  ridge? 
marking  the  position  of  the 
canals  within.  The  vestibule 
is  a  very  thin-walled  laterally 
compressed  sac,  distinctly 
marked  off  into  two  portions  ; 

A.,  anterior  vertical  semicircular  canal;    Am.,        the     Upper,      Or     UtricuhlS,     is 
ampulla ;    D.E.,    ductus     endolymphaticus ;     H., 
horizontal    semicircular   canal ;     L.,   lagena ;    P., 
posterior   vertical  semicircular   canal;    S.,  saccu- 
lus  ;  S  E.,  saccus  endolymphaticus  ;  U.,  utriculus. 


FIG.   86. — -Diagram  of  ear  of  primitive 
Craniate,  adapted  from  Wiedersheim. 


elongated  and  triangular  in 
shape,  the  lower,  or  sacculus, 
is  a  smaller  more  rounded  sac 
in  open  communication  with  it.  The  semicircular  canals  bearing 
ampullae,  and  coming  off  from  the  utriculus  are  well  developed 
and  present  certain  peculiarities.  The  anterior  and  posterior  canals 


SCYLLIUM    CANICULA  265 

join  up  on  the  dorsal  side,  and  open  by  a  common  aperture,  while 
the  posterior  canal  takes  the  form  of  an  almost  complete  ring  com- 
municating with  the  utriculus  by  a  single  opening.  The  ductus 
endolymphaticus,  coming  off  from  the  sacculus,  runs  dorso-mesially 
to  open  on  the  dorsal  surface  of  the  skull  by  an  aperture  situated  in  a 
depression  at  its  hinder  end.  When  treating  of  the  cranial  nerves 
we  saw  that  the  enervation  of  the  ear  and  lateralis  system  was 
closely  connected.  Furthermore,  the  actual  structure  of  the 
sensory  cells  in  the  ear  is  practically  identical  with  those  of  the 
neuromasts,  and  they  function  in  the  same  way,  for  in  each  the  actual 
receptive  process  is  surrounded  by  liquid,  the  vibrations  of  which 
bring  about  its  stimulation.  Lastly,  when  we  come  to  study  the 
development  of  the  embryo,  it  is  found  that  the  ear  and  the  lateral 
line  system  all  start  from  a  peculiarly  modified  patch  of  thickened 
ectoderm  in  the  hind  brain  region,  so  that  we  consider  the  ear  as 
simply  a  highly-specialised  portion  of  the  lateralis  system. 

This,  then,  completes  our  survey  of  the  dogfish,  a  simple 
vertebrate  animal  eminently  adapted  for  life  in  the  water,  and  one 
that  exhibits,  in  spite  of  certain  modifications,  the  main  plan  of  a 
Chordate  in  a  little  specialised  form.  It  is  further  of  interest,  since 
the  anatomical  relations  of  certain  of  its  organs,  blood-vessels,  etc., 
are  closely  approached  in  the  developing  embryo  of  the  higher 
animals,  including  man  himself. 


CHAPTER  XI 
LEPUS  CUNICULUS 

A  Mammal — Lepus  cuniculus,  the  Rabbit — Introduction — External  characters 
— Skin — Muscular  System— Skeleton  and  the  Skull  of  the  Dog. 

A  Mammal — Lepus  cuniculus,  the  Rabbit. 

The  Class  Mammalia  includes  the  highest  animals  alive  in  the 
world  and  reaches  its  culmination  in  man  himself  who,  while  not  so 
highly  specialised  structurally  as  some  other  forms,  is  nevertheless 
characterised  by  such  a  high  degree  of  mental  development  that  he 
must  be  regarded  as  the  dominant  form  in  the  class.  Mammals 
are  ccelomate,  chordate,  craniate  animals  and  so  possess  all  the 
general  characters  implied  by  those  terms.  In  certain  points  they 
resemble  the  frog  much  more  closely  than  the  dogfish.  Thus,  for 
example,  they  possess  an  endoskeleton  composed  mainly  of  bone 
and  modified  pentadactyl  limbs.  The  pericardium  lies  within  the 
coelom,  respiration  in  the  adult  takes  place  by  means  of  lungs  and 
not  gills  and  the  air  passage  opens  within  the  buccal  cavity  by  a 
pair  of  internal  nares.  Lastly  the  urinary  bladder  is  not  an  enlarge- 
ment of  the  kidney  duct,  but  an  outgrowth  of  the  cloaca  of  the 
embryo. 

The  differences  from  both  the  dogfish  and  the  frog  are  numerous 
and  important,  and  practically  constitute  the  diagnostic  characters 
of  the  class.  It  is  only  necessary  to  call  attention  to  the  main  ones 
here,  since  the  detailed  differences  will  appear  in  the  course  of  an 
examination  of  a  particular  example.  A  well-marked  exoskeleton 
is  present  in  the  form  of  hairs,  each  springing  from  an  encasing 
sheath  or  follicle,  constituting  in  most  species  a  more  or  less  complete 
covering  for  the  body  and  also  as  claws,  nails  or  hoofs  occurring  at 
the  ends  of  the  digits.  The  skin  also  bears  two  kinds  of  glands, 
distinguished  as  sweat  glands  and  sebaceous  glands,  both  arising  in 
connection  with  the  hair  follicles.  Certain  of  the  sweat  glands  on 
the  ventral  surface  are  modified  to  form  the  mammary  or  milk  glands, 
from  the  presence  of  which  the  class  receives  its  name.  In  all 
save  the  lowest  members  of  the  class  (Ornithorhynchus  and 
Echidna)  the  mammary  glands  open  in  groups  on  projections  of 

266 


LEPUS  CUNICULUS  267 

the  body  wall  termed  the  mammae  or  teats,  and  these  structures  with 
their  associated  glands  are  much  better  developed  in  the  female 
than  in  the  male.  Well  marked  external  ears  or  pinnae  are  also  present, 
and  typically  they  are  capable  of  being  moved  by  a  special  set  of 
muscles.  In  the  adult  of  higher  mammals  there  is  no  cloaca,  so  that 
the  anus  opens  quite  separately  from  the  urogenital  aperture  in  both 
sexes.  Both  openings  lie  on  a  characteristic  area  known  as  the 
perineum  on  the  posterior  ventral  abdominal  wall  in  the  region  of 
the  hinder  end  of  the  pelvic  girdle,  and  with  this  certain  perinea! 
glands  are  associated.  A  notochord  is-  of  course  present  in  the 
embryo,  but  with  the  complete  ossification  of  the  vertebrae  in  the 
adult  it  disappears  save  for  a  small  remnant  in  the  middle  of  the 
centra  of  certain  species.  The  vertebral  column  consists  of  a  mode- 
rate number  of  vertebrae  which  are  differentiated  into  five  regions, 
the  cervical,  the  thoracic,  the  lumbar,  the  sacral  and  the  caudal.  The 
first  cervical  vertebra,  termed  the  atlas,  is  modified  for  articulation 
with  the  skull,  while  the  second,  the  axis  or  epistropheus,  forms  a  very 
striking  joint  with  the  atlas  by  means  of  which  the  rotation  of  the 
skull  can  be  brought  about.  The  skull  itself  is  a  far  more  solid  and 
compact  stiucture  than  we  have  previously  encountered  and  has  the 
orbit  sunk  deeply  in  it,  so  that  this  cavity  becomes  largely  or  com- 
pletely surrounded  by  bones  on  the  internal  side.  The  suspensory 
apparatus  of  the  lower  jaw  is  completely  incorporated  with  the 
cranium,  so  that  the  jaw  actually  articulates  with  that  structure,  a 
type  of  suspension  that  is  termed  autostylic.  In  the  frog  the  internal 
nares  open  into  the  front  end  of  the  buccal  cavity,  but  in  the  mammal, 
owing  to  the  formation  of  a  sort  of  false  roof  to  the  mouth,  the 
palate,  partly  consisting  of  bone,  the  hard  palate,  and  partly  of 
mucous  membrane,  the  soft  palate,  the  internal  nares  come  to  open 
far  back  at  the  begining  of  the  pharynx.  No  columella  auris  is 
present,  but  its  place  is  taken  by  a  very  characteristic  chain  of 
tiny  bones,  the  auditory  ossicles.  During  the  life  of  the  animal  two 
distinct  sets  of  teeth  make  their  appearance  in  the  jaws  ;  the  first 
occurring  in  the  young  animal  being  termed  the  milk  teeth,  and  these 
are  later  replaced  by  the  so-called  permanent  teeth  of  the  adult. 
1  his  condition  is  termed  diphyodont,  to  distinguish  it  from  that  in 
the  dogfish,  where  there  are  many  successions  of  teeth.  Not  only 
this,  but  we  also  find  that  the  teeth  are  heterodont,that  is  to  say,  may 
be  differentiated  into  different  types  to  subserve  special  functions, 
and  in  a  typical  mammal  we  can  recognise  incisors,  canines,  premolars 
and  molars.  A  series  of  thin  curved  movable  bony  rods,  the 
ribs,  are  developed  and  they  articulate  on  the  dorsal  side  with  the 
vertebrae  and  on  the  ventral  side  with  tjie  sternum.  These  afford 
protection  to  the  heart  and  lungs,  and  as  the  hind  limbs  are  more 


268  AN   INTRODUCTION  TO  ZOOLOGY 

important  in  locomotion  we  find  the  pectoral  girdle  is  poorly  deve- 
loped. On  the  other  hand,  owing  to  the  greater  use  of  the  hind  limbs, 
the  pelvis  girdle  is  large  and  securely  attached  to  the  backbone. 

The  coelom  is  completely  divided  into  two  cavities,  by  the 
development  of  a  partly  membranous,  largely  muscular  arched 
partition,  the  diaphragm,  situated  in  the  region  of  the  hinder  end 
of  the  ribs.  Thus  we  can  recognise  an  anterior,  pleural  cavity, 
lying  in  a  part  of  the  body  termed  the  thorax,  containing  the  lungs 
and  having  within  it  the  pericardial  cavity  and  a  posterior  one, 
situated  in  the  abdomen,  termed  the  peritoneal  cavity,  which  contains 
the  rest  of  the  viscera.  The  movements  of  breathing  differ  com- 
pletely from  those  in  Rana.  Air  is  drawn  into  the  lungs  by  the 
enlargement  of  the  thorax  either  by  the  movement  of  the  ribs  or  the 
flattening  of  the  arch  of  the  diaphragm,  or  more  commonly  both 
combined. 

The  kidneys  are  metanephroi  and  the  ureters  open  directly  into 
the  bladder,  not  serving  for  the  passage  of  the  sperms  during  any 
part  of  their  course.  In  the  female,  the  lower  ends  of  the  oviducts 
are  modified  to  form  characteristic  structures,  the  uteri,  which  in 
some  cases  fuse  together  forming  a  single  uterus.  The  ova  (save  in 
Ornithorhynchus  and  Echidna)  are  minute  and  practically  yolkless, 
although  their  structure  and  mode  of  development  give  distinct 
indications  that  they  have  been  derived  from  eggs  with  a  large 
amount  of  yolk  such  as  we  find  in  reptiles.  Save  in  the  forms  just 
mentioned  the  eggs  are  never  passed  to  the  outside,  but  undergo 
development  within  the  uterus  to  the  walls  of  which  they  are  attached 
for  a  short  time  (Marsupials)  or  a  relatively  long  time  (as  in  higher 
mammals,  Eutheria)  by  a  very  characteristic  organ  termed  the 
placenta.  This  is  composed  of  an  intimate  union  of  tissues  from 
both  mother  and  embryo  and  serves  for  the  transference  of  food, 
oxygen,  and  excretory  products.  After  birth  the  young  animals 
are  quite  incapable  of  obtaining  food  for  themselves  and  are 
dependent  upon  the  milk  secreted  by  the  mammary  glands  of  the 
mother. 

The  heart  of  the  mammal  is  completely  divided  into  two  sides 
by  the  interatrial  septum  and  the  further  development  of  a  median 
partition,  the  septum  ventriculorum,  in  the  ventricle,  and  the  two 
sides  have  no  means  of  intercommunication.  More  than  that,  in 
order  that  the  blood  in  a  ventricle  may  return  to  it  again,  it  is  necessary 
for  it  to  leave  the  heart  twice,  once  to  go  to  the  tissues  and  return 
and  once  to  go  to  the  lungs  and  back.  We  refer  to  this  condition 
as  a  complete  double  circulation.  The  heart  contains  only  four 
chambers,  two  auricles,  or  better  atria,  and  two  ventricles,  there  being 
no  distinct  sinus  venosus  and  no  conus  arteriosus.  A  hepatic 


LEPUS   CUNICULUS  269 

portal  system  is  present  as  in  all  Craniates,  but  a  renal  portal  system 
is  absent.  The  mammals  are  warm  blooded,  that  is  to  say,  the  blood 
is  maintained  at  a  fairly  constant  temperature  somewhere  just 
below  100°  Fahr.,  and  this  is  quite  independent  of  the  variations 
of  the  temperature  of  the  surroundings. 

The  brain  is  extremely  well  developed  in  the  mammals, 
particularly  in  the  cerebral  hemisphere  which  in  some  animals, 
for  example  man,  forms  the  largest  part  of  the  brain.  Their  surfaces 
are  thrown  into  a  series  of  folds  or  gyri,  separated  from  one  another 
by  well-defined  grooves  or  sulci,  whereby  the  amount  of  space  at 
the  periphery  of  the  hemispheres,  in  which  the  nerve  cells  lie,  is 
greatly  increased.  The  main  part  of  this  increase  is  due  to  the 
formation  of  a  new  portion  of  the  roof  of  the  cerebrum  termed  the 
neo-pallium,  which  is  not  found  in  the  lower  animals.  The  optic 
lobe  on  each  side  is  divided  so  that  there  are  two  pairs  of  bodies, 
the  corpora  quadrigemina.  The  sense  organs  are  also  strongly 
developed,  and  in  the  internal  ear  we  find  a  spirally  coiled  structure, 
the  cochlea. 

With  this  short  discussion  of  the  general  characters  of  the  class, 
we  can  pass  on  to  consider  a  particular  example  in  the  rabbit. 
Lepus  cuniculus,  the  ordinary  wild  rabbit,  belongs  to  the  genus 
Lepus,  which  includes  among  other  forms  L.  timidus  the  hare,  and 
L.  variabilis,  the  Arctic  hare  or  Snowshoe  rabbit.  It  is  a  very  common 
animal  in  practically  all  parts  of  the  British  Isles,  and  is  the  species 
most  widely  kept  in  captivity  when,  as  is  well  known,  it  exhibits  a 
wide  range  of  variation  in  colour,  general  size  and  build,  and  so  on. 
This  phenomenon  of  variation  under  domestication  is  a  common  one, 
and  is  marked  in  cats,  dogs,  cattle,  pigeons,  fowls,  etc.,  to  a  greater 
or  less  extent  when  they  are  domesticated  by  man.  Years  ago  the 
rabbit  was  introduced  into  Australia,  where,  free  from  the  enemies 
and  other  checks  that  keep  its  numbers  down  in  its  native  lands,  it 
has  multiplied  to  such  an  extent  that  it  has  become  a  serious  menace 
to  the  farmers.  This  is  an  illustration  of  what  may  happen  when  the 
equilibrium  in  the  animal  life  of  a  given  area  that  is  established  in 
the  course  of  a  long  period  of  time,  is  upset  by  man  introducing  a 
new  animal. 

External  Features. 

Externally  we  can  readily  distinguish  in  the  body  of  the 
rabbit  a  head,  a  neck,  a  trunk,  a  tail  and  two  pairs  of  limbs,  all  of 
which  are  covered  with  a  dense  growth  of  fine  hair,  the  fur.  The 
manus  possesses  five  digits,  while  the  pes  has  only  four.  The  colour 
in  the  wild  form  is  a  dark  brownish-grey  above,  shading  off  to  a 
lighter  grey  on  the  sides,  and  becoming  practically  white  underneath. 


270  AN   INTRODUCTION   TO  ZOOLOGY 

The  underside  of  the  tail  and  adjacent  perineal  regions  of  the  body 
are  quite  white.  Altogether  it  is  very  inconspicuous  against  almost 
any  background  in  the  light  of  the  early  morning  or  evening,  when 
it  comes  out  to  feed.  It  passes  most  of  its  time  in  a  burrow  in  the 
ground,  and  a  number  live  close  together,  each  pair  with  their  own 
hole,  forming  a  community  termed  a  warren. 

The  front  end  of  the  head  is  drawn  out  into  a  blunt  snout.  The 
mouth  is  not  large,  but  noteworthy  in  that  the  upper  lip  is  cleft 
vertically  in  the  middle  line,  a  feature  that  enables  the  animal  to 
employ  its  teeth  for  gnawing  without  this  lip  getting  in  the  way. 
Above  the  mouth  are  the  openings  of  the  two  external  nares.  The 
upper  lips  bear  a  series  of  long  stout  hairs,  the  vibrissse,  whose  roots 
have  the  endings  of  certain  nerves  specially  related  to  them,  and  so 
constitute  a  sensory  apparatus  of  considerable  use  to  the  animal 
for  feeling  its  way  in  its  dark  underground  home.  The  eyes  are 
large,  but  not  prominent,  and  provided  with  well-developed  upper 
and  lower  lids.  At  their  inner  corner,  or  canthus,  is  the  nictitating 
membrane,  which  is  capable  of  being  drawn  partly  across  the  eye. 
External  ears,  or  pinnae,  are  strongly  marked  and  well  provided  with 
muscles ;  in  some  domestic  varieties  they  do  not  stand  upright  as 
in  the  wild  form,  but  droop  down  over  the  sides  of  the  head,  producing 
a  variety  known  as  a  lop-eared  rabbit. 

The  perineum  is  marked  by  bearing  white  fur.  On  it  open  the 
alimentary  canal  by  the  anus  and  the  urogenital  ducts,  and  it  also 
has  a  pair  of  small  perineal  pouches  and  two  small  bare  areas  on 
which  open  the  ducts  of  the  perineal  glands  to  the  secretion  of  which 
the  animal  owes  its  characteristic  odour.  The  rabbit,  like  all 
mammals,  is  dioecious,  and  so  the  perineal  areas  of  the  two  sexes 
are  different.  The  female  urogenital  aperture  is  a  slit-like  opening, 
the  vulva,  bearing  in  its  roof  at  the  ventral  end  a  rod-like  structure, 
the  clitoris.  The  corresponding  aperture  in  the  male  is  borne  at  the 
end  of  a  conical  rod-like  projection,  the  penis,  which  is  covered 
by  a  fold  of  skin,  the  prepuce,  so  that  at  first  sight  the  urogenital 
apertures  of  the  two  sexes  appear  somewhat  similar.  On  the 
perineum  of  the  male  at  each  side  of  the  base  of  the  penis  lies  a  small 
scrotal  sac,  within  which  is  contained  the  testis.  It  is  characteristic 
of  most  mammals  that  the  testes  leave  their  primitive  position  on  the 
dorsal  side  of  the  abdominal  cavity,  and  pass  into  a  single  or  paired 
outgrowth  of  the  peritoneal  cavity  which  projects  on  the  outside 
of  the  body  in  the  perineal  region.  The  last  of  the  external  features 
that  call  for  notice  are  the  mammary  glands.  These  number  four 
or  five  pairs,  situated  on  the  lines  joining  the  armpits,  or  axillae,  and 
the  corresponding  or  inguinal  regions  of  the  leg.  In  the  male, 
although  present,  they  are  hard  to  distinguish  ;  in  the  female, 


LEPUS   CUNICULUS  271 

however,  they  are  more  marked  and  become  large  after  the  birth  of 
the  young.  They  are  marked  externally  by  papilla-like  projections 
known  as  the  teats  or  mammae.  The  ducts  of  the  gland  open  on  to 
the  apex  of  this  projection,  and  so  form  an  outlet  through  which  the 
young  animal  can  suck  the  milk  that  is  its  only  food  for  the  first 
weeks  of  its  life. 

Integument  and  Muscular  System. 

The  skin  of  the  rabbit  fits  far  more  closely  to  the  under- 
lying muscular  tissue  than  in  the  frog,  for  there  are  no  subcutaneous 
lymph  sinuses.  A  certain  amount  of  independent  movement  is 
provided  for,  however,  by  the  development  of  a  layer  of  sub- 
cutaneous or  areolar  connective  tissue,  which  connects  the  skin  to 
the  body  wall.  The  structure  of  this  tissue  has  already  been  dealt 
with.  The  detailed  structure  of  the  skin  and  its  glands  also  differ 
in  Rana  and  Lepus,  although  both  are  composed  of  the  same  two 
fundamental  layers,  the  epidermis  and  the  dermis. 

The  epidermis  is  a  stratified  epithelium  divisible  into  two  layers  ; 
it  is  derived  from  the  ectoderm  of  the  embryo,  and  to  be  regarded  as 
forming  a  protective  layer  for  the  underlying  tissues.  Its  basal 
portion  is  composed  of  a  number  of  layers  of  polygonal  cells,  which 
are  composed  of  protoplasm  and  contain  nuclei.  These  constitute 
the  rete  mucosum  or  Malpighian  Layer,  and  the  cells  at  its  outer  side 
become  filled  with  secretory  granules.  The  outer  layer  of  the 
epidermis,  known  as  the  stratum  corneum,  is  many  layers  thick,  and 
the  cells  become  more  and  more  flattened  as  they  pass  outwards,  at 
the  same  time  their  nuclei  disappear  and  the  protoplasm  becomes 
replaced  by  a  substance  allied  to  keratin,  a  process  termed  cornifica- 
tion.  The  outermost  cells  form  small  dead  horny  scales  that  are 
constantly  being  shed.  As  noted  above,  this  is  a  protective  layer, 
and  in  certain  parts,  for  example  the  heel  of  man,  becomes  extremely 
thick. 

The  dermis  or  corium  is  composed  of  dense  connective  tissue, 
which  on  its  outer  side  is  thrown  up  into  a  series  of  folds  into  which 
pass  the  blood  vessels  nourishing  the  skin,  and  in  which  are  situated 
the  various  touch  corpuscles.  As  it  passes  inwards  the  texture  of  the 
dermis  becomes  looser,  and  it  incloses  islets  of  fatty  tissue,  and  finally 
it  merges  into  the  subcutaneous  tissue. 

Hairs,  the  characteristic  covering  of  the  mammalian  skin,  are 
the  products  of  the  epidermis,  and  are  borne  in  deep  tubular  pit-like 
downgrowths  of  that  layer  termed  the  hair  follicles.  Each  hair  is  a 
long  rod-like  structure  with  an  enlarged  root,  and  contains  an  axis 
or  medulla  of  large  angular  cells  containing  granules  and  sometimes 
air  globules.  Surrounding  this  is  a  fibrous  layer  composed  of  long 


272 


AN   INTRODUCTION  TO  ZOOLOGY 


fibrillated  cells,  which  in  dark-coloured  hair  contain  numerous 
granules  of  a  dense  pigment.  The  outside  is  covered  by  a  layer  of 
thin  imbricated  scales,  the  hair  cuticle.  At  the  bottom  of  the 
follicle  the  hair  expands  to  form  a  bulbous  root,  which  is  the  actual 
growing  point  and  composed  of  soft  protoplasmic  cells.  Into  this 
root  fits  a  projection  of  the  cutis  termed  the  hair  papilla,  which  is 
plentifully  supplied  with  blood-vessels.  Often  when  the  hair  is 
pulled  out  the  epidemal  portion  of  the  follicle  comes  with  it  so  that 
it  is  sometimes  termed  the  root  sheath.  Where  the  epidermis  is 


-M 


Sb 


FIG.  87. — Diagram  of  section  of  skin  of  mammal. 

C.,  corium  or  dermis  ;  E.,  epidermis  ;  F.,  fat ;  F.L.,  fibrous  layer  ;  H.,  hair  follicle  ;  H.C.,  hair 
cuticle  ;  M.,  medulla  ;  M.L.,  Malpighian  layer  ;  P.,  hair  papilla  ;  R.,  root  sheath  ;  Sb.,  sebaceous 
gland  ;  S.C.,  stratum  coraeum  ;  S.D.,  duct  of  sweat  gland  ;  Sg.,  sweat  gland. 

turned  in  to  form  the  follicle  it  is  composed  of  two  layers  as  on  the 
skin  itself  :  the  outer  one  corresponds  with  the  stratum  corneum,  and 
therefore  comes  to  lie  next  to  the  hair  itself,  and  is  termed  the  inner 
root  sheath,  being  in  itself  composed  of  three  layers  of  cells  ;  the 
outer  root  sheath  corresponds  with  the  rete  mucosum,  and  is  con- 
siderably thicker  at  the  upper  end,  but  thins  off  towards  the  base. 
Outside  these  epidermal  layers  the  cutis  furnishes  a  hyaline  layer  in 
the  form  of  a  fairly  thick  basement  membrane,  and  around  this  again, 
a  layer  composed  of  flattened  fibres  and  cells  circularly  arranged. 
The  nerves  of  the  corium  penetrate  the  hyaline  membrane  and 
arborise  on  the  outside  of  the  outer  root  sheath.  In  the  vibrissae 


LEPUS   CUNICULUS  273 

they  are  particularly  well  developed  and  exhibit  a  characteristic 
structure  :  such  sensory  hairs  also  possess  a  series  of  venous  sinuses 
at  their  bases. 

Primitively  connected  with  the  hair  follicle  of  the  mammal  are 
two  sets  of  glands,  the  sweat  and  the  sebaceous  glands.  The  latter 
are  small  saccular  glands  lined  by  a  secretory  epithelium  composed  of 
characteristic  large  .cells.  They  arise  singly  or  in  pairs  about  half- 
way down  the  follicle.  The  cells  secrete  a  fatty  substance  and  then 
themselves  disintegrate  in  order  to  produce  the  characteristic 
sebaceous  secretion.  The  sweat  glands  are  developed  from  higher 
up  the  follicle,  and  may  later  lose  connection  with  it  altogether. 
When  fully  formed,  they  consist  of  a  long  corkscrew-like  duct 
passing  deep  down  into  the  dermis  to  terminate  in  a  fairly  closely 
coiled  secretory  tubule.  The  secretory  portion  is  lined  by  a  cubical 
epithelium,  and  the  sweat  is  produced  in  the  cells  and  passed  out  into 
the  lumen.  In  man  these  glands  are  developed  all  over  the  general 
surface  of  the  body,  and  are  particularly  numerous  on  the  palms  of 
the  hand,  but  in  the  rabbit  they  are  restricted  to  certain  limited 
areas. 

The  mammary  glands  are  highly  modified  sweat  glands  pro- 
ducing their  secretion,  the  milk,  in  a  similar  manner,  and  they  are 
developed  in  connection  with  special  groups  of  hairs  along  the 
axillary-inguinal  line. 

The  muscular  system  of  the  rabbit  calls  for  little  special 
notice,  since  it  is  not  intended  to  enter  into  a  detailed  description  of 
the  individual  muscles.  It  is  divisible,  as  in  the  frog,  into  voluntary 
or  striate  muscles  and  involuntary  or  non-striate  muscles,  and  also 
the  heart  is  composed  of  characteristic  cardiac  muscle.  The  whole 
of  the  ventral  and  lateral  regions  of  the  trunk  and  the  neck  have 
a  thin  layer  of  cutaneous  muscle  attached  closely  to  the  inner  side 
of  the  skin,  and  so  enabling  the  animal  to  twitch  its  skin  quite 
independently  of  the  underlying  body  wall.  Recti  Abdomnis  and 
oblique  muscles  essentially  similar  to  those  in  Rana  are  found  in  the 
abdominal  region  ;  but  inside  the  oblique  is  a  further  thin  muscular 
layer,  the  transversalis  covered  by  the  peritoneum.  In  the  thoracic 
region,  however,  this  arrangement  is  broken  up  by  the  ribs,  and  we 
find  it  replaced  by  a  series  of  internal  and  external  intercostal 
muscles  which  play  an  important  part  in  respiration.  Yet  one 
further  structure  deserves  attention,  and  that  is  the  diaphragm. 
This  is  a  thin  arched  sheet  with  a  tendinous  centre  into  which  are 
inserted  radial  muscles  originating  in  the  ribs  and  on  the  vertebral 
column,  and  it  completely  separates  thoracic  and  abdominal 
cavities.  Two  particularly  strong  bands  known  as  the  pillars  of 
the  diaphragm  pass  dorsally  one  to  each  side  of  the  vertebral 

T 


274  AN   INTRODUCTION   TO   ZOOLOGY 

column.  Contraction  of  the  muscles  flattens  the  curvature  of  the 
diaphragm  and  so  enlarges  the  thoracic  cavity,  a  considerable  factor 
in  breathing. 

Skeleton. 

The  skeleton  in  the  very  young  mammal  is  entirely  carti- 
laginous, but  in  the  adult  comes  to  consist  almost  exclusively  of 
bone  which  may  be  cartilage  bone  replacing  the  cartilage  or  mem- 
brane bone  developed  in  the  membranes  around  the  cartilage.  In 
addition  to  this  we  find  in  the  mammals  a  third  variety,  the  sesamoid 
bones,  which  are  bony  nodules  developed  in  certain  tendons  where 
they  pass  over  a  joint,  and  they  serve  to  alter  the  direction  of  pull. 
The  skeleton  as  a  whole  is  composed  of  the  same  general  factors 
as  in  Rana,  and  like  it  is  divisible  into  an  Axial  portion  composed  of 
the  vertebral  column  and  skull  and  an  appendicular  portion 
comprising  the  limbs  and  girdles. 

The  vertebral  column  consists  of  a  long  chain  of  bones  about 
forty-five  in  number,  and  falls  into  five  definite  regions,  each  dis- 
tinguished by  certain  characteristic  features  :  these  are,  the  cervical 
with  seven  vertebrae,  the  thoracic  with  twelve  or  thirteen,  the 
lumbar  with  six  or  seven,  the  sacral  with  three  or  four,  and  the  caudal 
with  fifteen  or  sixteen.  A  typical  vertebra  consists  of  a  centrum, 
neural  arch  and  transverse  and  other  processes.  The  centrum  is  a 
short  bony  rod  that  ossifies  from  three  centres,  and  so  possesses  well- 
marked  epiphyses  which  do  not  become  firmly  fused  until  the  animal 
is  quite  old.  The  articulating  surfaces  of  the  centra  are  not  separated 
by  synovial  cavities  as  in  the  frog,  but  by  nbro-cartilaginous 
intervertebral  discs,  and  in  the  sacral  region  the  centra  are  actually 
fused  together.  The  neural  arch  consists  on  each  side  of  a  dorsal 
lamina  meeting  its  fellow  in  the  middle  line  at  the  neural  spine  and 
joined  to  the  centrum  by  a  narrow  portion,  the  pedicle,  so  leaving  at 
the  front  and  hinder  end  an  intervertebral  notch,  which  with  the 
similar  notch  on  the  adjoining  vertebra  leaves  an  intervertebral 
foramen  through  which  the  spinal  nerves  leave  the  neural  canal. 
Pre-  and  postzygapophyses  are  present  on  all  the  vertebrae  save  the 
first  two,  and  a  number  of  the  caudals  and  transverse  processes  on 
ah1  save  the  posterior  caudals. 

The  atlas  or  first  vertebra  differs  markedly  from  all  the  others, 
it  is  marked  by  the  presence  of  a  large  neural  canal  divided  in  life 
by  a  stout  transverse  ligament  (which  may.  persist  in  the  dried 
skeleton  in  a  shrivelled  condition  or  be  lost)  into  a  dorsal  portion, 
the  neural  canal  proper  and  a  smaller  ventral  portion  which  occupies 
much  of  the  space  usually  taken  up  by  the  centrum,  that  body  being 
much  reduced  in  size.  The  neural  spine  is  also  reduced,  being 


LEPUS   CUNICULUS 


275 


simply  a  low  ridge  most  marked  anteriorly.  At  the  front  end  are 
two  large  articular  facets  for  articulation  with  the  occipital  condyles 
of  the  skull.  The  transverse  process  is  a  wide  flat  lamina  grooved 
on  its  posterior  edge  for  the  second  spinal  nerve.  The  rib  is  repre- 
sented by  a  flat  plate  of  bone  joining  the  transverse  process  and 
leaving  a  triangular  aperture  for  the  passage  of  the  vertebral  artery. 


Cc  OF1 


FIG.  88. — Vertebrae  of  Lepus. 


I.,  Lumbar  vertebra,  side  view  ;  II., lumbar  vertebra,  front  end  ;  III.,  thoracic  vertebra,  side 
view  ;  IV.,  atlas,  dorsal  view,  slightly  reduced;  V.,  cervical  vertebra,  posterior  end;  VI.,  axis  or 
epistropheus,  side  view. 

A.,  anapophysis  ;  C.,  C'.,  facets  for  articulation  with  capitulum  of  rib  ;  Ce.,  centrum  ;  C.R., 
cervical  rib  ;  H.,  hypapophysis  ;  I.,  intervertebral  notch  ;  L.,  lamina  ;  L.F.,  lateral  surface  for 
articulatioVi  with  atlas;  L.F'.,  facet  for  articulation  with  lateral  surface  of  axis;  M.,  metapo- 
physis  ;  N.C.,  neural  canal ;  N.S.,  neural  spine  ;  O.,  odontoid  process  ;  O.F.,  articular  surface  on 
odontoid  process  ;  O.F'.,  articular  facet  for  articulation  with  odontoid  process  ;  P.,  prezygapo- 
physis  ;  Pe.,  pedicle  ;  P.O.,  postzygapophysis  ;  T.,  facet  for  articulation  with  tuberculum  of  rib 
T.P.,  transverse  process  ;  V.,  vertebrarterial  canal. 

This  aperture,  the  vertebrarterial  canal,  passes  upwards  through  a 
foramen  in  the  neural  canal  through  which  the  artery  enters  and  the 
first  spinal  nerve  leaves  the  canal. 

The  second  vertebra,  the  axis  or  epistropheus,  has  a  broad  flat 
centrum  produced  anteriorly  into  a  peg-like  odontoid  process,  which 
is  really  the  main  part  of  the  centrum  of  the  atlas  that  fuses  with  the 
centrum  of  the  axis  during  development.  The  neural  spine  forms  a 
large  vertical  spine  that  projects  forward  some  way  over  the  atlas. 
The  anterior  end  of  the  centrum  bears  two  facets  for  articulation, 
with  corresponding  surfaces  in  the  first  vertebra,  and  the  posterior 
end  bears  zygapophyses  that  are  quite  typical  and  have  their  facets 


276  AN   INTRODUCTION  TO  ZOOLOGY 

facing  outwards  and  downwards.  The  transverse  process  is  short, 
but  stout,  being  in  reality  a  portion  of  a  rib  fused  with  a  true  trans- 
verse process  and  the  combined  structure  is  perforated  by  a 
vertebrarterial  canal  through  which  the  vertebral  artery  runs. 

The  remaining  cervicals  are  characterised  by  a  small  flattened 
centra,  only  moderately  developed  neural  spines  and  stout  trans- 
verse processes  formed  by  the  fusion  of  part  of  a  rib  with  the 
transverse  process,  and  this  combined  structure  is  pierced  by  the 
vertebrarterial  canal. 

The  thoracic  vertebrae  are  readily  distinguished  by  bearing 
movably  articulating  ribs.  Their  neural  spines  are  very  strongly 
developed,  forming  long  stout  flattened  rods,  of  which  the  tenth  is 
approximately  vertical,  while  those  in  front  slope  backwards  and 
those  behind  forwards.  The  centrum  is  short  and  thick,  and  bears 
at  the  front  and  hinder  ends  a  small  facet  which,  with  that  of  the 
next  vertebra,  constitutes  an  articulating  area  for  the  head  of 
the  rib.  In  the  last  three  or  four  this  surface  lies  entirely  on  the 
anterior  end  of  one  centrum,  and  is  not  shared  by  the  two.  The 
transverse  processes  are  well  developed,  and  have  an  articular 
surface  on  their  under  face,  for  the  tubercle  of  the  rib  :  these  facets 
are  lacking  from  the  last  three  or  four  vertebrae.  On  the  neural 
arch  of  the  ninth  vertebra  in  the  series  a  pair  of  slightly  lateral 
wing-like  processes  passing  dorsally  appear.  These  are  the  meta- 
pophyses,  and  from  the  tenth  onwards  they  are  joined  with  the 
prezy  gapophy  ses . 

The  lumbar  vertebrae  possess  stout  centra ;  their  neural  spines 
are  not  so  high,  but  are  blade-like,  and  their  metapophyses  well 
developed.  The  transverse  processes  are  short  flattened  rods 
projecting  outwards  and  downwards.  In  addition  to  these  other 
processes  are  also  developed,  the  hypapophyses,  triangular  blade- 
like  projections  from  the  mid  ventral  line  of  the  centrum,  and  the 
anapophyses,  small  short  processes  running  backwards  from  the 
hinder  end  of  the  neural  arch  below  the  postzygapophyses  but  above 
the  inter  vertebral  notches. 

The  sacral  vertebrae  are  all  fused  together,  but,  nevertheless, 
the  lines  of  junction  between  them  can  be  easily  identified.  Strictly 
speaking,  only  those  vertebrae  articulating  with  the  ilia,  generally 
one  but  at  most  two,  can  be  termed  the  sacrum,  but  it  is  customary 
to  apply  the  term  to  the  whole  of  the  vertebrae  fused  with  them  to 
form  one  mass. 

The  caudal  vertebrae  rapidly  decrease  in  size  and  complexity, 
losing  their  processes,  and  even  towards  the  end  of  the  tail,  their 
neural  arches,  so  that  the  terminal  members  of  the  series  are  repre- 
sented only  by  their  rod-like  centra. 


LEPUS  CUNICULUS 


277 


Ribs  and  Sternum. 

The  ribs  in  the  rabbit  number  twelve  or  sometimes  thirteen 
pairs,  and  together  with  the  backbone  and  the  sternum  constitute 
a  bony  basket  in  which  lies  the  thorax.  Each  rib  is  a  curved,  some- 
what flattened  rod  of  bone  passing  out  laterally  and  slightly  back- 
wards from  the  vertebral  column.  The  dorsal  end  is  enlarged  to 


M 


5r 


FIG.  89 — Rib  of  Lepus  viewed 
from  anterior  aspect. 

C.,  capitulum  ;  S.,  sternal  portion ; 
T.,  tuberculum  ;  V.,  vertebral  por- 
tion. 


FIG.  90. — Sternum  of  Lepus  from 
ventral  surface. 

M.,  manubrium ;  S.,  sternal  portions  of 
ribs  ;  St.,  sternebrap ;  V.,  vertebral  por- 
tions of  ribs  ;  X.,  xiphisternum. 


form  a  knob-like  head  or  capitulum  which  articulates  with  the 
facets  on  the  centra.  Just  lateral  to  this  is  given  off  a  dorsal  pro- 
jection, the  tuberculum,  which,  as  we  have  seen,  articulates  with 
a  facet  on  the  under  side  of  the  transverse  process  of  a  thoracic 
vertebra ;  but  such  a  tubercle  is  absent  on  the  posterior  three  or 
four  pairs  which  have  only  a  capitulum.  To  the  lateral  extremity 
of  each  bony  vertebral  part  of  the  rib  is  attached  a  short  bar  of 
calcified  or  incompletely  ossified  cartilage,  the  sternal  portion, 


278  AN   INTRODUCTION   TO  ZOOLOGY 

almost  at  right  angles.  This  passes  inwards  towards  the  mid-ventral 
line  The  first  seven  pairs  of  these  directly  join  the  sternum,  the 
next  two  almost  reach  it,  and  are  each  connected  with  the  rib  in 
front.  The  remaining  pairs  are  not  so  complete  and  not  so  attached, 
and  in  consequence  sometimes  termed  the  floating  ribs. 

The  sternum  is  a  moderately  wide  flattened  rod  lying  in  the  mid 
ventral  line  and  divided  into  six  segments,  or  sternebrse.  The  first, 
termed  the  manubrium,  is  larger  than  the  rest  and  possesses  a  marked 
ventral  ridge-like  keel ;  to  it  is  joined  the  first  pair  of  ribs.  The 
posterior  segments,  the  xiphisternum,  is  a  long  slender  one,  ter- 
minating in  a  thin  flattened  rounded  cartilaginous  plate.  It  has 
been  found  that  in  the  embryo  the  sternebrae  develop  from  the 
ventral  ends  of  the  ribs,  and  it  is  a  matter  of  doubt  whether  they 
are  to  be  regarded  as  strictly  homologous  with  the  segments  of  the 
sternum  in  Rana. 

Skull. 

The  skull  of  a  mammal  is  modified  to  adapt  it  for  the  acqui- 
sition of  its  food,  and  in  the  case  of  the  rabbit,  a  Rodent  or  gnawing 
animal,  this  process  of  modification  has  been  carried  so  far  that 
it  has  resulted  in  a  highly  specialised  structure  that  is  considerably 
removed  from  what  may  be  considered  a  generalised  mammalian 
skull.  For  that  reason  it  is  better  to  study  another  form,  say 
the  dog,  in  order  to  get  a  general  idea  of  the  structure  of  a  skull, 
and  so  it  is  only  necessary  here  to  refer  to  certain  of  its  main  features 
in  the  rabbit. 

For  the  purposes  of  description  we  can  divide  the  skull  into  a 
larger  posterior  cranial  region  and  a  smaller  anterior  facial  region. 
At  each  side  of  the  former,  towards  the  front,  lies  a  large  hollow,  the 
orbit,  in  which  the  eye  is  situated,  while  behind  this  the  auditory 
capsule  is  actually  fused  with  the  cranium  proper.  The  actual 
portion  occupied  by  the  brain  case  is  relatively  small,  since  in  their 
middle  regions  the  orbits  are  only  separated  by  a  thin  bony  partition, 
the  interorbital  septum,  perforated  by  the  optic  foramen,  and  the 
brain  does  not  extend  directly  in  front  of  or  below  this.  The  various 
cartilage  and  membrane  bones  comprising  the  skull  remain  more  or 
less  distinct  throughout  life,  being  united  along  their  edges  by  wavy 
junctions,  the  sutures.  In  the  following  account  the  cartilage  and 
membrane  bones  will  be  distinguished  by  placing  the  letters  c  or 
m  in  brackets  after  their  names. 

The  facial  region  consists  mainly  of  an  elongated  nasal  capsule. 
This  is  roofed  by  the  paired  somewhat  long  nasal  bones  (m)  and  its 
antero-lateral  and  ventro-lateral  wells  formed  by  the  premaxillse 
(m).  Each  premaxilla  carries  two  incisor  teeth,  the  anterior  of 


LEPUS   CUNICULUS 


279 


which  is  broad,  long  and  curved  and  the  posterior  immediately 
behind  it  is  rounder,  smaller  and  less  curved.  The  enamel  layer 
is  more  strongly  developed  on  the  front  than  on  the  hinder  edge 
of  the  anterior  incisor,  with  the  result  that  use  keeps  it  sharpened 
as  the  hinder  edge  wears  away  quicker  than  the  front,  always 
preserving  a  chisel-like  cutting  surface.  A  long  narrow  continuation 
of  the  premaxilla,  the  nasal  process,  runs  backwards  alongside  the 
nasal  bone.  The  space  left  on  the  ventral  side  of  the  nasal  capsule 
between  the  premaxillae  is  in  part  filled  by  two  median  ventral 


PP 


Tb. 


FIG.  91. — Lepus.     Lateral  view  of  skull.    - 

A.,  superior  incisor  ;  B.,  inferior  incisor  ;  C.,  nasal ;  D.,  pre-maxilla  ;  E.,  mandible  ;  F.,  angle 
of  mandible  ;  G.,  zygomatic  process  of  maxilla  ;  H.,  frontal ;  I.,  parietal ;  I.F.,  infra-orbital 
foramen  ;  J.,  squamosal ;  Ju.,  jugal ;  K.,  interparietal ;  L.,  supra-occipital ;  La.,  lachrymal  ; 
M.,  optic  foramen  ;  MX.,  maxilla  ;  N.,  external  auditory  meatus  ;  O.C.,  occipital  condyle  ;  P.P., 
paroccipital  process  ;  Pr.,  premolar  ;  T.B.,  tympanic  bulla. 

backwardly  running  plates,  the  palatine  processes,  one  from  each 
premaxilla.  At  the  hinder  end  the  two  nasal  cavities  are  separated 
by  a  complete  bony  plate  coming  from  the  mesethmoid  bone  (c)  and 
termed  the  septum  narium  ;  this  is  continued  in  the  living  animal, 
to  the  front  end  of  the  cavities  as  a  cartilaginous  lamina  which  is 
generally  absent  in  prepared  skulls.  Practically  the  whole  of  the 
nasal  cavity  on  each  side  is  filled  by  a  number  of  extremely  delicate 
much  coiled  bones,  the  turbinals  (c),  termed  the  ethmo-turbinals, 
maxillo-turbinals  or  naso-turbinals  according  to  the  bone  to  which 
they  are  attached.  Much  of  the  wall  of  the  postero-lateral  and 
postero-ventral  regions  of  the  olfactory  capsule  is  completed  by 
processes  of  the  maxilla  (m)  proper,  the  main  mass  of  which,  how- 
ever, lies  in  the  cranial  region. 


280  AN   INTRODUCTION  TO  ZOOLOGY 

The  roof  of  the  cranial  region  of  the  skull  is  composed  of  paired 
frontal  (m)  and  parietal  (m)  bones  and  posteriorly  by  a  small  inter- 
parietal  (m)  bone.  The  f rentals  are  large  bones  forming  not  only 
the  roof  but  also  the  sides,  and  right  in  front  also  the  floor  of  the  brain 
case.  In  front  they  meet  the  nasals  and  behind  the  parietals.  They 
send  a  very  thin  process  forwards  which  separates  the  nasal  process 
of  the  premaxilla  from  the  maxilla  for  some  distance,  and  again  over 
each  orbit  they  send  out  a  prominent  crescent-shaped  supra-orbital 
process.  The  parietals  are  somewhat  smaller  bones  meeting  the 
frontals  anteriorly,  the  interparietal  and  supra-occipitals  posteriorly 
and  the  squamosals  along  the  side.  The  median  suture  where 
the  nasals,  frontals  and  parietals  of  the  two  sides  meet  is  termed 
the  sagittal  suture.  The  interparietal  is  a  small  oval  bone  bounded 
by  the  parietals  in  front  and  the  supra-occipitals  behind. 

The  side  wall  of  the  cranial  region  comprises  a  number  of  bones. 
At  the  antero- ventral  end  lies  the  maxilla  (m).  This  is  a  complex 
bone  whose  main  mass  forms  the  upper  jaw  and  bears  a  series  of 
six  teeth  ;  three  are  termed  pre-molars  and  three  molars.  The  two 
sets  are,  in  Lepus,  indistinguishable  in  structure  and  consequently 
all  may  be  described  as  molariform.  They  are  the  grinding  teeth 
and  between  them  and  the  incisors  there  is  a  long  gap,  the  diastema. 
Teeth  corresponding  with  the  canine  teeth  of  other  mammals  are 
lacking.  The  antero-lateral  plate  of  the  maxilla,  forming  a  large 
part  of  the  side  of  the  nasal  capsule,  is  cancellous.  On  the  ventral 
side  the  maxilla  sends  in  a  short  stout  palatine  process  which  joins  its 
fellow  in  the  middle  line,  and  these,  with  similar  ad  joining  processes 
of  the  palatine  bone,  constitute  the  bony  floor  of  the  narial  passage  : 
this  floor  is  the  hard  palate  and  forms  also  part  of  the  roof  of  the 
mouth.  From  the  side  of  the  maxilla  is  given  off  a  stout  zygomatic 
process,  which  also  continues  upwards  as  an  orbital  process  forming 
the  front  wall  of  the  orbit.  The  remainder  of  the  anterior  orbital 
wall  is  completed  by  a  small  lachrymal  bone  (m)  which  is  interpolated 
between  the  orbital  process  of  the  maxilla  and  the  frontal  bone  and 
is  perforated  by  a  canal  for  the  duct  of  the  lachrymal  gland.  The 
inner  wall  of  the  orbit  is  completed  by  the  orbito-sphenoid  (c), 
palatine  (m),  squamosal  (m)  and  the  alisphenoid  (c)  bones.  The 
orbito-sphenoid  is  a  lamella  of  bone  surrounding  the  orbital  fora- 
men ;  it  is  bounded  above  by  the  frontal,  in  front  by  the  maxilla, 
below  by  the  palatine  and  behind  by  the  squamosal.  The  palatine 
is  an  almost  vertical  plate  of  bone  forming  the  side  walls  of  the 
posterior  end  of  the  narial  passage.  At  its  front  end  it  gives  off  a 
palatine  process  which,  as  we  have  seen,  takes  part  in  the  formation 
of  the  hard  palate.  In  addition,  the  palatine  also  forms  the  mid 
ventral  part  of  the  inner  orbital  wall  where  it  joins  the  maxilla  in 


LEPUS   CUNICULUS  281 

front  and  the  alisphenoid  behind.  The  squamosal  is  a  large  bone 
whose  anterior  portion  takes  part  in  the  formation  of  the  hinder  wall 
of  the  orbit.  Its  lateral  portion  helps  to  form  the  side  wall  of  the 
hinder  region  of  the  cranial  cavity.  Laterally  it  gives  off  a  con- 
spicuous zygomatic  process,  on  the  postero- ventral  surface  of  which  is 
a  smooth  facet  for  the  articulation  of  the  lower  jaw.  The  alisphenoid 
is  a  wing-shaped  bone,  part  of  which  is  embedded  in  the  posterior 
corner  of  the  orbit.  From  its  ventral  border  arises  a  small  trans- 
versely lying  plate  of  bone,  the  external  pterygoid  process,  perforated 
by  three  holes  ;  the  innermost  is  for  the  passage  of  the  internal 
maxillary  artery  and  vein  and  the  outermost  for  branches  of  the 
mandibular  trunk  of  the  trigeminal  nerve.  At  the  innermost 
ventral  corner  of  the  alisphenoid  bone  is  a  large  vertical  slit,  the 
foramen  lacerum  anterium  or  sphenoidal  fissure,  which  permits  the 
exit  of  the  third,  fourth  and  sixth  cranial  nerves  and  the  first  two 
branches  of  the  fifth.  The  external  lower  border  of  the  orbit  is 
composed  of  the  zygomatic  processes  of  the  maxilla  and  the 
squamosal  and  a  thin  strip  of  bone  lying  in  the  vertical  plane,  the 
malar  or  jugal  (m),  which  joins  them.  The  three  structures  together 
forming  a  curved  zygomatic  arch  or  zygoma. 

The  extreme  front  end  of  the  cranial  cavity  is  bounded  by  a 
vertical  transverse  plate  of  bone,  the  mesethmoid  (c),  which  is  not 
visible  externally  and  completely  separates  the  cranial  and  nasal 
cavities.  The  septum  narium,  already  dealt  with,  is  a  median  for- 
ward projection  of  this  bone  and  its  lateral  wings,  termed  the 
cribriform  plates,  are  perforated  by  a  number  of  holes  which  transmit 
the  various  branches  of  the  olfactory  nerve. 

The  base  of  the  cranial  region  between  the  palatines  is  formed  by  a 
thin  vertical  plate,  the  presphenoid  (c),  which  forms  the  lower  border 
of  the  orbital  foramen.  Within  the  skull,  on  the  floor  of  the  cranial 
cavity,  it  is  produced  into  a  small  transverse  anterior  clinoid  process. 
Immediately  behind  this  bone  is  the  basisphenoid  (c),  a  median 
triangular  fairly  thick  bone  which  is  bounded  laterally  by  the  ali- 
sphenoids  and  posteriorly  by  the  basioccipital.  It  is  perforated  in 
the  mid-ventral  line  by  a  small  hole,  the  pituitary  foramen.  On 
the  inner  floor  of  the  cranial  cavity  it  is  produced  upwards  into  a 
transverse  post-clinoid  process,  and  between  this  and  the  anterior 
process  in  a  depression,  the  sella  turcica,  within  which  lies  the  pituitary 
body.  Just  at  the  junction  of  the  basi  and  alisphenoid  is  attached 
a  small,  thin,  vertical  bone,  the  pterygoid,  which  is  joined  to  the 
palatine  in  front  and  passes  downwards  in  a  free  hamular  process. 

The  auditory  capsule  is  composed  of  a  series  of  bones  which  in 
the  adults  fall  into  two  portions,  the  periotic  and  the  tympanic.  The 
periotic  is  an  irregular  mass  comprising  developmentally  three  separate 


282  AN   INTRODUCTION   TO   ZOOLOGY 

elements,  the  pro-otic,  the  epi-otic  and  the  opisth-otic  and  is  divisible 
into  two  secondary  regions,  a  very  dense  hard  petrous  portion  enclosing 
the  membranous  labyrinth  and  a  more  cancellous  mastoid  portion 
which  is  visible  on  the  outside  of  the  skull  and  bounded  posteriorly 
by  the  exoccipital  bone.  The  surface  of  the  periotic  facing  the 
inside  of  the  cranium  has  a  deep  pit-like  depression,  the  floccular 
fossa,  in  which  is  contained  a  projecting  portion  of  the  cerebellum 
termed  the  floccular  lobe.  Below  this  are  two  openings  side  by  side  ; 
the  hinder  one  is  the  meatus  auditorius  internus  through  which  the 
auditory  nerve  passes  from  the  brain  to  the  inner  ear,  and  the  other 
foramen  is  the  aquaeductus  Fallopii,  transmitting  the  facial  nerve. 
The  outer  surface  of  the  petrous  portion  is  perforated  by  two  holes 
which,  however,  cannot  be  seen  until  the  tympanic  bone  has  been 
removed.  The  anterior  one  is  the  fenestra  ovalis  and  the  posterior 
one  is  the  fenestra  rotunda ;  both  lead  to  the  internal  ear  from  the 
tympanic  cavity.  The  hollow  tympanic  bone  has  the  shape  of  a 
short-necked  flask.  Its  expanded  part,  termed  the  bulla,  contains 
the  tympanic  cavity,  while  the  neck  is  the  meatus  auditorius  externus 
or  external  ear  aperture.  Across  the  base  of  the  meatus  in  life  lies  the 
tympanic  membrane,  which  is  tightly  stretched  over  an  incomplete 
bony  ring.  The  Eustachian  tube  leaves  the  tympanic  cavity  by  an 
aperture  at  the  an tero- ventral  corner  of  the  tympanic  bulla  and 
opens  into  the  posterior  part  of  the  narial  passage.  Just  next  to 
this,  between  the  tympanic  bone  and  the  alisphenoid,  is  the  foramen 
lacerum  medium  for  the  transmission  of  the  mandibular  branch  of 
the  trigeminal  nerve.  On  the  postero- ventral  aspect  of  the  bulla 
is  a  round  foramen  through  which  the  internal  carotid  artery  passes 
into  the  cranial  cavity.  Between  the  postero-lateral  border  of  the 
bulla  and  the  mastoid  portion  of  the  periotic  is  the  stylomastoid 
foramen,  through  which  runs  the  main  branches  of  the  facial  nerve. 
Across  the  tympanic  cavity  stretches  a  string  of  bones  termed  collec- 
tively the  auditory  ossicles.  The  outermost  of  these,  the  malleus  (c), 
is  a  hammer-shaped  bone  with  its  blade-like  handle  attached  to  the 
tympanic  membrane  and  also  articulating  with  the  second  bone, 
the  incus  (c) .  This  is  a  pyrif orm  bone  produced  behind  into  a  short 
process  and  below  into  a  short  stalk  bent  inwards.  To  the  end  of 
this  is  attached  a  small  disc  of  bone,  the  OS  orbiculare  (c) ,  which  in 
turn  articulates  with  the  arch  of  a  stirrup-shaped  bone,  the  stapes  (c) . 
The  basal  oval-shaped  part  of  this  bone  is  attached  to  the  membrane 
closing  the  fenestra  ovalis.  By  means  of  this  chain  of  bones  the 
vibrations  of.  the  tympanic  membrane  are  transmitted  to  the  peri- 
lymph  surrounding  the  membranous  labyrinth. 

The  posterior  end  of  the  cranial  region  is  composed  of  a  ring  of 
four  bones,  termed  the  occipital  segment.     The  dorsal  member  is 


LEPUS   CUNICULUS  283 

the  supra-occipital  (c),  a  large  median  bone  joining  the  parietals  in 
front,  the  squamosals  and  periotics  at  the  side  and  the  exoccipitals 
ventro-laterally,  while  its  mid  ventral  portion  forms  part  of  the  wall 
of  the  foramen  magnum.  Its  postero-dorsal  portion  is  marked  by 
strong  ridges  into  which  are  inserted  the  muscles  raising  the  head. 
The  lateral  bones  are  the  exoccipitals  (c),  which  form  the  major  part 
of  the  wall  of  the  foramen  magnum.  They  bear  posteriorly  strong 
curved  smooth  ridges,  the  occipital  condyles,  which  articulate  with 
the  condylar  facets  of  the  atlas.  Laterally  each  bone  is  produced 
downwards  as  a  paroccipital  process  behind  the  periotic  bone. 
Between  the  posterior  border  of  the  tympanic  bulla  and  the  exocci- 
pital  is  an  irregular  fissure,  the  foramen  lacerum  posterius,  through 
which  passes  the  ninth,  tenth  and  eleventh  cranial  nerves  and  the 
internal  jugular  vein.  The  twelfth  nerve  leaves  the  cranium  by  two 
branches  which  pass  through  the  body  of  the  condyle,  and  the 
condylar  foramina  that  transmit  them  are  best  seen  on  the  inside 
of  the  condyles.  The  basal  part  of  this  ring  of  bones  is  completed 
by  a  large  basioccipital  (c)  ;  it  forms  the  ventral  wall  of  the  foramen 
magnum  and  contributes  a  small  part  of  each  condyle. 

The  lower  jaw  or  mandible  (m)  consists  of  a  pair  of  flattened 
triangular  masses  of  bone,  thin  and  broad  posteriorly,  but  getting 
thicker  and  narrower  as  they  pass  forward  till  they  finally  unite 
in  the  symphysis  menti.  At  the  anterior  end  each  half  or  ramus 
bears  a  single  incisor  tooth  ;  this  is  followed  by  a  diastema,  and  this 
by  two  premolars  and  three  molars  all  molarif orm  and  similar  to  those 
in  the  upper  jaw.  Each  ramus  gives  off  posteriorly  a  broad,  thin 
ascending  coronoid  process  which  passes  up  to  articulate,  by  means  of 
a  transverse  expansion  of  its  upper  end,  with  the  under  side  of  the 
zygomatic  process  of  the  squamosal.  The  postero- ventral  border  of 
the  ramus  is  known  as  the  angle  and  its  edge  is  slightly  incurved.  On 
the  inner  side  of  each  ramus  just  behind  and  below  the  last  molar 
tooth  is  an  aperture,  the  inferior  dental  foramen,  through  which  a 
branch  of  the  mandibular  nerve  enters  the  bone  to  supply  the  teeth. 
The  hyoid  bone  (c)  is  a  small  bone  embedded  in  the  muscles 
of  the  base  of  the  tongue  between  the  hinder  ends  of  the  rami. 
It  possesses  a  body  and  two  pairs  of  cornua.  The  anterior  cornua 
represent  the  persistent  remains  of  the  hyoid  arch  of  the  embryo 
and  the  posterior  cornua,  also  termed  thyrohyals,  the  lower  ends 
of  the  first  branchial  arch. 

Appendicular  Skeleton. 

We  now  pass  on  to  consider  the  remaining  part  of  the 
skeleton  comprising  the  pectoral  and  pelvic  girdles  and  the  fore 
and  hind  limbs.  All  of  its  main  bones  are  cartilage  bones  save 


284 


AN   INTRODUCTION  TO  ZOOLOGY 


only  the  clavicle  which  is  a  membrane  bone,  but  in  addition  there 
are  also  developed  in  certain  of  the  tendons,  sesamoid  bones. 

Pectoral  Girdle. 

Compared  with  Rana  we  find  the  pectoral  girdle  much 
reduced  and  consisting  mainly  of  two  elements,  a  dorsal  and  a 


^o-'S  W- 


I  tMjii 

&    i*:s'°*?Z' 


ii! 


1 


ventral.  The  scapula,  or  shoulder  blade,  is  a  thin  flat  triangular 
plate  of  bone.  The  apex  of  the  triangle  bearing  the  glenoid  facet, 
lies  laterally  over  the  first  rib  and  the  remainder  passes  back  dorsally 


LEPUS  CUNICULUS 


285 


towards  the  middle  line.  The  bone  is  thickened  in  the  region 
of  the  glenoid  facet  which  bears  on  its  edge  a  small  inwardly  pro- 
jecting coracoid  process,  the  representative  of  part  of  the  coracoid 
portion  of  the  girdle  of  the  frog.  The  outer  surface  of  the  scapula 
bears  a  prominent  keel-shaped  spine  which  increases  in  depth  as 
it  passes  to  the  glenoid  facet.  Just  before  reaching  this  it  becomes 
free  from  the  plate  and  continues  as  a  slender  flattened  rod,  the 
acromion,  from  the  end  of  which  is  given  off  at  right  angles  a  back- 
wardly  projecting  process,  the  metacromion.  The  supra-scapula 
of  the  frog  is  represented  in  Lepus  by  a  cartilaginous  supra-scapular 
border  attached  to  the  dorsal  edge  of  the  scapula.  The  clavicle 
or  collar  bone  of  the  rabbit  is  a  small  slender  curved  rod  of  bone 
attached  by  fibrous  tissue  to  the  coracoid  process  at  the  one  end 
and  to  the  sternum  at  the  other. 

Pelvic  Girdle. 

As  in  the  frog,  the  pelvic  girdle  has  undergone  a  rotation 
from  its  original  plane,  until  instead  of  being  approximately  at 


FIG.  93. — A,  Right  scapula  of  Lepus,  dorsal  aspect;    B,  Left  os  innominatum 
Lepus,  ventral  aspect. 

a.,  acromion  ;  ac.t  acetabulum  ;  a.s.,  articular  surface  for  sacrum  ;  c.,  cotyloid  ;  c.p.,  coracoid 
process;  g.t  glenoid  facet ;  g.f.,  gluteal  fossa  ;  »./.,  iliac  fossa ;  U.,  ilium;  is.,  ischium  ;  i.t.,  ischia 
tuberosity  ;  m.,  metacromion  ;  o.f.,  obturator  foramen  ;  p.,  pubis  ;  p.s.,  pubic  symphysis  ;  s., 
spine  ;  s.s.,  supra-scapular  cartilage. 

right  angles  to  the  vertebral  column  it  comes  to  lie  almost  parallel 
with  it.  The  girdle  is  composed  of  two  halves  only  feebly  united 
in  the  mid- ventral  line  ;  each  is  roughly  in  the  form  of  a  P,  and  is 
termed  the  OS  innominatum.  This,  however,  really  consists  of 


286  AN   INTRODUCTION   TO   ZOOLOGY 

three  elements,  the  ilium,  the  ischium  and  the  pubis,  closely  fused 
together  and  all  taking  part  in  the  formation  of  the  acetabulum,  a 
deep  cavity  in  which  the  head  of  the  femur  articulates.  The  ilium 
is  the  anterior  dorsal  bone  of  the  girdle  on  each  side  and  is  flattened 
in  the  vertical  plane.  Its  inner,  sacral  surface,  is  roughened  for 
junction  with  the  sacrum.  The  outer  surface  is  marked  by  a 
longitudinal  ridge,  leaving  a  shallow  dorsal  depression,  the  gluteal 
fossa,  and  a  similar  ventral  one,  the  iliac  fossa.  It  forms  about  one- 
half  of  the  acetabulum.  The  ischium  is  the  second  largest  bone  in 
the  girdle  passing  backwards  from  the  acetabular  cavity,  of  which 
it  forms  about  one- third,  in  the  same  line  as  the  ilium.  About 
half-way  back  it  gives  off  dorsally  a-  blunt  ischial  spine  and  it 
terminates  in  an  expanded  roughened  enlargement,  the  ischial 
tuberosity.  The  smallest  bone  is  the  pubis,  whose  contribution  to 
the  acetabulum,  less  than  one-third,  usually  ossifies  separately  and 
is  termed  the  cotyloid  bone.  It  passes  ventrally  as  a  thin  strip  of 
bone  to  meet  its  fellow  in  the  middle  line  in  the  pubic  symphysis, 
continuing  this  union  backwards  until  it  joins  the  inner  portion  of 
the  expanded  end  of  the  ischium.  Save  for  this  posterior  junction, 
only  visible  in  young  animals,  the  pubis  and  ischium  are  separated 
by  a  large  hole,  the  obturator  foramen. 

Fore  Limb. 

The  fore  limb  of  the  rabbit  is  composed  of  the  same  main 
divisions  as  in  the  frog,  but  it  is  more  highly  specialised,  and  it 
differs  considerably  in  the  details  of  its  structure. 

The  humerus  is  a  long  bone  with  a  well-developed  rounded  head, 
articulating  with  the  glenoid  facet  of  the  scapula,  and  an  enlarge- 
ment, the  trochlea,  at  its  distal  end  articulating  with  the  radius  and 
ulna.  Near  the  head  are  two  roughened  projections,  an  outer  or 
greater  tuberosity,  and  an  inner  or  lesser  tuberosity,  and  they  are 
separated  by  a  bicipital  groove  for  the  tendrons  of  the  biceps  muscle. 
The  trochlea  is  a  ridged  pulley-like  surface  just  above  which  on  each 
side  is  a  deep  supra  trochlear  fossa,  and  these  two  fossae  communicate 
with  one  another  by  means  of  a  supra  trochlear  foramen.  The 
posterior  of  the  two  is  considerably  the  larger  and  often  termed  the 
olecranon  fossa,  since  it  lodges  part  of  the  olecranon  process  when  the 
arm  is  fully  extended.  The  radius  and  ulna  are  almost  of  equal 
size,  and  although  not  actually  fused  are  immovably  articulated. 
The  radius  is  the  inner  bone  and  articulates  proximally  with  the 
trochlear  notches,  while  distally,  where  it  is  somewhat  enlarged,  it 
touches  the  carpal  bones.  The  ulna  has  a  small  distal  end,  but  its 
proximal  extremity  is  enlarged  and  marked  by  a  deep  groove,  the 
sigmoid  notch,  in  which  lies  part  of  the  trochlea,  while  it  is  continued 


LEPUS   CUNICULUS 


287 


on  beyond  the  joint  as  a  laterally  compressed  olecranon  process. 
The  carpus  or  wrist  consists  of  nine  bones.  The  three  proximal 
carpalia,  i.e.  the  radiale,  intermedium  and  ulnare,  are  three  small 
bones  articulating  with  the  radius  and  ulna  and  sometimes  termed 
the  navicular  (scaphoid) ,  lunate  (semilunar)  and  triquetral  (cuneiform) 
respectively.  The  distal  carpalia  in 
order  from  the  radial  to  the  ulnar  side 
are  the  greater  multangular  (trapezium), 
the  lesser  multangular  (trapezoid),  the 
capitate  (os  magnum),  and  the  hamate 
(cuneiform)  ;  the  last  named  represent- 
ing the  fusion  of  two  separate  elements. 
Between  the  proximal  and  distal  car- 
palia lies  another  small  bone,  the 
centrale,  and  on  the  under  side  of  the 
carpus  articulating  with  the  ulna  and 
navicular  is  a  small  pisiform  bone  usually 
regarded  as  being  a  sesamoid  bone. 
There  are  five  metacarpals,  of  which 
the  first  is  markedly  smaller  than  the 
rest.  The  digits  are  also  five  in  number, 
of  which  the  innermost,  the  thumb  or 
pollex,  is  much  shorter  than  the  others, 
possessing  only  two  phalanges  as  against 
three  in  the  remainder.  (7  B  *  IV 

Hind  Limb.  ll        »    III 

The   hind   limb   is    noticeably    FlG    94._Left  forefoot  of 
larger  than  the  fore  limb  and  plays  a       Lepus  viewed  from  extensor 
far  more  important  part  in  locomotion. 
The   femur   is   a  long  bone  with   a 


un. 


surface. 


C.,  capitate  ;    Ce.,  centrale  ;   H., 
hamate  ;   I.,  intermedium  ;  M.,  great 


-     —  hamate;   I.,  intermedium  ;  M.,  great 

Cylindrical   Shaft    and    tWO    enlarged    ex-      multangular;   ».,  lesser  multangular; 
J  .  .  metacaral     P.      halanx     R. 


., 

M.C.,  metacarpal  ;   P.,  phalanx  ;   R., 
radiale,     navicular;       Ra.,     radius; 


.  .  ,          f     ,  , 

tremitieS.        On     the     inner    Side    OI     trie      radiae,     navcuar;         a.,     raus; 
-,       •  i  ,•  RE.  epiphysis  of  radius  ;   U.,  ulnare, 

proximal    end    is    a    large    projecting    triquetrai  ;  U.E.,  epiphysis  of  ulna  ; 
rounded    head    which    lies    partly    en-    ^^.;  u%:;  <Sl£?  pb* 
closed    within    the    acetabular    cavity. 

On  the  outer  side  of  the  proximal  end,  opposite  the  femur,  is 
a  large  rough  mass,  the  great  trochanter,  and  between  that  and 
the  head  on  the  hinder  side  is  a  deep  trochanteric  fossa.  Just 
below  the  head  are  two  smaller  projections,  one  on  each  side  ; 
that  on  the  inside  being  the  lesser  trochanter  and  that  on  the  outside 
the  third  trochanter.  The  distal  end  of  the  bone  is  swollen  out  to 
form  two  large  condyles  separated  by  an  intercondylar  notch  ;  it 
articulates  with  the  tibia  and  fibula.  Over,  the  front  of  this  joint 


288 


AN   INTRODUCTION   TO  ZOOLOGY 


lies  a  large  sesamoid  bone,  the  patella  or  knee-cap,  and  behind  it  are 
another  series  of  small  bones,  the  fabellse. 

The  tibia  and  fibula  are  two  long 
bones,  the  former  much  larger  than 
the  latter,  which  are  free  proximally 
but  fused  distally.  The  tibia  is  a 
long  stout  bone  with  an  enlarged 
proximal  extremity  for  articulation 
with  the  femur  and  bearing  on  its 
anterior  surface  a  prominent  cnemial 
crest,  while  its  distal  end  articulates 
with  the  talus.  The  fibula  is  a 
slender  bone,  free  proximally  but 
fusing  distally  with  the  tibia  and 
also  bearing  a  facet  for  articulation 
with  the  calcaneum.  The  tarsalia  in 
Lepus  have  been  reduced  to  six 
bones.  The  proximal  row  consists 
of  two  bones,  of  which  the  inner 
represents  the  tibiale  and  inter- 
medium fused  and  is  often  termed 
the  talus,  while  the  outer  or  fibulare 
is  termed  the  calcaneum.  The 
centrale  or  navicular  is  a  bone  con- 
tinuing the  line  of  the  talus  and 
sending  forward  a  slender  process 
on  the  under  surface  of  the  foot. 
There  are  but  three  distal  carpalia, 
the  first  or  ento-cuneiform  is  alto- 
gether lacking  as  a  separate  element, 
but  is  probably  fused  with  the  second 
metatarsal  so  that  the  innermost  is 
really  the  second  or  meso-cuneif  orm ; 
the  next  is  the  third  ecto-cuneiform 
and  the  remaining  bone  represents 
two  tarsalia  fused,  and  is  termed  the 
cuboid.  Only  four  metatarsals  are 
present,  the  first,  corresponding  with 
the  metatarsal  of  our  big  toe,  is 
absent,  as  indeed  is  the  whole  toe. 
The  remainder  are  long  bones,  the  innermost  bearing  a  back- 
wardly  projecting 'corner  which,  as  has  just  been  pointed  out,  is 
probably  the  innermost  distal  tarsal.  Each  metatarsal  is  followed 
by  a  digit  composed  of  three  phalanges. 


FIG.  95 — Left  hind  foot  of  Lepus, 
viewed  from  extensor  surface. 

C.,  centrale,  navicular  ;  Cu.,  cuboid  ; 
EC.,  ecto-cuneiform ;  En.,  process  prob- 
ably representing  ento-cuneiform ;  F., 
fibiale,  calcaneum  ;  M.,  meso-cuneiform  ; 
Me.,  metatarsal ;  P.,  phalanx  ;  T.,  talus  — 
tibiale  +  intermedium ;  Un.,  ungual  pha- 
lanx ;  II.-V.,  digits. 


LEPUS   CUNICULUS  289 

The  distal  phalanges  in  both  fore  and  hind  limbs  are  of  a 
characteristic  bent  pointed  shape,  and  are  covered  by  the  hollow 
horny  claws  so  that  they  are  termed  the  ungual  phalanges. 

The  Pentadactyl  Limb. 

Having  now  studied  the  fore  and  hind  limbs  of  both  frog 
and  rabbit  we  are  in  a  position  to  consider  the  relation  of  one  to 
the  other  and  to  the  limbs  of  air-breathing  vertebrates  in  general. 


Fb     Ce    MT 

FIG.  96. — Idealised  diagram  of  pentadactyl  limbs. 

_C.,  pre-axial  centrale  ;  Ce.,  post-axial  centrale  ;  D.C.,  distal  carpalia  ;  D.T.,  distal  tarsalia  ; 
F.,  fore  limb  ;  Fb.,  fibulare  ;  Fe.,  femur ;  F.H.,  head  of  femur  ;  Fi.,  fibulare  ;  H.,  hind  limb  ; 
H.H.,  head  of  humerus  ;  Hu.,  humerus  ;  I.,  intermedium  ;  L.,  axis  of  h'mb  ;  M.,  main  axis  of 
body  ;  M.C.,  metacarpal ;  M.T.,  metatarsal ;  Po.,  post-axial  border  of  limb  ;  Pr.,  pre-axial  border 
of  limb  ;  R.,  radius  ;  Ra.,  radiale  ;  T.,  tibia  ;  Ti.,  tibiale  ;  U.,  ulna  ;  Ul.,  ulnare  ;  I.,  pollex  ; 
I'.,  hallux  ;  II.-V.,  digits. 

Comparison  will  show  that  not  only  are  the  fore  and  hind  limbs  in 
each  animal  composed  of  homologous  parts,  but  also  that  there  is  a 
fundamental  similarity  between  those  of  Lepus  and  Rana.  Indeed, 
we  can  go  further  than  this  and  sketch  the  plan  of  a  primitive  limb 
from  which  most  probably  those  of  all  vertebrates  in  the  classes 
Amphibia,  Reptilia,  Aves  and  Mammalia  have  been  derived  and 
of  which  they  can  certainly  be  regarded  as  modifications.  This 
ideal  type  is  known  as  the  typical  pentadactyl  limb  and  is  constituted 
in  the  following  way.  The  proximal  element  is  a  long  cylindrical 
bone,  the  humerus  or  femur,  enlarged  proximally  to  form  a  head 
for  articulation  with  the  girdle  and  enlarged  distally  for  articulation 
with  the  succeeding  elements.  The  next ''port ion  is  composed  of 
two  bones,  a  radius  and  ulna  or  a  tibia  and  fibula,  which  articulate 

u 


290  AN   INTRODUCTION   TO  ZOOLOGY 

distally  with  the  groups  of  bones  forming  the  carpus  or  tarsus  as 
the  case  may  be.  These  groups  consist  primitively  of  ten  bones 
arranged  in  three  rows.  The  first  or  proximal  row  comprises  three 
bones,  the  radiale  and  ulnare  or  the  tibiale  and  fibulare,  articulating 
with  the  correspondingly  named  limb  bones,  and  between  them 
in  each  case  is  the  third  element,  an  intermedium.  The  second 
row  consists  of  two  central  bones,  the  centralia  ;  in  most  animals 
but  one  of  these  is  represented  either  freely  or  fused,  but  there  is 
evidence  to  show  that  primitively  there  were  two.  In  the  distal 
row  are  five  bones,  the  distal  carpalia  or  tarsalia,  and  again  there  is 
frequently  loss  or  fusion  of  these.  Each  of  them  is  followed  by 
a  metacarpal  or  metatarsal,  which  are  longer  bones  contained  within 
the  palm  of  the  hand  or  sole  of  the  foot.  Following  these  are  the 
five  digits  composed  of  a  series  of  phalanges.  In  each  case  the  first 
digit,  the  thumb  or  pollex  and  the  big  toe  or  hallux,  is  shorter  than 
the  remainder  and  composed  of  but  two  phalanges,  whereas  the  others 
are  each  composed  of  three.  Ihis,  then,  is  the  structure  of  the 
primitive  pentadactyl  limb,  but  it  also  has  definite  relations  to  the 
body  as  a  whole.  In  its  original  position,  when  extended,  it  was 
related  to  the  main  or  vertebral  axis  in  the  same  way  as  our  own 
arm  would  be  if  it  were  held  straight  out  to  the  side  at  right  angles 
to  the  body  with  the  thumb  uppermost.  An  imaginary  line  drawn 
lengthwise  through  the  limb,  i.e.  through  the  proximal  bone,  the 
intermedium  and  the  middle  digit,  is  regarded  as  the  axis  of  the  limb. 
The  parts  of  the  limb  on  the  same  side  as  the  radius  and  pollex  or 
tibia  and  hallux  are  termed  pre-axial,  those  on  the  opposite  side 
e.g.  ulna  or  fibula,  etc.,  are  post-axial.  The  digits  are  numbered  one 
to  five  from  the  preaxial  side,  and  so  we  have  in  both  limbs  a 
phalangeal  formula  of  2.3.3.3.3.  In  the  primitive  animals  this  was 
the  position  of  the  limbs  which  were,  however,  bent  or  flexed  at 
right  angles  at  the  elbow  or  knee  so  as  to  allow  the  palm  of  the  hand 
or  sole  of  the  foot  to  reach  the  ground.  The  surface  on  the  inner 
side  of  the  bend  is  termed  the  flexor  surface,  while  that  on  the  outer 
side  is  the  extensor  surface.  However,  in  the  rabbit  and  in  mammals 
generally  the  limbs  have  undergone  a  certain  amount  of  rotation. 
Taking  the  arm  from  the  position  just  described  and  swinging  it  to 
the  front,  still  at  right  angles  to  the  body,  it  will  be  seen  that  in 
order  for  the  palm  of  the  hand  to  touch  the  ground  it  is  necessary 
to  turn  the  hand,  and  with  it  the  forearm,  through  an  angle  of  90° 
so  that  the  thumb,  instead  of  pointing  upwards,  points  inwards  and 
the  palm  downwards.  In  this  position  the  hand  is  said  to  be  in 
pronation,  while  in  the  original  position  it  is  in  supination.  The  fore 
limb  of  Lepus  therefore  is  permanently  in  a  position  of  pronation, 
i.e.  the  forearm  and  hand  are  twisted  with  regard  to  the  arm. 


LEPUS   CUNICULUS  291 

The  bringing  of  the  hallux  to  the  inside  of  the  hind  limb  is  not 
accomplished  in  the  same  way,  but  this  is  done  by  the  rotation  of 
the  limb  as  a  whole,  whereby  the  original  dorsal  border  becomes 
the  anterior  border,  but  there  is  no  extra  twisting  of  the  lower  part 
of  the  limb. 

This  ideal  type  of  limb  is  only  met  with  in  a  few  primitive 
animals  and  in  all  others  a  certain,  often  very  considerable,  amount 
of  modification  has  occurred.  One  of  the  first  changes  is  a  fusion 
or  loss  of  certain  of  the  carpal  and  tarsal  elements,  and  another 
common  one  is  a  loss  or  reduction  of  the  digits. 

The  Skull  of  a  Mammal — The  Dog,  Canis  familiaris. 

For  the  study  of  a  typical  mammalian  skull  we  may  take 
that  of  the  dog  which,  although  not  ideal,  is  fairly  generalised  and 
readily  obtainable.  While  it  is  to  the  dog's  skull  that  the  details 
of  the  following  description  apply,  it  is  well  to  bear  in  mind  that  the 
skull  of  all  the  higher  Mammalia  is  constructed  upon  the  same 
general  plan  and  the  differences  are  mainly  small  points,  and 
particularly  in  the  relative  sizes  of  the  parts.  As  in  the  frog,  the 
skull  of  the  mammal  is  first  laid  down  in  cartilage.  In  the  adult 
it  comes  to  consist  of  certain  parts  of  this  chondrocranium  that  have 
ossified  and  so  became  cartilage  bones,  to  which  have  been  added 
other  membrane  bones  laid  down  in  the  surrounding  tissues.  Again, 
in  Canis  as  in  Rana  the  entire  skull  consists  of  the  cranium  or  brain 
case,  the  olfactory,  optic  and  auditory  capsules,  the  upper  and 
lower  jaws  and  the  hyoid  apparatus,  and  to  laciliate  description 
we  may  deal  with  these  various  parts  separately.  The  various 
bones  in  an  old  skull  are  closely  joined  together  by  small  inter- 
locking projections  so  that  the  line  of  union  is  indicated  by  wavy 
lines  termed  sutures,  or  it  may  be  that  they  are  so  closely  fused  as 
to  be  indistinguishable.  For  purposes  of  study,  therefore,  it  is  well 
to  examine  a  moderately  young  specimen. 

Cranium. — The  cranium  is  a  large,  hollow  box  somewhat  oval  in 
shape  and  occupying  slightly  more  than  the  posterior  half  of  the 
skull.  The  floor  or  base  is  more  or  less  straight,  while  the  roof  is  well 
arched.  In  the  hinder  part  of  adult  skulls  we  find  a  prominent 
ridge  in  the  mid-dorsal  line  ;  it  is  termed  the  sagittal  crest  and 
furnishes  attachment  for  the  large  temporal  muscles  moving  the 
lower  jaw.  The  bones  composing  the  cranium  will  be  found  to 
fall  into  three  segments  corresponding  with  three  median,  unpaired 
bones,  namely,  the  basi-occipital,  the  basi-sphenoid  and  the  pre- 
sphenoid,  which  together  form  the  basi-cranial  region  of  the  skull. 

The  occipital  segment  is  composed  of  cartilage  bones  of  which 
the  mid- ventral  is  the  basi-occipital,  a  flat  plate  at  the  hinder  end 


AN   INTRODUCTION  TO  ZOOLOGY 


of  the  floor  of  the  skull  ending  freely  behind  where  it  forms  the 
ventral  boundary   of  the   foramen   magnum,   the  large   aperture 


:     Hi  3  -iS 

•-     g  12  .3  g 


through  which  the  medulla  passes  backwards  to  become  continuous 
with  the  spinal  cord.     Lying  latero-dorsally  on  each  side  of  this 


LEPUS   CUNICULUS  293 

are  the  ex-occipital  bones,  which  form  the  larger  part  of  the  circum- 
ference of  the  foramen.  Each  has  its  hinder  margin  produced 
into  a  well-marked  curved  occipital  condyle,  which  serves  for  articula- 
tion with  the  atlas  or  first  vertebra.  A  large  occipital  sinus  lies 
within  the  condyle,  and  its  anterior  and  posterior  openings  show 
clearly  inside  the  brain  case.  At  its  antero-lateral  margin  the  bone 
is  produced  ventrally  into  a  strong  par-occipital  process,  giving  a 
surface  for  muscle  attachment,  and  internal  to  this  it  is  perforated 
by  a  small  hole,  the  condylar  foramen,  which  is  the  exit  for  the 
hypogossal  nerve.  The  line  of  junction  between  the  basi-  and  ex- 
occipital  bones  is  obliterated  in  the  adult  skull,  but  clearly  visible 
in  young  examples.  The  dorsal  border  of  the  foramen  magnum 
is  formed  by  a  median  bone,  the  supra-occipital,  constituting  the 
flat,  almost  vertical  plate  at  the  hinder  end  of  the  cranium.  In 
an  old  dog  it  is  completely  fused  with  a  forward  prolongation  that 
appears  as  a  median  dorsal  tongue  of  bone  which,  however,  ossifies 
separately,  and  in  some  animals  remains  throughout  life  a  distinct 
membrane  bone,  the  interparietal. 

The  mid- ventral  unit  of  the  parietal  segment  is  the  basi-sphenoid, 
a  flat  plate  of  somewhat  thicker  cartilage  bone  more  or  less  cancellous 
within.  Ventrally  it  is  fairly  level,  but  inside  the  cranium  it  is 
hollowed  in  the  middle  to  form  a  depression,  the  sella  turica,  in 
which  lies  the  pituitary  body.  In  some  mammals  its  floor  is  per- 
forated by  a  small  opening,  the  pituitary  foramen,  which  is  not  present 
in  Canis.  From  the  sides  of  this,  extending  upwards  and  outwards, 
arise  the  wing-like  alisphenoids,  also  cartilage  bones.  Ventrally 
this  bone  gives  off  a  vertical  bony  plate,  the  external  pterygoid 
process,  to  the  inner  edge  of  which  is  attached  the  palatine  bone. 
The  alisphenoid  itself  is  important  because  of  the  perforations  or 
foramina  connected  with  it.  The  base  of  the  pterygoid  process  is 
pierced  by  the  alisphenoid  canal,  through  which  the  external 
carotid  artery  runs.  The  lower  anterior  margin  of  the  portion 
of  the  alisphenoid  in  the  orbit  forms  the  posterior  limit 
of  a  well-marked  aperture  running  vertically,  the  foramen 
lacerum  anterius.  Through  this  pass  the  third,  fourth  and  sixth 
cranial  nerves  supplying  the  muscles  of  the  eye  and  also  the 
ophthalmic  branch  of  the  fifth  nerve.  In  its  basal  region  the 
alisphenoid  is  perforated  by  two  openings  ;  one  is  approximately 
round,  the  foramen  rotundum,  serving  for  the  exit  of  the  maxillary 
branch  of  the  fifth  nerve,  and  the  other  a  slightly  larger,  oval,  more 
posterior  hole,  the  foramen  ovale,  for  the  mandibular  branch  of  the 
trigeminal.  The  dorsal  portion  of  the  cranium  in  this  region  is 
formed  by  the  large  square-shaped  membrane  bones,  the  parietals, 
meeting  in  the  middle  line  in  a  junction  termed  the  sagittal  suture, 


294  AN   INTRODUCTION  TO  ZOOLOGY 

save  at  the  hinder  end  where  they  are  separated  by  the  interparietal 
portion  of  the  supra-occipital. 

Encasing  the  anterior  end  of  the  brain  is  the  frontal  segment 
whose  median,  basal  cartilage  bone  is  the  pre-sphenoid.  It  is  a  narrow 
bone  flat  ventrally,  but  of  irregular  outline  within  the  cranium ; 
inside  it  is  cancellous.  Internally  it  just  touches  the  lower  border 
of  the  optic  foramen.  To  the  side  of  the  pre-sphenoids  are  attached 
the  thin  wing-like  orbito-sphenoids,  also  cartilage  bones.  They 
form  part  of  the  inner  wall  of  the  orbit  and  contribute  the  greater 
part  of  the  margin  of  the  optic  foramen.  At  their  posterior  corner 
they  also  participate  in  forming  the  front  border  of  the  foramen 
lacerum  anterius.  Dorsally  the  cranial  roof  is  composed  of  the 
large  membrane  bones,  the  frontals,  which  not  only  meet  in  the 
middle  line  continuing  the  sagittal  suture,  but  also  pass  forwards 
and  ventrally  for  some  distance.  Dorso-laterally  the  frontal 
bone  gives  off  a  projection,  the  post-orbital  process,  extremely  well 
developed  in  some  other  mammals,  which  marks  the  hinder  limit 
of  the  orbit.  When  the  skull  is  viewed  from  above  a  large  arch, 
the  jugal  or  malar  arch  is  seen  to  sweep  out  on  each  side  of  the 
cranium.  The  space  within  this  arch  may  be  divided  into  two,  the 
so-called  orbital  fossa,lodging  the  eye  in  front,  and  the  temporal  fossa, 
which  is  filled  in  life  by  the  great  temporal  muscle  whose  origin 
on  the  sagittal  crest  has  already  been  noted. 

Finally,  the  space  at  the  front  end  of  the  cranium  between  the 
pre-sphenoid  and  the  frontals  is  closed  in  by  a  median  vertical 
plate  of  spongy  cartilage  bone,  the  ethnoid.  Lateral  expansions  of  this 
form  the  cribriform  plates,  which  are  vertical  bones  perforated  by 
a  large  number  of  holes  for  the  passage  of  numerous  olfactory 
nerves.  The  bone  itself  is  continued  forward  in  the  middle  line 
as  a  thin  vertical  lamina,  the  mesethmoidal  plate  or  septum  narium, 
part  of  which  always  remains  cartilaginous,  and  this  separates  the 
two  olfactory  capsules. 

The  front  part  of  the  skull,  occupying  less  than  half  its 
length,  consists  in  the  main  of  the  olfactory  capsules  and  the  bones 
connected  with  it.  The  capsule  consists  of  an  upper  space,  the 
olfactory  chamber,  and  a  lower  tube-like  portion,  the  narial  passage. 
The  roof  of  these  capsules  is  made  partly  of  prolongations  of  the 
frontal  bones,  the  nasal  processes,  but  mainly  of  two  long  narrow 
membrane  bones,  the  nasals,  whose  front  ends  form  the  dorsal 
limit  of  the  anterior  nares.  Their  floor  in  the  mid- ventral  line  is 
composed  of  a  small  elongated  membrane  bone,  the  vomer, 
representing  a  pair  of  bones  fused.  The  remaining  bones 
participating  in  the  formation  of  the  nasal  capsule,  namely,  the 
palatines,  maxillae  and  pre-maxillae,  are  better  dealt  with  in 


LEPUS   CUNICULUS  295 

connection  with  the  upper  jaw  and,  as  already  noted,  its  hinder  limit 

AC.  P. 

rr. 


PO.P 


M<     °* 

FIG.  99. — Lateral  view. 


C.F. 


FIG.  100. — Medium  long  section. 
Skull  of  dog,  Canis  familiaris. 

Art.,  articular  facet  for  lower  jaw  ;  A.S.,  alisphenoid  ;  B.Oc.,  basi-occipital  ;  Ca.,  canine  ; 
C.F. ,  carotid  foramen  ;  E.A.M.,  external  auditory  meatus  ;  E.T.,  ethmo-turbinals  ;  Eth.,  ethmoid  ; 
Ex. Oc.,  ex-occipital ;  F. A.,  internal  auditory  foramen  ;  F.L.A.,  foramen  lacerum  anterius  ;  F.L.M., 
foramen  lacerum  medium  ;  F.O.,  foramen  ovale  ;  F.M.,  foramen  magnum  ;  F.R.,  Foramen  rotun- 
dum  ;  Fr.,  frontal ;  Inc.,  incisors  ;  Inf.O.F.,  infra-orbital  foramen  ;  J.,  jugal ;  La.,  lachrymal ; 
M.,  molars  ;  Mx.T.,  maxillo-turbinals  ;  N.,  nasal ;  N.S.,  nasal  septum  ;  Oc.Co.,  occipital  condyle  ; 
Oc.S.,  occipital  sinus  ;  O.F.,  optic  foramen  ;  O.S.,  orbito-sphenoid  ;  P.,  palatine  ;  Pa.,  Parietal  ; 
Per.,  peri-otic  ;  P.M.,  pre-molars  ;  P.Mx.,  pre-maxilla  ;  P.Oc.P.,  par-occipital  process  ;  P.S.,  pre- 
sphenoid  ;  Pt.,  pterygoid  ;  Sq.,  squamosal ;  S.T.,  sella  turica  ;  St.F.,  stylomastoid  foramen  ; 
Su.Oc.,  supra-occipital ;  Ty.B.,  tympanic  bulla  ;  Vo.,  vomer. 

is  marked  by  the  cribriform  plate.     The  main  part  of  the  posterior 
end  of  the  cavity  of  the  capsule  is  filled  by  a  very  complexly  folded 


296  AN   INTRODUCTION   TO  ZOOLOGY 

and  coiled  series  of  bony  lamellae  consituting  the  so-called  ethno- 
turbinal  bones,  since  they  are  fused  behind  with  the  ethmoid  bone. 
They  are  covered  by  the  olfactory  membrane,  which  is  well  supplied 
with  sense  cells  and  nerves,  and  so  serve  to  provide  a  much  greater 
area  for  the  sensory  epithelium  than  would  be  possible  without  the 
folding.  The  most  dorsal  of  these  lamellae  lie  close  to  the  nasal  bones 
with  which  they  become  fused  in  some  mammals,  and  then  they  are 
termed  the  naso-turbinals.  A  similar  series  of  even  more  slender 
lamellae  are  to  be  found  in  the  anterior  part  of  the  nasal  cavity,  and 
as  they  are  fused  with  the  maxillae  they  are  distinguished  as  the 
maxillo-turbinals. 

The  optic  capsule,  well  developed  in  bony  fishes,  is  very  much 
reduced  in  the  mammal  and  comes  to  consist  of  a  single  membrane 
bone,  the  lachrymal,  on  each  side.  This  is  a  small  bone  lying  on 
the  anterior  border  of  the  orbit  at  the  ventral  limit  of  the  junction 
between  the  frontal  bone  and  the  maxilla.  It  is  perforated  by  the 
lachrymal  foramen,  through  which  passes  the  duct  of  the  lachrymal 
or  tear  gland.  In  some  mammals  it  is  but  loosely  attached  to  the 
surrounding  bones,  but  in  the  dog  it  is  more  firmly  inserted. ' 

The  auditory  capsule  consists  of  a  group  of  bones  more  or 
less  firmly  joined  together  and  lying  laterally  to  the  basi-occipital 
and  immediately  in  front  of  the  par-occipital  process.  The  most 
obvious  part  externally  is  the  tympanic,  a  cartilage  bone  which 
is  swollen  out  ventrally  to  form  the  well-marked  tympanic  bulla. 
At  the  hinder  median  border  of  the  bulla  lies  an  obliquely  directed 
aperture,  the  foramen  lacerum  posterius,  through  which  the  ninth, 
tenth  and  eleventh  cranial  nerves  leave  the  skull  in  company  with 
the  internal  jugular  vein.  Behind  its  postero-dorsal  corner  is  a 
somewhat  round  hole,  the  stylomastoid  foramen,  serving  for  the 
exit  of  the  main  part  of  the  seventh  cranial  nerve.  Antero-laterally 
to  the  bulla  lies  an  irregular  opening,  the  foramen  lacerum  medium, 
which  on  closer  examination  is  seen  to  be  double.  The  more  median 
of  the  two  perforations  is  the  foramen  caroticum,  through  which 
the  internal  carotid  artery  reaches  the  brain  and  slightly  to  the 
outside  of  this  is  the  orifice  of  the  Eustachian  canal  for  the 
Eustachian  tube.  On  its  anterior,  upper  surface  the  tympanic 
bone  is  produced  into  a  short  neck  with  a  wide  opening,  the  external 
auditory  meatus.  Across  the  bottom  of  the  neck  in  life  is  stretched 
the  tympanic  membrane  supported  by  an  incomplete  bony  ring. 

The  internal  portion  of  the  auditory  capsule  is  formed  by  an 
irregular  cartilage  bone,  the  peri-otic,  which  is  regarded  as  being 
composed  of  three  separate  elements,  the  pro-otic,  the  epi-otic  and 
the  opisth-otic,  and  lodges  the  internal  ear.  In  the  adult  the  single 
bone  formed  by  this  fusion  can  be  divided  into  two  more  or  less 


LEPUS   CUNICULUS  297 

well-marked  parts,  the  petrous  portion  on  the  inside  and  the  mastoid, 
which  is  produced  posteriorly  into  a  mastoid  process  lying  next  to  the 
par-occipital  process.  An  oval  opening,  the  internal  auditory  meatus, 
lies  on  the  middle  of  the  inner  surface  of  the  per-iotic  bone  ;  into  this 
go  the  eighth  nerve  supplying  the  internal  ear  and  the  seventh  nerve 
which  reaches  the  outside  of  the  skull  through  the  stylomastoid 
foramen ,  as  has  been  noted  above .  On  the  outer  surface  of  the  peri-otic 
bone  facing  the  tympanic  cavity,  and  so  not  seen  unless  the  tympanic 
bulla  is  removed,  are  two  small  holes,  the  fenestra  ovalis  and  fenestra 
rotunda,  which  place  the  inner  ear  in  communication  with  the 
cavity.  Within  and  stretching  across  the  tympanic  cavity  itself 
is  a  chain  of  small  bones  connected  with  the  function  of  hearing. 
The  malleus  is  a  small  bone  consisting  of  a  blade-like  portion,  the 
manubrium,  which  is  attached  to  the  tympanic  membrane,  and  a 
more  solid  body.  The  latter  part  articulates  with  the  next  bone  in 
the  series,  the  incus,  and  this  also  has  a  tiny  process,  the  end  of 
which  fits  on  to  a  small  bony  disc,  the  OS  orbiculare.  The  last 
bone,  the  stapes,  is  shaped  somewhat  like  a  stirrup  with  a  central 
perforation.  At  one  end  it  joins  the  os  orbiculare  and  the  other 
is  flattened  to  form  a  basal  plate  that  closes  the  fenestra 
ovalis. 

To  turn  now  to  the  jaws,  which  although  laid  down  in 
cartilage  in  the  embryo,  are  entirely  replaced  by  membrane  bones 
in  the  adult. 

The  pterygoids  are  thin  lamellae  of  bone  lying  almost  in  the  vertical 
plane  ;  their  bases  lie  upon  the  pre-sphenoid  and  basi-sphenoid, 
laterally  they  pass  dov/nwards  on  the  inside  of  the  alisphenoid  to 
which  they  are  closely  apposed  and  they  project  freely  beyond  this 
for  a  short  distance  only.  Thus  in  looking  at  the  skull  from  the 
side  only  the  small  triangular  points  of  these  bones  are  visible. 
At  their  anterior  end  they  are  attached  to  the  next  bone  in  the 
series,  the  palatine.  This  is  an  irregular  and  much  larger  bone. 
Its  lateral  portion  which  abuts  on  to  the  pterygoid  continues  upwards 
and  outwards  to  form  a  considerable  part  of  the  antero-ventral 
wall  of  the  orbit,  where  it  articulates  with  the  alisphenoid  behind, 
and  the  orbito-sphenoid  and  frontal  above.  It  stretches  forward 
to  the  lachrymal  and  contributes  partly  to  the  mesial  margin  of 
the  hinder  opening  of  the  infra-orbital  canal.  Later  in  its  orbital 
region  it  joins  the  maxilla.  Near  the  line  of  junction  with  the 
maxilla  lie  two  small  perforations  in  the  palatine  bone.  The  upper 
of  these  leads  into  the  narial  passage  and  transmits  the  posterior 
nasal  branch  of  the  trigeminal,  while  the  lower  is  for  the  palatine 
branches  of  the  same  nerve  and  the  palatine  artery  which  pass 
through  the  posterior  palatine  foramen  of  the  maxilla  and  several 


298  AN   INTRODUCTION   TO  ZOOLOGY 

other  small  apertures  in  both  the  palatine  and  maxilla  in  its  neigh- 
bourhood. A  short  way  up  the  inner  part  of  this  main  lateral 
plate  is  given  off  a  horizontal  lamina  which  passes  inwards,  coming 
in  contact  with  the  pre-sphenoid  for  a  short  distance  and  running 
forward  along  the  vomer  for  some  way.  This  lamina  forms  the 
dorso-lateral  wall  of  the  narial  passage  and  separates  it  from  the 
orbit.  More  ventrally  the  main  lateral  plate  gives  off  another 
horizontal  extension,  the  palatal  process,  which  passes  inwards  and 
forwards  to  the  level  of  the  fourth  pre-molar,  to  join  with  its  fellow 
in  the  middle  line  and  form,  not  only  the  ventral  wall  of  the  narial 
passage,  but  also  the  posterior  portion  of  the  hard  bony  roof  of  the 
mouth.  The  posterior  nares  therefore  come  to  be  bounded  by  the 
palatine  bones  save  for  a  small  part  on  their  dorsal  side  which  is 
filled  in  by  the  vomer. 

The  maxillae  are  two  large  bones  forming  the  major  portion  of 
the  lateral  walls  of  the  nasal  capsules.  The  main  mass  or  body  of 
the  maxilla,  termed  the  alveolar  portion  of  the  maxilla,  completes 
the  ant ero- ventral  wall  of  the  orbit.  Right  in  the  front  corner 
it  is  perforated  by  the  hinder  opening  of  the  infra-orbital  canal, 
through  which  the  infra-orbital  branch  of  the  trigeminal  nerve  passes 
to  be  distributed  to  the  upper  lip  and  vibrissae.  It  makes  its  exit 
by  the  well-marked  infra-orbital  foramen  just  in  front  of  and  below 
the  orbit.  From  the  body  of  the  maxilla  a  frontal  wing  passes 
upwards  helping  to  form  the  roof  of  the  nasal  chamber  and  touching 
the  pre-maxillae,  the  nasal,  the  frontal  and  lachrymal  bones.  The 
body  also  gives  off  a  palatine  lamella  which  joins  with  its  fellow 
in  the  mid- ventral  line,  constituting  a  large  part  of  the  hard  palate. 
At  its  hinder  edge  this  plate  of  bone  joins  the  palatine  in  a  curved 
suture  running  in  a  postero-lateral  direction,  and  about  half-way  along 
this  suture  occurs  the  posterior  palatine  foramen  which,  as  already 
noted,  transmits  the  palatine  nerves  and  arteries.  The  front  end 
of  the  palatine  plate  touches  the  pre-maxillae  and  forms  the  posterior 
walls  of  the  anterior  palatine  foramina.  The  junction  of  the  body 
of  the  maxilla  and  the  palatine  plate  is  termed  the  alveolar  border, 
since  in  it  lie  the  alveoli  of  the  canine,  premolar  and  molar  teeth. 
From  the  postero-lateral  border  of  the  body  of  the  maxilla  a* 
short  pointed  zygomatic  process  runs  backwards,  taking  part  in  the 
formation  of  the  zygomatic  arch. 

The  pre-maxillse  are  the  most  anterior  of  the  bones  of  the  skull, 
completing  the  front  portion  of  the  upper  jaw,  and  they  bear  the 
alveoli  of  the  incisor  teeth.  They  are  joined  laterally  to  the  maxillae 
and  in  the  middle  line  with  one  another,  thus  forming  the  lower  and 
side  borders  of  the  anterior  nares.  Postero-dorsally  each  sends  off 
a  long  narrow  tongue,  the  nasal  process,  which  lies  between  the  nasal 


LEPUS   CUNICULUS  299 

bone  and  the  maxilla  on  each  side.  They  also  give  oft  ventrally 
a  small  palatine  process  in  the  middle  line  which  form  the  mesial 
borders  of  the  anterior  palatine  foramina,  long  oval  apertuies  whose 
postero-lateral  rims  are  completed  by  the  maxillae,  through  which 
pass  the  naso-palatine  branches  of  the  facial  nerve. 

The  malar  or  jugal  bone  is  a  thin  flat  bone  attached  to  the  body 
of  the  maxilla  near  the  zygomatic  process.  It  runs  out  posteriorly, 
intimately  connected  with  this  process  and  unites  with  a  similarly 
named  projection  from  the  squamosal  bone.  At  its  upper  dorsal 
corner  it  bears  a  slight  prominence,  the  post-orbital  process,  opposite 
to  the  similarly  named  structure  on  the  frontal  bone.  These  two 
processes  are  but  little  marked  in  Cams,  but  in  other  mammals  they 
may  be  much  more  strongly  developed  and  even  join  one  another 
so  that  superficially,  at  any  rate,  the  orbital  and  temporal  fossae  are 
completely  separated. 

The  squamosal  bone  is  a  broad  scale-like  bone  at  the  lower 
hinder  end  of  the  temporal  fossa.  Posteriorly  it  is  bounded  by  the 
ex-occipital  and  supra-occipital,  above  by  the  parietal,  antero- 
ventrally  by  the  alisphenoid,  and  the  hinder  part  of  its  ventral 
border  articulates  with  the  tympanic  bone.  Immediately  in  front 
of  the  bulla  it  sends  out  a  stout  perpendicular  process,  the  under 
surface  of  which  is  hollowed  and  smooth  to  form  an  articular  surface, 
the  glenoid  facet,  for  the  lower  jaw.  At  the  hinder  edge  of  the  facet 
is  a  projection  running  downwards,  the  post-glenoid  process,perf orated 
behind  by  the  post-glenoid  foramen  for  a  branch  of  the  lateral 
sinus.  This  outgrowth,  the  zygomatic  process,  then  turns  forwards 
and  joins  the  jugal  by  a  long  oblique  suture,  thus  completing  the 
zygoma  or  zygomatic  arch. 

The  mandible,  or  lower  jaw,  consists  of  two  elongated  somewhat 
triangular  bony  plates,  the  rami,  compressed  laterally,  which  unite 
in  front  by  roughened  surfaces,  forming  the  mandibular  symphysis. 
Their  union  is  not  complete  save  in  quite  old  specimens,  and  they 
generally  fall  apart  in  prepared  skeletons.  The  upper  or  alveolar 
border  bears  the  teeth.  At  its  post ero- ventral  extremity  it  juts 
out  to  form  a  well-marked  projection,  the  angular  process.  Just 
above  this  and  separated  from  it  by  a  notch  is  a  strong  transverse 
ridge,  the  condyle,  rounded  in  the  antero-posterior  direction  and 
serving  for  articulation  with  the  facet  of  the  squamosal  bone.  From 
the  structure  of  the  condyle  and  the  facet  it  is  clear  that  the  jaw  is 
only  capable  of  a  simple  up-and-down  motion.  Between  the  condyle 
and  the  last  tooth  the  upper  edge  of  the  ramus  is  thrown  up  into 
a  laterally  compressed,  backwardly  curved  wing,  the  coronoid 
process.  Below  this  and  in  front  of  the  condyle  on  the  outer  side 
of  the  jaw  is  a  hollow  space  on  to  which  the  masseter  muscle  of 


300 


AN   INTRODUCTION   TO  ZOOLOGY 


the  mandible  is  inserted.  On  the  inner  side  of  the  ramus  just  below 
and  in  front  of  the  condyle  lies  the  inferior  dental  foramen,  which 
allows  the  inferior  dental  nerve,  a  branch  of  the  trigeminal,  and  the 
similarly  named  artery  that  accompanies  it  to  enter  the  substance 
of  the  bone.  A  terminal  branch  of  the  same  nerve  leaves  the  outer 
surface  of  the  jaw  by  the  mental  foramen,  just  behind  and  below 


B 


FIG.  101. — -Hyoid  bone  of 
Canis. 

B.,  body  of  hyoid,  basi-hyal ;  C.,  cera- 
to  -hyal ;  E.,  epi-hyal ;  S.,  stylo-hyal ; 
C +  E  +  S=  anterior ;  TM  thyro-hyal,  pos- 
terior cornu. 


FIG.  102. — Longitudinal  section  of  a 
molar  tooth. — From  Owen. 

k.,  crown  ;  n.,  neck  ;  /.,  fangs  ;  e.,  enamel ; 
d.,  dentine  ;  c.,  cement  ;  p.,  pulp  cavity. 


the  canine  tooth,  and  is  distributed  to  the  lip  and  surrounding 
tissues. 

The  hyoidean  apparatus  is  composed  of  a  median  portion, 
the  basi-hyal,  and  a  pair  of  anterior  and  a  pair  of  posterior  cornua. 
The  basi-hyal,  or  body  of  the  hyoid,  is  a  small  transverse  flattened 
bar  of  cartilage  bone  somewhat  thickened  and  turned  up  at  its 
ends.  The  posterior  cornua  are  short  rods  of  bone  articulating  with 
the  outer  extremities  of  the  basi-hyal.  They  pass  upwards  and 
are  attached  to  the  thyroid  cartilage  of  the  larynx  and  hence  are 
termed  the  thyro-hyals.  The  anterior  cornua  are  longer  rods 
passing  in  a  curved  manner  upwards  and  outwards  and  forming 
a  connection  with  the  cranium  in  the  region  of  the  tympanic  bone. 
This  ossifies  in  four  parts  ;  the  most  dorsal  is  the  tympano-hyal,  a 


LEPUS   CUNICULUS  301 

small  bony  mass  lying  in  between  the  tympanic  and  peri-otic  bones 
just  in  front  of  the  stylomastoid  foramen.  It  is  hardly  distinguish- 
able in  most  dogs,  though  plain  in  some  other  mammals.  The 
remaining  portions  of  the  cornua  passing  from  dorsal  to  ventral 
end  are  the  stylo-hyal,  the  epi-hyal  and  the  cerato-hyal,  which  joins 
the  body  near  the  thyro-hyal. 

A  typical  tooth  consists  of  a  crown,  the  portion  standing  out 
from  the  gum ;  a  root,  which  is  inserted  in  the  jaw  and  may  possess 
one  or  more  fangs ;  and  a  slightly  constricted  region,  the  neck,  joining 
the  other  two.  Within  the  tooth  is  a  hollow,  the  pulp  cavity,  con- 
taining the  pulp,  which  is  connective  tissue  richly  supplied  with 
blood-vessels  and  a  nerve. 

The  main  mass  of  the  tooth  is  composed  of  the  hard  dentine. 
The  crown  is  capped  with  a  still  harder  substance,  the  enamel,  and 
the .  fangs  are  coated  with  cement.  This  is  a  softer  substance 
resembling  bone,  in  that,  it  possesses  lacunae  and  canaliculi,  but  it 
has  no  Haversian  canals. 


CHAPTER   XII 
LEPUS   CUNICULUS—  (continued) 

Digestive  System — Respiratory  System — Circulatory  System — The  Mammalian 
Heart — Urogenital  System — Ductless  Glands. 

Digestive  System. 

The  digestive  system  is  composed  of  the  alimentary 
canal  and  its  related  glands,  and  in  the  rabbit  consists  of  the  same 
main  parts  as  in  Rana. 

The  mouth  is  bordered  by  very  freely  movable  lips  of  which  the 
upper,  as  has  been  noted,  is  cleft.  The  buccal  cavity  is  narrow  and 
long  as  compared  with  the  frog.  Within  the  jaws  are  the  teeth 
whose  form  and  distribution  we  have  already  considered.  The 
roof  of  the  buccal  cavity  is  composed  of  a  mass  of  tissue  termed  the 
palate.  The  nasal  cavities,  instead  of  communicating  by  the  internal 
nares  with  the  front  end  of  the  buccal  cavity,  pass  into  long  passages, 
the  narial  canals,  which  open  by  the  nares  back  in  the  pharyngeal 
region,  and  it  is  the  palate  that  constitutes  the  partition  separating 
the  buccal  cavity  from  these  canals.  The  front  part  of  this  roof 
contains  osseous  structures,  namely,  the  palatine  processes  of  the 
pre-maxillae,  the  maxillae  and  the  palatine  bone ;  it  is  consequently 
termed  the  hard  palate.  The  hinder  part,  on  the  contrary,  is  com- 
posed entirely  of  membrane  and  connective  tissue  and  so  is  designated 
the  soft  palate.  Right  at  the  front  end  of  the  palate  are  a  pair  of 
grooves  terminating  in  small  apertures,  just  behind  the  posterior 
incisors,  which  lead  into  the  naso-palatine  canals,  and  these  form  a 
means  of  communication  between  the  anterior  portions  of  the 
nasal  chambers  and  the  buccal  cavity.  In  the  sides  of  the  hinder 
end  of  the  soft  palate  are  two  small  pits,  the  tonsils.  The  floor  of 
the  mouth  cavity  is  mainly  occupied  by  the  long  prominent  muscular 
tongue,  which  unlike  that  in  Rana  is  attached  posteriorly  and  free 
in  front.  It  is  covered  with  tiny  white  spots  that  mark  the  position 
of  the  taste  papillae.  On  the  sides  of  the  posterior  end  of  the  tongue, 
level  with  the  last  molar  teeth,  are  a  pair  of  oval  patches  crossed  by 
parallel  ridges  and  termed  the  papillae  foliatae.  Just  above  and 

302 


LEPUS   CUNICULUS  303 

behind  these  are  a  pair  of  small  white  spots  surrounded  by  shallow 
circular  grooves  and  termed  the  circumvallate  papillae. 

Opening  into  the  buccal  cavity  by  means  of  ducts  are  four  pairs 
of  salivary  glands  whose  function  is  to  produce  the  -saliva — the 
action  of  which  on  the  food  has  already  been  discussed  in  connection 
with  digestion.  The  infra-orbital  glands  are  fairly  large  irregularly 
lobate  glands  situated  just  below  the  eyeball.  Their  ducts  pass 
downwards  to  open  on  the  inside  of  the  cheek  nearly  opposite  to  the 
second  pre-molar  tooth.  The  parotid  gland  is  a  soft  mass  on  each 
side  just  below  the  skin  and  lying  in  front  of  and  below  the  base 
of  the  external  ear,  between  it  and  the  mandible.  Its  duct,  the 
Stenonion  duct,  passes  close  beneath  the  skin  parallel  with  the 
zygomatic  arch  to  open  on  the  inside  of  the  cheek  near  the  preceding 
duct.  The  sub-maxillary  gland  is  a  compact,  red,  ovoidal  structure 
situated  just  inside  the  angle  of  the  mandible,  and  its  canal,  Wharton's 
duct,  runs  along  the  inner  side  of  the  lower  jaw  to  open  midway 
between  the  posterior  incisors  and  the  base  of  the  tongue.  The 
fourth  pair  are  the  sub-lingual  glands,  elongated  flattened  red 
bodies  situated  on  the  inner  side  of  the  ramus,  between  it  and 
Wharton's  duct,  near  the  openings  of  which  their  own  ducts 
terminate. 

The  organs  of  Jacobson  are  two  small  tubular  bodies 
embedded  in  the  front  end  of  the  hard  palate  just  above  the  palatine 
processes  of  the  pre-maxillae.  They  communicate  with  the  nostrils 
in  front  and  by  means  of  the  naso-palatine  canals  with  the  buccal 
cavity.  Their  functional  significance  is  not  apparent. 

The  buccal  cavity  passes  backwards  into  the  pharynx  and 
the  point  of  transition  is  marked  dorsally  by  the  termination  of  the 
soft  palate,  so  that  the  posterior  nares  or  ends  of  the  narial  passages 
also  lead  into  it,  uniting  to  form  one  posterior  nasal  chamber  just 
before  so  doing.  Into  this  chamber  open  the  Eustachian  tubes 
leading  from  the  tympanic  cavities  of  the  ears.  On  the  floor  of  the 
pharynx  is  a  fairly  large  opening,  the  glottis,  leading  into  the  larynx 
and  so  allowing  air  to  reach  the  lungs.  From  the  front  wall  of 
this  opening,  a  thin  bilobed  flap  of  cartilage,  the  epiglottis,  projects 
upwards  into  the  pharynx.  In  swallowing,  this  flap  is  bent  down 
and  so  closes  the  glottis,  preventing  the  entry  of  food.  The  pharynx 
continues  on  into  the  oesophagus.  It  will  be  seen  then,  that  the 
Eustachian  tubes,  internal  nares  and  laryngeal  opening  are  all  in 
close  proximity  in  Lepus,  and  this  is  the  case  in  mammals  generally, 
hence  certain  diseases  are  liable  to  affect  ear,  nose  and  throat  at 
the  same  time. 

The  oesophagus  is  a  narrow  dilatable  tube  running  along 
the  neck  and  through  the  thoracic  cavity  close  below  the  vertebral 


304 


AN   INTRODUCTION  TO  ZOOLOGY 


column,  to  pass  through  the  diaphragm.  A  short  distance  inside 
the  abdominal  cavity  it  opens  by  an  aperture  termed  the  cardia 
into  the  stomach  on  the  left  side  of  its  anterior  wall.  Its  walls  are 
fairly  thick,  containing  non-striate  muscle  fibres  and  it  is  lined  by 
a  mucous  membrane.  The  stomach  itself  is  a  wide  dilated  sac-like 
structure,  rounded  and  much  larger  at  the  left  or  cardiac  end  and 
narrower  and  smaller  at  the  right  or  pyloric  end.  It  is  asymmetri- 
cally situated,  its  main  mass  lying  to  the  left  of  the  middle  line. 
It  terminates  on  the  right  in  a  constriction,  the  pylorus,  where 
it  is  narrowed  down  to  a  small  opening  leading  on  into  the  intestine. 
This  passage  is  guarded  by  a  ridge  of  the  mucous  membrane  and  a 
circle  of  muscles,  constituting  the  pyloric  sphincter,  which  enables 
the  stomach  to  be  shut  off  from  the  intestine  during  the  preliminary 
stages  of  digestion.  By  means  of  its 
muscle  layers  the  stomach  is  able  to  keep 
the  food  in  it  churned  up  until  it  is  ready 
to  be  passed  on.  The  first  part  of  the 
intestine  is  the  duodenum,  and  this  takes 
the  form  of  a  long  U-shaped  loop,  held 
together  by  an  omentum  and  running 
almost  the  whole  length  of  the  abdominal 
cavity  on  the  right  side. 

The  duodenum  passes  on  into  a  very 
similar  shaped  tube,  the  small  intestine  or 
ileum,  about  seven  or  eight  feet  long,  and 
thrown  into  a  series  of  folds  held  together 
by  omenta.  On  its  walls  appear,  here  and 
there,  small  granular  areas  slightly  darker 
in  colour  than  the  remainder.  These  are 
known  as  Peyer's  patches,  and  while,  like 
the  tonsils,  they  consist  of  lymphoid 
nodules,  their  exact  significance  has  not 
yet  been  ascertained.  The  walls  of  the 
small  intestine  will  be  found  to  be  thrown 

up  into  a  number  of  small,  closely  set,  blunt  papillae  the  villi, 
giving  it  the  appearance  of  the  "  pile "  of  a  carpet.  Each  is 
supplied  with  blood-vessels  and  also  lymphatics,  and  by  their 
means  a  large  part  of  the  absorption  is  carried  on  ;  the  blood- 
vessels taking  up  the  soluble  food  and  the  lymphatics  the 
emulsified  fat.  The  ileum  terminates  in  a  swollen  portion,  the 
sacculus  rotundas,  whose  walls  are  similar  in  structure  to  the 
patches,  and  this  opens  into  the  next  part  of  the  intestine  a  short 
distance  beyond  the  proximal  end  of  the  latter.  This  part  of  the 
gut,  the  large  intestine  or  colon,  is  about  one  and  a  half  feet  long,  and 


FIG.  103. — Two  intestinal 
villi.  Magnified  i  oo  di- 
ameters.— -From  Quain. 

a.,  b.,  and  c..  lacteals  ;  d., 
blood-vessels. 


LEPUS  CUNICULUS  305 

noticeably  wider  than  the  preceding  portions.  Its  walls  are  pursed 
up  into  a  series  of  marked  sacculations,  arranged  at  first  in  three 
longitudinal  rows,  then  in  two  rows,  then  in  one  and  finally  dis- 
appearing altogether  and  leaving  its  wall  smooth.  This  then  passes 
over  into  the  terminal  portion  of  the  gut,  the  rectum,  a  tube  about 
two  and  a  half  feet  long,  about  the  same  diameter  as  the  small 
intestine  ;  it  is  slightly  bent  at  first,  but  finally  runs  straight  back- 
wards through  the  pelvic  cavity  to  open  externally  at  the  anus. 
Throughout  its  length  it  is  usually  marked  by  the  presence  of  a 
series  of  pill-shaped  bodies,  the  faeces. 

At  the  junction  of  the  ileum  and  colon  is  a  large  blindly  ending 
tube,  the  coecum,  which  is  not  represented  in  the  frog  or  dogfish. 
It  is  a  little  longer  and  much  wider  than  the  colon  and  its  greenish 
coloured,  thin  walls  exhibit  about  twenty-five  turns  of  a  spiral 
constriction.  Proximally  it  opens  into  the  colon  and  distally  it 
ends  in  a  narrower  thick-walled  vermiform  appendix,  about  four 
inches  long,-  whose  walls  are  granular  in  texture  and  pinkish  in 
colour.  Animals  confined  to  a  vegetable  diet,  i.e.  herbivores, 
have  a  more  specialised  alimentary  canal  than  carnivorous  animals, 
and  we  find  that  they  possess  either  a  complicated  stomach  com- 
posed of  several  chambers,  or  if  the  stomach  be  simple  as  in  Lepus 
there  is  present  a  large  coecum. 

The  digestive  glands  comprise,  in  addition  to  those  in 
the  walls  of  the  stomach  and  intestine,  the  liver  and  pancreas. 

The  liver  is  a  large  brown  five-lobed  mass  lying  anterior  to  the 
stomach  with  its  front  surface  curved  convexly  to  fit  on  to  the 
diaphragm,  and  its  hinder  surface  hollowed  out  for  the  various 
parts  of  the  stomach.  It  is  attached  to  the  dorsal  body  wall  by 
a  fold  of  mesentery,  and  also  to  the  diaphragm  by  a  vertical  fold 
of  peritoneum,  the  suspensory  ligament,  which  also  marks  the 
division  of  the  gland  into  right  and  left  halves.  The  two  median 
lobes,  one  on  each  side  of  the  ligament,  are  termed  the  right  and 
left  central  lobes  respectively.  Outside  the  left  central  lobe  and 
between  it  and  the  cardiac  end  of  the  stomach  lies  the  left  lateral 
lobe.  Externally  to  the  right  central  lobe  is  the  caudate  lobe,  which 
is  applied  to  the  anterior  surface  of  the  pyloric  end  of  the  stomach 
and  postero-dorsally  is  hollowed  to  fit  over  the  front  end  of  the 
right  kidney.  The  remaining  lobe,  the  Spigelian,  is  shaped  so  as 
to  fit  closely  against  the  median  antero-dorsal  surface  of  the  stomach. 
The  gall-bladder  is  a  longish  thin-walled  sac  partially  embedded 
in  the  ventro-median  border  of  the  right  central  lobe,  and  from  it 
a  duct  about  two  inches  long  passes  backwards  to  open  into  the 
duodenum  just  a  short  distance  beyond  the  pylorus.  From  each 
lobe  of  the  liver  comes  a  hepatic  duct  to  open  into  this  main  duct 

x 


306  AN   INTRODUCTION   TO  ZOOLOGY 

in  which  therefore  two  portions  can  be  distinguished,  a  cystic 
duct  coming  from  the  bladder,  and  a  common  bile  duct  after  the 
various  hepatic  ducts  have  joined  it. 

The  pancreas  in  Lepus  is  unlike  that  in  Scylliwn  or  Rana,  in 
that  it  does  not  form  one  discrete  solid  mass.  On  the  contrary 
it  takes  the  form  of  a  number  of  scattered  glandular  nodules  generally 
distributed  throughout  the  omentum  of  the  duodenal  loop.  They 
appear  almost  like  little  lumps  of  fat  slightly  tinged  with  pink  and 
are  more  closely  aggregated  towards  the  anterior  end.  Here  the 
pancreatic  duct  starts  and  thence  runs  backwards,  receiving  small 
ducts  from  the  various  nodules  to  open  into  the  distal  loop  of  the 
duodenum  about  three  inches  beyond  the  bend. 

While  not  actually  forming  part  of  the  alimentary  system  we 
find,  as  in  the  dogfish  and  frog,  the  spleen  closely  attached  to  the 
stomach  by  a  gastro-splenic  omentum.  It  has  the  form  of  an 
elongated  strip  of  tissue  of  dark  red  colour  lying  in  the  cardiac 
region  of  the  stomach. 

A  general  outline  of  the  digestive  processes  in  a  vertebrate 
and  the  parts  played  therein  by  the  various  parts  of  the  alimentary 
canal  have  already  been  considered  when  dealing  with  the  frog,  so 
that  it  is  not  necessary  to  repeat  them  again  here. 

Respiratory  System. 

As  pointed  out  already  the  glottis,  protected  by  the 
epiglottis,  lies  on  the  floor  of  the  pharynx  ;  it  leads  into  a  cavity, 
the  larynx,  or  organ  of  voice.  The  wall  of  the  hollow  is  supported 
by  cartilages  of  which  the  first  is  termed  the  thyroid,  and  this  takes 
the  form  of  a  wide  band  incomplete  dorsally.  The  second  cartilage 
is  the  cricoid  which  forms  a  complete  loop,  narrow  ventrally  and 
broad  dorsally.  Articulating  with  the  antero-dorsal  edge  of  the 
cricoid  is  a  pair  of  small  cartilages,  the  arytenoids.  Stretching  across 
the  laryngeal  cavity  dorso-ventrally  are  a  pair  of  membranous 
flaps  strengthened  by  fibre-elastic  ligaments  ;  these  are  termed  the 
vocal  cords,  since  their  vibrations  produce  the  voice.  They  are 
attached  ventrally  to  the  thyroid  and  dorsally  to  the  cricoid 
cartilages.  The  presence  of  a  fairly  long  neck  makes  it  impossible 
for  the  larynx  to  lead  directly  into  the  lungs  as  in  Rana,  and 
consequently  we  find  a  longish  straight  tube,  the  trachea  or  wind- 
pipe, connecting  the  two.  The  trachea  lies  on  the  ventral  side,  of 
the  oesophagus  between  it  and  the  skin  of  the  neck.  Through- 
out the  whole  of  its  length  it  is  strengthened  by  a  series  of 
cartilaginous  rings  incomplete  dorsally,  which  keep  it  open  and 
nevertheless  allow  for  a  certain  amount  of  distention  of  the  adjacent 
oesophagus.  It  enters  the  thoracic  cavity  and  passes  dorsal  to  the 


LEPUS   CUNICULUS 


307 


heart  about  half-way  along  which  it  divides  into  two  branches,  the 
bronchi,  one  going  to  each  lung.  These  divide  up  into  smaller  and 
smaller  tubes,  the  bronchioles,  also  supported  by  small  cartilaginous 
rings,  thus  constituting  a  system  of  vessels  through  which  the  air 
is  conveyed  to  all  parts  of  the  lungs.  The  lungs  themselves  are 
soft  spongy  pink  bodies,  and  not  hollowed  sacs  as  in  Rana,  lying 
freely  in  the  thoracic  cavity  unattached  save  where  the  bronchi 
and  blood-vessels  enter  them.  Each  is  divided  into  two  main 
lobes,  but  there  are  on  the  right  side  two  small  accessory  lobes  of 
which  the  posterior  and  smallest  lies  in  the  middle  line  closely 
applied  to  the  oesophagus  just  behind  the  heart.  The  ultimate 
branches  of  the  bronchioles  finally  open  into  saccular  chambers, 
the  alveoli,  where  the  actual  respiratory  exchanges  take  place. 

The  peritoneum  lines  the  thoracic  cavity  and  is  reflected  round 
each  lung  separately  and  is  termed  the  pleura,  being  divided  into 
the  parietal  pleura  lining  the  wall  of  the  cavity  and  the  visceral 
pleura  around  the  lungs.  Thus  we  can  see  that  each  lung  has  its 
own  pleural  cavity,  and  further  the  two  lungs  are  separated  in  the 
middle  line  by  the  two  layers  of  the  pleura,  constituting  the  media- 
stinum and  enclosing  between  them  the  mediastinal  space.  Within 
this  lie  the  oesophagus,  heart,  main  blood-vessels,  the  end  of  the 
trachea  and  the  beginnings  of  the  bronchi,  and  the  space  is  shaped 
so  as  to  accommodate  them. 

The  mechanism  of  breathing  differs  much  from  that  in 
the  frog  owing  to  the  presence  of  the  ribs  and  sternum  making  an 
airtight  compartment  for  the  lungs.  The  ribs  lie  somewhat 
obliquely  and  by  means  of  the  intercostal  muscles  they  can  be  raised 
in  such  a  manner  as  to  enlarge  the  thoracic  cavity,  and  the  relaxation 
of  these  muscles  allows  the  chest  to  return  to  its  original  size. 
This  operation  brings  about  what  is  termed  costal  breathing. 
Further,  the  floor  of  the  cavity  is  formed  by  the  arched  diaphragm 
which,  when  its  muscles  contract,  flattens,  and  so  increases  the 
capacity  of  the  thorax,  the  return  is  accomplished  by  the  relaxing 
of  these  muscles  again  and  this  brings  about  diaphragmal  breathing. 
In  normal  respiration  both  types  play  their  part,  and  so  air  is  drawn 
into  the  lungs  by  the  enlargement  of  the  cavity.  Their  emptying 
is  brought  about  in  part  by  the  elasticity  of  the  lungs  themselves, 
but  also  by  a  return  to  their  original  positions  of  the  ribs  and 
diaphragm. 

Circulatory  System. 

The  circulatory  system  consists  of  the  same  main  divisions 
as  in  the  frog,  but  it  diifers  considerably  in  points  of  detail  and 
represents  a  higher  stage  of  development. 


308  AN   INTRODUCTION  TO  ZOOLOGY 

The  centre  of  the  system  is  the  heart,  and  while  the  heart  of  the 
rabbit  is  typically  mammalian,  yet  owing  to  its  small  size  it  is  not 
easy  to  examine,  and  after  considering  the  rest  of  the  circulatory 
system  we  shall  return  to  consider  a  mammalian  heart  as  exemplified 
in  a  larger  animal  such  as  the  pig  or  sheep.  It  is  sufficient  to  note 
only  the  main  points  here.  The  heart  consists  of  four  chambers, 
two  atria  with  auricular  appendages  and  two  ventricles,  a  right  and 
a  left  in  each  case,  and  the  cavities  of  one  side  of  the  heart  have  no 
means  of  communicating  directly  with  those  of  the  other.  Into 
these  chambers  open  or  from  them  come  off  the  main  vessels,  there 
being  no  structures  corresponding  with  the  sinus  venosus  or  conus 
arteriosus  of  Rana. 

Arterial  System. 

The  arterial  system  is  divided  into  two  portions,  the 
pulmonary  and  the  systemic.  The  pulmonary  artery  takes  origin 
from  the  antero-median  corner  of  the  right  ventricle,  passes  forwards 
and  arches  over  to  the  dorsal  side  of  the  left  atrium  where  it  divides 
into  two  branches,  one  going  to  each  lung.  The  base  of  the  pulmonary 
artery  is  guarded  by  three  tough  membranous  pocket-shaped 
semilunar  valves. 

The  systemic  arteries  are  all  served  by  a  single  large  trunk,  the 
aorta  or  aortic  arch,  which  arises  from  the  left  ventricle  and,  passing 
forwards  dorsal  to  the  pulmonary  artery  arches,  over  the  atrium, 
and  the  left  bronchus  reaches  the  left  side  of  the  vertebral  column 
along  which  it  runs  backwards  as  the  dorsal  aorta.  Upon  entering 
the  abdomen  it  takes  up  a  median  position  below  the  centra.  Just 
before  the  pulmonary  artery  divides  into  two  it  is  joined  to  the 
aorta  by  a  band  of  fibrous  tissue,  the  ligamentum  arteriosum,  which 
in  the  embryo  is  an  open  tube,  the  ductus  arteriosus,  and  is  the 
remnant  of  the  connection  between  these  vessels  in  the  embryo 
and  also  in  the  primitive  vertebrate,  just  as  the  ligamentum  caroticum 
of  Rana  is  the  persistent  remains  of  another  of  these  connections. 
The  base  of  the  aorta  is  guarded  by  a  series  of  three  pocket-shaped 
semilunar  valves,  similar  to  those  in  the  pulmonary  artery,  which 
prevent  the  blood  passing  back  again  into  the  ventricle.  Be- 
hind two  of  these  valves  are  the  openings  of  two  small  coronary 
arteries  which  carry  blood  to  the  tissues  of  the  heart  itself.  Just 
at  the  point  where  the  aorta  is  arching  over  it  gives  rise  to  a  short 
stout  innominate  artery,  turning  off  to  the  right  side,  which  after 
a  short  course  divides  into  the  right  common  carotid  and  right  sub- 
clavian  arteries.  The  right  common  carotid  artery  runs  forward 
alongside  the  trachea,  giving  off  branches  to  it  and  other  tissues 
of  the  neck  up  to  the  level  of  the  angle  of  the  jaw.  Here  it  divides 


LEPUS   CUNICULUS 


309 


LE 


into  the  internal  carotid,  which  enters  the  cranium  through  the 
carotid  foramen  and  is  dis- 
tributed to  the  brain,  and  the 
external  carotid,  which  rami- 
fies over  the  whole  of  the 
right  side  of  the  face  and 
head.  The  right  sub-clavian 
passes  straight  outwards  to- 
wards the  right  fore  limb. 
A  short  distance  along  this 
artery  is  given  off  a  small 
vertebral  artery.  This  runs 
forwards  and  inwards  to  pass 
into  the  vertebrarterial  canal 
supplying  the  spinal  cord  and 
also  the  posterior  end  of  the 
brain.  A  little  way  further 
on  arises  an  intercostal  artery 
which,  passing  backwards, 
gives  off  side  branches  to  the 
first  few  intercostal  spaces. 
Right  close  to  it  arises 
another  small  artery,  the 
anterior  epigastric  (internal 
mammary),  which  .runs  late- 
rally down  the  inside  of  the 
ventral  thoracic  wall.  Ihe 
sub-clavian  then  gives  off 
certain  branches  to  the 
shoulder  girdle  and  axillary 
region,  and  as  the  brachial 
artery  goes  on  into  the  arm 
which  it  supplies. 

A  short  distance  further 
along  the  main  aortic  arch 
gives  rise  to  the  left  common 
carotid  artery,  which  passes 

,1          ,         -i  j  j-          u.,  genital ;     ti.,  nepatic  :    i. A.  innominate ;    i.c, 

Up     the     trachea    and     IS    dlS-     intercostal ;   I.I.,  internal  iliac ;   I.L.,  ilio-lumbar 

tributed  in  a  similar  manner 
to  the  right.  Beyond  this 
again,  as  the  aorta  reaches 

,  .         .,  .         .  external  carotid  ;  K.  1.,  nglu  x^v^.^^xv,..,^ ,  *x. 

the  Vertebral  COlumn  it  gives     right  sub-clavian ;  V.,  vesicular  ;  V.A.,  vertebral. 

off  the  left  sub-clavian  artery, 

which    runs     outwards    to     become    the    brachial    after    giving 


FIG.  104. — Diagram  of  the  arterial 
system  of  Lepus. 

A.E.,  anterior  epigastric  ;  A.M.,  anterior  mesen- 
teric  ;   B.,  brachial;  C.,  coeliac;  C.I.,  common  iliac 
D.A.,  dorsal  aorta ;  E.I.,  external  iliac  ;  F.,  femoral 
G.,  genital ;     H.,  hepatic  ;    I.A.  innominate  ;    I.C. 


L.,  lieno-gastric  ;  L.C.,  left  common  carotid  ;  L.E. 
left  external  carotid  ;  L.I.,  left  internal  carotid 
L.S.,  left  sub-clavian ;  M.S.,  median  sacral ;  P.E. 
posterior  epigastric ;  P.M.,  posterior  mesenteric 
R.,  renal  ;  R.C.,  right  common  carotid  ;  R.E.,  right 
external  carotid  ;  R.  I.,  right  internal  carotid  ;  R.S., 


310  AN   INTRODUCTION   TO  ZOOLOGY 

off   the  same  branches   as  the  right  with  the   exception   of  the 
intercostal. 

The  aorta  then  runs  on  giving  off  a  short  series  of  paired 
intercostal  arteries  to  the  posterior  intercostal  spaces,  and  then  some 
distance  behind  the  diaphragm  gives  off  a  large  median  cceliac 
artery  which,  after  a  short  course  in  the  mesentery,  divides  into  a 
hepatic  trunk  going  to  the  liver  and  a  lieno-gastric  branch  supplying 
the  stomach  and  spleen.  A  second  large  artery,  the  anterior 
mesenteric,  arises  just  posterior  to  the  foregoing  and  divides  up  to 
go  to  the  duodenum,  pancreas,  small  intestine,  ccecum  and  colon. 
This  is  followed  by  a  pair  of  renal  arteries  which,  after  giving  off 
a  branch  to  the  musculature  of  the  dorsal  body  wall,  proceeds  to 
the  kidney.  The  right  vessel  arises  a  short  distance  ahead  of  the 
left.  Some  distance  further  back  arise  a  similar  asymmetrical 
pair  of  genital  arteries  differently  named  in  the  two  sexes.  In 
the  female  the  ovarian  arteries  pass  out  laterally  to  the  ovaries,  but 
in  the  male,  owing  to  the  migration  of  the  testes,  the  spermatic 
arteries  run  outwards  for  a  short  distance  and  then  back  and  into 
the  scrotal  sacs.  The  posterior  mesenteric  artery  is  a  small  median 
vessel  originating  just  before  or  just  behind  the  genitals  and  going 
to  the  hinder  part  of  the  rectum.  Posterior  to  this  the  median 
sacral  artery  is  given  off  from  the  dorsal  side  of  the  aorta  and  it 
runs  parallel  with  this  to  the  tail :  it  is  probably  to  be  regarded  as 
being  actually  the  continuation  of  the  aorta  itself.  Shortly  after 
this  the  main  vessel — the  so-called  aorta — bifurcates  to  form  the 
common  iliac  arteries,  which  pass  out  at  an  angle  laterally  towards 
the  legs.  A  short  way  along  each  vessel  gives  off  on  its  anterior 
side  an  ilio-lumbar  artery  to  the  dorsal  body  wall.  This  is  followed 
by  a  pair  of  arteries,  the  internal  iliac  and  the  vesicular,  on  the 
posterior  wall.  The  former  spreads  over  the  dorsal  wall  of  the 
pelvic  cavity  and  the  latter  supplies  the  bladder.  Another  small 
artery,  the  posterior  epigastric,  is  given  off  from  the  anterior  wall  of 
the  main  trunk  which  may  now  be  termed  the  external  iliac.  It 
passes  up  the  ventral  abdominal  wall.  Finally  the  external  iliac 
passes  on  into  the  leg  as  the  femoral  artery. 

Venous  System. 

The  veins  of  the  body  can  be  considered  as  falling  into 
three  more  or  less  independent,  but  nevertheless  related  groups  of 
vessels  and  dealt  with  under  the  headings  :  the  pulmonary  system, 
the  hepatic  portal  system  and  the  system  of  the  caval  veins. 

The  pulmonary  veins  bring  the  blood  back  from  the  lungs. 
Each  one  is  formed  by  the  union  of  two  main  trunks  coming  from  those 
bodies  quite  close  to  the  heart  and  they  open  into  the  right  atrium. 


LEPUS  CUNICULUS  311 

The  hepatic  portal  system  consists  of  a  series  of  veins 
coming  from  all  parts  of  the  alimentary  canal  which  ultimately 
unite  to  form  a  large  vessel,  the  hepatic  portal  vein,  which,  passing 
forward  in  the  mesentery  near  the  post-caval  vein,  divides  into 
branches  going  to  the  various  lobes  of  the  liver.  Its  various  factors 
can  readily  be  made  out  in  a  freshly  killed  animal  when  they  are 
usually  full  of  blood.  Four  main  trunks  can  be  readily  distinguished. 
The  posterior  mesenteric  vein  comes  from  the  hinder  portion  of  the 
rectum,  and  lies  mainly  in  the  mesorectum.  The  anterior  mesenteric 
vein  is  a  larger  trunk  formed  by  the  union  of  numerous  vessels 
coming  from  the  front  part  of  the  rectum,  the  ilium,  the  coccum 
and  the  colon.  The  duodenal  loop  and  the  pancreas  are  drained 
by  a  single  vessel,  the  duodenal  vein,  and  the  blood  from  the  stomach 
and  spleen  is  returned  by  a  lieno-gastric  vein. 

Three  main  systemic  or  caval  veins  are  present,  two 
anterior  and  one  posterior.  The  right  pre-caval  vein  (vena  cava 
anterior  dextra)  is  formed  by  the  junction  of  the  jugular  and  sub- 
clavian  veins  just  in  front  of  the  first  rib.  It  runs  back  on  the 
inner  side  of  the  right  lung,  and  opens  into  the  dorsal  wall  of  the 
right  atrium.  The  azygos  vein  (azygos  cardinal)  is  a  small  vessel 
running  forward  along  the  right  side  of  the  vertebral  column  from 
the  posterior  end  of  the  thorax,  receiving,  as  it  does  so,  factors  from 
the  hinder  intercostal  spaces.  Finally  it  passes  round  the  oesophagus 
to  open  into  the  pre-caval,  just  before  this  enters  the  atrium.  In 
spite  of  its  small  size  it  is  of  interest,  since  it  represents  the  persistent 
remnant  of  the  right  posterior  cardinal  vein  of  the  lower  vertebrates 
such  as  the  dogfish,  while  all  trace  of  it  has  disappeared  in  the  frog. 
The  anterior  phrenic  vein  is  a  small  vessel  bringing  blood  from  the 
ventral  portion  of  the  diaphragm,  and  opening  into  the  ventral 
side  of  the  pre-caval.  The  right  anterior  intercostal  vein  is  a  short 
dorsally  situated  vessel  receiving  factors  from  the  anterior  four  or 
five  intercostal  spaces  and  opening  close  to  the  preceding  vein. 
The  right  anterior  epigastric  vein  (internal  mammary)  lies  on  the 
ventral  thoracic  wall  in  close  proximity  to  the  corresponding  artery, 
and  it  joins  the  caval  vein  at  the  level  of  the  first  rib. 

The  right  sub-clavian  vein  is  a  stout  trunk  which  drains  the 
right  fore  limbs  and  shoulder  girdle,  being  termed  the  brachial  vein, 
in  the  upper  arm. 

The  right  external  jugular  is  a  large  vein  running  fairly  super- 
ficially the  whole  length  of  the  neck,  from  just  behind  the  angle 
of  the  jaw  where  it  is  formed  by  the  confluence  of  the  anterior  and 
posterior  facial  veins,  to  the  point  where  it  joins  the  sub-clavian. 
It  drains  the  whole  of  the  facial  region  and  receives  factors  from  the 
oesophagus,  trachea  and  various  tissues  of  the  neck.  It  has 


312 


AN   INTRODUCTION  TO  ZOOLOGY 


previously  been  pointed  out  that  the  internal  jugular  vein  leaves 

the  cranial  cavity  through  the 
foramen  lacerum  posterius  in 
company  with  the  ninth,  tenth 
and  eleventh  cranial  nerves. 
From  this  point  it  runs  back 
along  the  side  of  the  trachea 
to  enter  the  external  jugular 
vein  just  before  this  joins  the 
sub-clavian,  and  the  trunk 
formed  by  the  union  of  these 
two  vessels  is  sometimes 
designated  the  common  jugu- 
lar vein. 

The  leit  pre-caval 
vein  (vena  cava  anterior 
sinistra)  'is  constituted  in  a 
similar  way  to  the  right, 
with  the  one  important  ex- 
ception that  it  receives  no 
azygos  vein.  After  its  for- 
mation in  front  of  the  first 
rib  it  enters  the  thoracic 
cavity,  passes  back  along  the 
inner  side  of  the  left  lung  and 
crosses  between  the  heart  and 
the  root  of  the  left  lung  to 
enter  the  left  dorsal  wall  of 
the  right  atrium. 

The  post-caval  vein 
(vena  cava  posterior)  is  a  large 
trunk  formed  at  the  posterior 
end  of  the  abdominal  cavity 
by  the  union  of  the  two  in- 
ternal iliac  veins.  Thence  it 
runs  forward  beneath  the 
vertebral  column  slightly  to 
one  side  of  and  close  to  the 
aorta  up  to  the  liver,  in  the 
dorsal  surface  of  which  it 
becomes  embedded.  It  enters 
the  thorax  through  an  aper- 
ture in  the  central  tendon  of 
the  diaphragm,  and  runs  forward  slightly  to  the  right  of  the  middle 


FIG.  105. — Diagram  of  the  venous 
system  of  Lepus. 

A.,  azygos  ;  A.F.,  anterior  facial ;  E.I.,  external 
iliac  ;  F.,  femoral ;  G.,  genital ;  H.,  hepatic  ;  I.,  in- 
tercostals  ;  1. 1.,  internal  iliac  ;  I.L.,  ilio-lumbar  ; 
L.E.,  left  anterior  epigastric  ;  L.Ex.,  left  external 
jugular ;  L.I.,  left  internal  jugular ;  L.P.,  left 
pre-caval;  L.S.,  left  sub-clavian;  P.,  phrenics ; 
P.C.,  ppst-caval ;  P.P.,  posterior  facial;  R.,  renal; 
R.A.,  right  anterior  intercostal  ;  R.E.,  right  anterior 
epigastric ;  R.Ex.,  right  external  jugular ;  R.I., 
right  internal  jugular  ;  R.P. ,  right  pre-caval ;  R.S., 
right  sub-clavian  ;  V.,  vesicular. 


LEPUS   CUNICULUS  313 

line  and  opens  into  the  postero-dorsal  wall  of  the  right  atrium. 
The  internal  iliac  veins  drain  the  back  of  the  thighs,  and  run  in  the 
dorsal  wall  of  the  pelvic  cavity,  at  the  anterior  end  of  which  they 
unite  to  form  the  post-caval.  A  short  distance  in  front  of  this 
the  two  external  iliacs  enter  the  main  trunk.  They  are  large 
vessels  coming  from  the  hind  limb  along  the  pre-axial  side  of 
which  they  run  as  the  femoral  veins.  In  the  abdominal  cavity 
they  receive  small  veins,  including  a  vesicular,  from  the  urinary 
bladder,  and  in  the  female  rabbit  also  branches  from  the  uterus. 
Just  in  front  of  this  again  the  post-caval  receives  the  paired 
ilio-lumbar  veins,  which  bring  blood  back  from  the  posterior 
abdominal  waUs.  Still  more  anteriorly  are  the  genital  veins, 
the  spermatic  veins  in  the  male  and  ovarian  veins  in  the  female. 
In  the  female  they  pass  practically  straight  outwards,  but  in  the 
male  they  pass  outwards  a  short  way  and  then  backwards  in  company 
with  the  corresponding  artery  into  the  scrotal  sacs.  The  renal  veins 
which  open  more  anteriorly,  although  paired,  are  asymmetrical, 
the  right  being  shorter,  about  three-quarters  of  an  inch  long  and 
in  front  of  the  left.  A  certain  amount  of  variation  is  to  be  found  in 
the  veins  of  the  posterior  end  of  the  abdomen,  for  the  left  spermatic 
vein  sometimes  enters  the  ilio-lumbar  or  the  renal  of  the  same  side 
instead  of  entering  the  post-caval  separately.  Large  hepatic  veins 
return  blood  from  the  various  lobes  of  the  liver,  and  enter  the  post- 
caval  as  it  passes  through  this  gland.  As  a  rule  about  four  large 
trunks  can  be  distinguished  coming  from  the  liyer.  The  diaphragm 
is  drained  by  the  small  phrenic  veins  running  in  its  substance,  the 
most  ventral  of  these  open  into  the  pre-caval  veins,  but  the  others 
flow  into  the  post-caval  as  it  passes  through  the  central  tendon. 

It  will  be  noticed  that  there  is  a  much  closer  correspondence 
between  the  arteries  and  veins  in  Lepus  than  in  Rana,  and  indeed 
over  the  greater  part  of  the  system  we  find  that  the  main  vessels 
of  the  two  systems  accompany  one  another. 

Attention  has  already  been  called  to  the  fact  that  the 
heart  is  a  very  important  organ,  but  as  it  has  only  been  dealt  with 
in  broad  outline,  we  may  return  to  the  question  here. 

Mammalian  Heart. 

The  heart  of  the  mammal  is  an  important  organ,  and  as  it 
is  so  small  in  the  rabbit  it  is  advisable  to  study  its  structure  in  a 
larger  animal  such  as  the  pig  or  the  sheep.  It  should  be  borne  in 
mind  that  there  are  slight  differences  in  the  relationship  of  the  main 
veins  to  the  atria  and  the  main  branches  of  the  arteries  in  the  three 
types.  They  are  only  of  minor  importance,  however,  and  do  not 
affect  the  general  plan,  so  that  they  need  not  be  considered  here. 


314  AN   INTRODUCTION   TO  ZOOLOGY 

In  making  a  practical  examination  of  a  heart  care  should  be  taken 
to  choose  a  specimen  that  has  not  been  damaged  in  removal  from 
the  body,  and  one  that  has  the  bases  of  the  main  arteries  and  veins 
left  as  long  as  possible.  If  a  sheep's  heart  be  chosen,  it  is  often 
advisable  to  remove  the  greater  part  of  the  fat  attached  to  it  before 
starting  dissection.  The  following  description  is  based  mainly  on 
the  heart  of  the  sheep. 

The  mammalian  heart  is  a  somewhat  conical  structure  lying 
almost  in  the  mid -ventral  line  of  the  thorax  with  its  broader  end,  the 
basis  cordis  or  base,  directed  anteriorly  and  to  the  right,  and  the 


DA 


SV 

^&.  *AJT  .  ^mffl;// 

RV 

LV 


A 

FIG.  106. — Ventral  view  of  sheep's  heart  with  atria  collapsed. 

A.,  apex  ;  D.A.,  dorsal  aorta  ;  F.,  fatty  tissue  ;  I.,  innominate  artery  ;  L.A.,  left  auricle  ;  L.V., 
left  ventricle  ;  P.,  pulmonary  artery  ;  R. A.,  right  auricle  ;  R.V.,  right  ventricle  ;  S.V.,  sinus  longi- 
tudinalis  ventralis. 

bluntly-pointed  apex  cordis  pointing  posteriorly  and  to  the  left. 
It  consists  of  four  entirely  separate  chambers,  a  right  and  left 
ventricle  and  a  right  and  left  atrium,  and  when  empty  the  main  part 
of  its  bulk  consists  of  the  two  large  fleshy  ventricles  with  parts  of 
the  atria  appearing  as  flaps  upon  them,  and  separated  by  the 
deep  coronary  sulcus.  In  the  body  the  heart  is  enclosed  in  a 
double-walled  sac,  the  pericardium,  whose  inner  layer  is  tightly 
attached  to  the  wall  of  the  heart,  and  whose  loose  outer  layer 
is  generally  removed  in  taking  the  heart  from  the  body.  The  line 
where  the  inner  layer  is  reflected  to  form  the  outer  will  be  seen 


LEPUS   CUNICULUS  315 

in  the  at  rial  region  around  the  bases  of  the  main  blood-vessels. 
Between  the  two  layers  is  the  pericardia!  space,  rilled  in  life  with  a 
lymph-like  pericardial  fluid. 

On  the  ventral,  more  rounded  surface  of  the  heart  a  shallow  groove 
containing  small  blood-vessels,  the  sulcus  longitudinalis  ventralis, 
runs  from  a  point  on  the  base  to  the  left  of  the  middle  line  obliquely 
across  to  the  right  side,  a  short  distance  above  the  apex.  A  some- 
what similar  but  shallower  groove  also  containing  blood-vessels, 
the  sulcus  longitudinalis  dorsalis,  is  present  on  the  dorsal,  flatter 


L.V 


A 

FIG.  107. — Dorsal  view  of  sheep's  heart  with  atria  collapsed. 

A.,  apex  ;  C.S.,  coronary  sulcus  ;  D.A.,  dorsal  aorta  ;  F.,  fatty  tissue  ;  L.A.,  left  auricle  ; 
L.V.,  left  ventricle  ;  P.A.,  pulmonary  arteries  ;  P.C.,  pre-caval  veins  ;  Po.,  post-caval  veins  ;  P.V., 
pulmonary  vein  ;  R.A.,  right  auricle  ;  R.V.,  right  ventricle  ;  S.D.,  sinus  longitudinalis  dorsalis. 

side  of  the  heart.  These  two  external  grooves  mark  the  position  of 
an  internal  partition  separating  right  and  left  ventricles.  The 
atria  are  marked  off  from  the  ventricles  by  a  very  deep  cleft,  the 
coronary  sulcus,  that  runs  completely  round  the  heart  save  where  it 
is  interrupted  by  the  main  blood-vessels. 

The  two  atria  when  inflated  appear  on  the  external  dorsal 
surface  almost  as  one  large  thin- walled  sac,  but  they  are  nevertheless 
completely  separated  internally  by  the  septum  atriorum.  Each 
atrium  consists  of  a  hollow  sac  with  a  thin  wall,  and  bears  at  its 
outer  corner  a  thicker  appendage,  the  auricle,  internally  marked  by  a 


3i6  AN   INTRODUCTION  TO  ZOOLOGY 

series  of  interlacing  muscular  ridges,  the  musculi  pectinati.  It  is 
these  appendages  that  show  prominently  externally  when  the  atria 
are  deflated. 

Into  the  right  atrium  open  two  large  veins,  the  anterior  and 
posterior  caval  veins,  whose  openings  are  separated  by  a  ridge,  the 
tuberculum  intervenosum,  and  a  smaller  vessel,  the  coronany  vein, 
bringing  back  the  blood  from  the  heart  itself.  A  sinus  venosus, 
such  as  we  find  in  the  frog,  is  not  present,  it  having  been  absorbed 
into  the  atrium  in  the  course  of  the  evolution  of  the  mammal.  Into 
the  left  atrium  open  the  two  pulmonary  veins.  The  septum 
atriorum  is  a  fairly  thin  smooth-walled  partition  that  completely 
separates  the  two  atrial  cavities,  and  in  it  is  an  oval  area  much 


FIG.  1 08. — The  base  of  the  heart. — From  Quain. 

The  auricles  h'ave  been  cut  away,  and  the  valves  are  closed.  The  pericardium  has  also  been 
removed  to  expose  the  muscular  fibres. 

i  and  i',  right  ventricle  ;  2,  left  ventricle  ;  3,  wall  of  right  auricle  ;  4,  wall  of  left  auricle  ; 
5,  5',  and  5",  the  tricuspid  valve  ;  6  and  6',  the  mitral  valve  ;  7,  pulmonary  artery  ;  8,  aorta ; 
9  and  9',  coronary  arteries. 

thinner  than  the  rest  and  semi-transparent.  This  is  termed  the 
fossa  ovalis,  and  in  the  embryo  it  was  an  aperture,  leading  directly 
from  one  cavity  to  the  other.  Each  atrium  opens  into  its  corre- 
sponding ventricle  by  an  atrio-ventricular  orifice  guarded  by  strong 
valves.  On  the  right  the  valve  is  composed  of  three  large,  tough, 
membranous  flaps,  and  so  is  termed  the  tricuspid  valve,  while  on  the 
left  there  are  but  two  similar  flaps  constituting  the  bicuspid  or  mitral 
valve,  from  its  somewhat  fanciful  resemblance  to  a  bishop's  mitre. 

The  ventricles  stand  in  marked  contrast  with  the  auricles 
on  account  of  their  very  thick,  strongly  muscular  walls.  Their 
cavities  are  completely  separated  by  the  septum  ventriculorum, 
which  is  also  thick  and  muscular.  A  transverse  section  through  the 


LEPUS  CUNICULUS  317 

ventricular  region  shows  a  noticeable  difference  between  the  two 
sides.  The  cavity  of  the  left  ventricle  appears  almost  circular  in 
outline,  and  its  walls  are  noticeably  thicker  than  the  right.  The 
cavity  of  the  right  ventricle  appears  crescent-shaped  and  partly 
wrapped  around  the  left,  so  that  the  latter  ventricle  with  the  septum 
ventriculorum  occupies  by  far  the  larger  part  of  the  heart.  The 
walls  of  both  ventricles  are  seen  to  be  thrown  up  internally  into  a 
series  of  prominent  rounded  muscular  ridges,  the  trabeculae  carneee. 
In  order  to  strengthen  them  the  edges  of  the  atrio- ventricular  valves 
are  each  provided  with  a  number  of  tough  ligamentous  cords,  the 
chordae  tendinese,  which  take  their  origin  from  raised  pyramidal 
muscular  projections,  the  musculi  papillares,  on  the  walls  of  the 
ventricles. 

From  the  anterior  median  end  of  the  left  ventricle  comes  off  the 
large  aorta,  the  main  artery  of  the  body,  and  at  its  base  lie  a  series 
of  three  pocket-shaped  semilunar  valves,  which  can  close  it  com- 
pletely. Behind  two  of  the  valves  are  small  apertures  leading  to 
the  two  coronary  arteries,  which  supply  the  actual  tissue  of  the  heart 
itself.  The  large  pulmonary  artery  originates  from  the  anterior 
median  end  of  the  right  ventricle,  and  this,  too,  is  guarded  at  its  base 
by  three  semilunar  valves  similar  to  those  in  the  aorta.  The 
pulmonary  artery  soon  divides  into  two  main  branches,  one  going  to 
each  lung.  The  conus  arteriosus  no  longer  exists  a  separate  element 
in  the  mammalian  heart,  it  has  been  absorbed  into  the  ventricle. 

We  may  now  consider  quite  briefly  the  action  of  the  heart. 
Blood  from  the  body  is  brought  back  by  the  Venae  cavae  into  the 
right  atrium,  which,  therefore,  contains  non-aerated  blood.  This 
is  then  passed  into  the  right  ventricle,  and  it  leaves  this  through  the 
pulmonary  artery,  so  that,  as  there  is  no  communication  between 
one  side  of  the  heart  and  the  other,  the  right  side  of  the  heart  and 
all  the  vessels  connected  with  it  contain  nothing  but  non-aerated 
blood.  In  the  lungs  the  blood  is  aerated  and  brought  back  by  the 
pulmonary  veins  to  the  left  atrium,  whence  it  passes  to  the  left 
ventricle,  and  thence  via  the  aorta  to  all  parts  of  the  body.  The 
left  side  of  the  heart  and  its  vessels,  therefore,  contain  only  aerated 
blood.  We  see  then  that  in  the  vertebrate  series  we  have  three 
possible  arrangements  in  the  blood  circulatory  system.  Firstly, 
in  the  fish,  as  exemplified  by  Scyllium,  the  blood  goes  from  the 
single  ventricle  to  the  gills  to  be  oxygenated,  thence  to  the  whole 
of  the  body,  and  finally  back  to  the  single  atrium,  by  the  ductus 
Cuvieri ;  no  other  course  is  possible.  THis  is  then  fitly  termed  a 
single  or  a  branchial  circulation.  Secondly,  in  the  mammal,  as  we 
have  seen,  blood  from  the  body  is  brought  to  the  right  side  of  the 


AN   INTRODUCTION  TO   ZOOLOGY 


heart,  taken  thence  to  the  lungs,  back  again  to  the  left  side  of  the 
heart,  and  thence  to  the  body.  Here,  again,  there  is  no  alternative 
course  ;  to  reach  the  same  point  again  any  given  portion  of  blood 

must  return  twice  to  the 
heart,  it  must  pass  through 
both  the  pulmonary  and 
systemic  circulations.  Such 
a  condition  we  characterise 
as  a  complete  double  circu- 
lation. Lastly,  in  Rana,  we 
have  a  condition  in  some 
respects  intermediate  be- 
tween these  two.  While 
we  have  both  a  pulmonary 
and  a  systemic  circulation, 
each  related  to  a  separate 
atrium,  yet  there  is  a  possi- 
bility of  a  mixture  of  blood 
in  either  the  ventricle  or 
the  conus  arteriosus.  It  is 
conceivable,  therefore,  that 
any  given  portion  of  blood 
might  only  go  to  the  heart 
once  before  returning  again 
to  the  same  point.  We 
indicate  this  by  describing 
the  blood  circulatory  system 
of  the  frog  as  incompletely 
double. 

As  in  all  land-dwelling 
vertebrates  the  blood  in  a 
mammal  is  kept  circulatory 
through  the  lungs  and  the 
body  by  means  of  the 
alternate  contraction  and 
expansion,  the  two  con- 
ditions passed  through 
being  termed  systole  and 
diastole  respectively.  The 
former  commences  practi- 
cally simultaneously  in  the  two  atria,  and  drives  the  blood  from 
these  chambers  into  the  ventricles.  When  these  latter  are  rilled 
and  the  pressure  in  them  exceeds  that  in  the  atria,  the  blood 
attempts  to  flow  back,  but  is  prevented  by  the  closure  of  the 


FIG.  109. — Diagram  illustrating  the  circu- 
lation.— From  Furneaux. 

i,  right  atrium  ;  2,  left  atrium  ;  3,  right  ventricle  ; 
4,  left  ventricle  ;  5,  vena  cava  superior  ;  6,  vena  cava 
inferior  ;  7,  pulmonary  arteries  ;  8,  lungs  ;  9,  pul- 
monary veins;  10,  aorta;  u,  alimentary  canal ;  12, 
liver  ;  13,  hepatic  artery  ;  14,  portal  vein  ;  15,  hepatic 
vein. 


LEPUS   CUNICULUS  319 

atrio-ventricular  valves.  Following  the  sharp  systole  the  atria 
pass  through  a  relatively  long  period  of  slow  distension  or  diastole. 
This  period  can  only  be  considered  long  when  compared  with  the 
time  occupied  by  systole  ;  actually,  of  course,  the  whole  time 
occupied  by  the  two  is  measured  in  fractions  of  a  second.  In  a 
similar  manner  the  ventricle  undergoes  a  sharp  systole  followed  by  a 
longer  diastole,  and  naturally  the  end  of  ventricular  diastole  coin- 
cides with  atrial  systole.  The  atrial  diastole,  however,  is  not 
similarly  related  to  ventricular  systole,  and  there  is  a  period  of 
time  when  all  chambers  of  the  heart  are  in  diastole.  The  con- 
traction of  the  ventricles  expels  the  blood  through  the  aorta  and 
pulmonary  artery.  These  vessels  having  elastic  walls  enlarge 
with  the  inflow  of  blood,  and  as  soon  as  the  ventricular  con- 
traction relax,  their  elasticity  tends  to  drive  the  blood  back  into 
the  ventricle,  a  proceeding  that  is  stopped  by  the  rapid  closure 
of  the  semilunar  valves.  It  will  be  recalled  that  behind  two 
of  the  aortic  valves  lie  the  apertures  of  the  coronary  arteries, 
and  the  blood  is  driven  into  the  coronary  circulation  by  this  same 
reverse  wave  of  pressure  that  shuts  the  valves.  The  elasticity  of 
the  walls  of  the  two  main  arteries  is  sufficient  to  drive  the  blood 
onwards  to  the  body  or  lungs  as  the  case  may  be. 

In  listening  to  the  heart  beating  within  the  thorax  two  distinct 
sounds  can  readily  be  made  out.  The  first  is  a  low,  dull  sound,  and 
the  second  a  higher,  sharper  one.  The  latter  is  found  to  be  due  to 
the  closure  of  the  semilunar  valves  at  the  bases  of  the  two  arterial 
trunks,  while  the  former  is  the  result  of  several  causes,  the  principal 
among  which  are  the  tensing  of  the  mass  of  the  ventricular  muscles, 
marking  the  beginning  of  systole  and  the  practically  simultaneous 
closure  of  the  atrio-ventricular  valves.  It  should  be  obvious  then 
that  imperfect  structure  or  functioning  of  either  sets  of  valves  will 
affect  the  quality  of  the  corresponding  heart  sound,  and  so  can  be 
detected  by  listening  to  the  beat  and  comparing  its  sound  with  those 
made  by  the  normal  heart. 

Urogenital  System. 

As  in  the  frog  we  find  that  the  excretory  and  reproductive 
systems  are  closely  related  anatomically,  and  while  the  sperms  do 
not  actually  utilise  the  kidney  ducts,  the  ends  of  the  reproductive 
and  excretory  ducts  are  closely  related  in  the  two  sexes,  and  the 
same  external  aperture  serves  for  the  emission  of  the  urine  and 
the  genital  products.  This  external  opening  is  independent  of  the 
opening  of  the  alimentary  canal,  and  as  it  is  concerned  with  the 
reproductive  functions  it  naturally  differs  considerably  in  the  two 
sexes. 


320  AN   INTRODUCTION   TO  ZOOLOGY 

The .  kidneys  in  the  male  lie  in  the  abdominal  cavity 
attached  to  the  latero-dorsal  wall  of  the  coelom,  and  as  has  been 
noticed  in  dealing  with  their  vascular  supply,  the  right  kidney  lies 
somewhat  in  front  of  the  left.  Each  kidney  is  a  flattened  ovoidal 
body  of  dark  red  colour  with  a  distinct  notch,  the  hilus,  on  its  inner 
surface  marking  the  point  at  which  the  blood-vessels  join  it  and  the 
ureter  leaves  it.  On  cutting  off  the  ventral  half  of  the  kidney  it 
will  be  seen  that  the  substance  of  which  it  is  composed  is  distinguish- 
able into  two  portions.  The  outer  part,  or  cortex,  is  somewhat 
granular  in  appearance  and  marked  by  a  series  of  small  spots  ;  it  is 
composed  mainly  of  the  much  looped  and  twisted  secretory  portions 
of  the  uriniferous  tubules,  and  the  spots  indicate  the  positions  of  the 
Malpighian  bodies.  The  inner  part,  or  medulla,  is  seen  to  be  striated 
and  it  consists  almost  entirely  of  the  collecting  parts  of  the  tubules 
which  are  nearly  straight  and  cut  lengthwise.  It  is  drawn  up  into 
a  pointed  pyramidal  projection,  termed  the  pyramid,  upon  the  blunt 
apex  of  which  the  tubules  open.  Inside  the  hilus  is  a  crescentic 
cavity,  the  pelvis  of  the  kidney,  which  represents  the  dilated  ex- 
tremity of  the  ureter,  and  lies  within  the  body  of  the  gland.  The 
ureters  are  long,  fairly  narrow  tubes  passing  along  the  dorsal  body 
wall  to  enter  directly  into  the  bladder  at  the  level  of  the  front  end 
of  the  pelvic  symphysis.  The  urinary  bladder  itself  is  a  large 
dilatable,  muscular,  thin-walled  sac  in  which  the  urine  is  stored  as  it 
is  secreted,  and  can  be  emptied  at  the  will  of  the  animal.  The  path 
from  the  bladder  will  be  considered  when  treating  of  the  repro- 
ductive organs,  but  it  should  be  noted  in  passing  that  the  ureters 
in  Lepus  open  directly  into  the  bladder,  and  not  into  the  cloaca  as 
in  Rana.  We  find  in  the  embryo,  however,  that  they  originally 
open  into  the  dorsal  wall  of  the  rectum,  from  the  opposite  side  of 
which  is  given  off  the  ventral  outgrowth  destined  to  become  the 
bladder.  In  the  course  of  development,  however,  as  the  urogenital 
opening  becomes  separated  from  the  end  of  the  alimentary  canal, 
the  openings  gradually  migrate  round  the  rectum  until  they  pass 
on  to  the  dorsal  wall  of  the  bladder  itself. 

During  the  embryonic  history  of  the  excretory  organs  two  other 
kidneys  make  their  appearance  ;  the  first  is  the  pronephros,  and 
the  second  is  the  mesonephros  or  Wolffian  body,  but  both  of  these 
disappear  save  for  certain  remnants  which,  however,  lose  their 
excreting  function  and  become  related  to  the  reproductive  organs. 
The  functional  kidney  in  the  adult  is  the  third  to  appear,  and  it  is 
termed  the  metanephros.  It  differs  from  those  preceding  it  in  that 
its  tubules  are  never  segmentally  arranged,  but  all  open  together 
at  one  point  ;  consequent  upon  this  we  find  that  from  the  very 
beginning  its  tubules  are  not  related  to  the  posterior  segments  of  the 


LEPUS   CUNICULUS 


32i 


is 


B 


body,  but  it  is  a  compact  discrete   organ.     Then,  too,  the  tubules 
do  not  possess  nephrostomic  funnels  opening  into  a  lymph  cavity 
as  we  find  in  the  mesonephros  of  Rana  ;  and,  lastly,  the  ureter  i' 
also   a  separate  structure,  not 
related  in  any  way  to  the  old 
segment al  duct. 

The  essential  male  re- 
productive organs  are  the  testes. 
In  the  embryo  these  are  situated 
on  the  latero-dorsal  abdominal 
wall  just  behind  the  kidneys, 
and  like  them  are  asymmetrical, 
the  right  lying  in  front  of  the 
left.  As  the  animal  reaches 
maturity,  however,  they  migrate 
backwards  and  finally  lie  in 
extensions  of  the  body  cavity, 
the  scrotal  sacs,  which  appear 
externally  as  small  pouch-like 
projections  of  the  body  wall  in 
the  perineal  region.  The  aper- 
ture joining  the  cavity  of  the 
sac  to  the  abdomen  is  termed 
the  inguinal  canal,  and  the  two 
cavities  are  always  in  open 
communication  through  it.  Of 
course  the  testis  was  attached 
to  the  body  wall  in  its  original 
position  by  a  fold  of  peri- 
toneum, and  there,  too,  it 
received  its  vascular  and  ner- 
vous connections.  These  ob- 
viously could  not  be  broken, 
and  so  we  find  a  strand  of 
connective  tissue  with  a  nerve, 
an  artery  and  a  vein,  attached  to  each  testis  and  passing  up 
through  the  inguinal  canal  to  the  primitive  position  of  that  body. 
The  strand  so  formed  is  termed  the  spermatic  cord. 

The  testes  themselves  are  oval  pinkish- white  bodies  lying  wholly 
within  the  scrotal  sacs  ;  it  is  possible,  however,  to  pull  them  through 
the  inguinal  canal  into  the  abdominal  cavity  by  means  of  the 
spermatic  cord,  but  in  so  doing  the  scrotal  sac  will  also  be  inverted, 
since  they  aie  attached  to  its  posterior  wall.  Attached  along  the 
inner  side  of  each  testis  is  an  irregular  convoluted  tubular  mass, 

Y 


FIG.  no. — Diagram  of  male  urogeni- 
tal  system  of  Lepus. 

A.,  anus  ;  B.;  urinary  bladder  ;  C.,  Cowper's 
gland  ;  C.S.,  corpus  spongipsum  ;  B.C.,  caput 
epididymis  ;  E.Ca.,  cauda  epididymis  ;  G.,  guber- 
naculum  ;  K.,  kidney  ;  P.,  penis  ;  Pe.,  perineal 
gland  ;  Po.,  prostate  gland  ;  Pr.,  prepuce  ;  P.S., 
pelvic  symphysis  ;  R.,  rectum  ;  R.G.,  rectal 
gland  ;  S.C.,  spermatic  cord  ;  S.S.,  scrotal  sac  ; 
T.,  testis  ;  U.,  ureter  ;  U.M.,  uterus  masculinus  ; 
Ur.,  urethra  ;  V.D.,  vas  deferens. 


322  AN   INTRODUCTION   TO  ZOOLOGY 

the  epididymis,  in  which  three  regions  may  be  distinguished.  The 
first  is  a  rounded  mass  of  this  tissue,  the  caput  epididymis,  closely 
adherent  to  the  anterior  end  of  the  testes  near  the  attachment  of 
the  spermatic  cord.  The  second  is  a  narrow  band  passing  along  the 
medial  side  of  the  testis,  and  the  third  is  a  larger,  somewhat  conical 
mass,  the  cauda  epididymis,  attached  to  the  posterior  end  of  the 
testis.  It  is  the  cauda  that  is  attached  to  the  wall  of  the  scrotal  sac 
by  means  of  a  short  strand  of  connective  tissue,  the  gubernaculum. 

Developmentally  we  find  that  the  epididymis  is  the  persistent 
highly  modified  remains  of  the  mesonephros  of  the  embryo,  and  its 
tubules  grow  into  the  testis  constituting  the  vasa  efferentia.  The 
vas  deferens  is  then  the  old  mesonephric  duct.  The  vasa  deferentia 
arise  from  the  cauda  epididymis,  pass  through  the  inguinal  canal  into 
the  abdominal  cavity  to  a  position  on  the  outside  of  the  ureters. 
They  loop  over  the  ureters  ventrally  and  then  run  back  on  the  dorsal 
side  of  the  bladder  to  open  into  its  neck  below  the  ureters.  From 
this  point  on  there  is  one  common  duct  for  both  the  urinary  and 
genital  products,  and  it  is  termed  the  urethra.  A  median,  short 
thick-walled  sac,  the  uterus  masculinus,  opens  into  the  urogenital 
canal,  as  the  urethra  may  be  called,  a  little  lower  down,  and  the 
canal  then  passes  on  ventral  to  the  end  of  the  rectum,  and  imme- 
diately dorsal  of  the  pelvic  symphysis  to  be  continued  outside  the 
body  in  the  penis.  The  penis  or  male  copulatory  organ  is  a  short 
rod  of  tissue  projecting  from  the  ventral  end  of  the  perineum,  but 
it  is  not  conspicuous,  since  its  free  end  is  enclosed  in  a  fold  of  skin, 
termed  the  prepuce.  The  posterior  wall  of  the  penis  is  formed  of  a 
very  characteristic  vascular  tissue,  the  corpus  cavernosum,  while  its 
ventral  wall  contains  two  closely  apposed  rods  of  firmer  tissue,  the 
corpora  spongiosa,  which  diverge  in  the  substance  of  the  abdominal 
wall  and  are  attached  one  to  each  ischium.  The  dorsal  wall  of  the 
urethra  just  below  the  uterus  masculinus  bears  a  gland,  the  prostate, 
composed  of  four  or  five  lobes  which  open  into  the  urogenital  canal 
on  each  side  of  a  median  papilla,  the  verumontanum,  lying  just  below 
the  aperture  of  the  uterus.  Still  lower  down  are  a  pair  of  small 
ovoid  bodies,  Cowper's  glands,  one  on  each  side  of  the  dorsal  surface 
of  the  urethra. 

The  rectum  is  a  muscular  thin-walled  tube  lying  close  -to  the 
urethra,  but  opening  to  the  exterior  by  an  entirely  separate  aperture, 
the  anus.  On  its  dorso-lateral  walls  it  bears  a  pair  of  long,  pale 
yellow  bands  of  tissue,  the  rectal  glands.  On  each  side  between 
rectum  and  urethra  is  a  small  perineal  gland  of  a  dark  colour  which 
opens  on  to  a  shallow  depression,  the  perineal  sac  on  the  perineal  area. 
It  produces  a  secretion  which  is  responsible  for  the  characteristic 
smell  of  the  rabbit. 


LEPUS  CUNICULUS 


323 


The  urinary  organs  of  the  female  are  practically  the  same 
as  in  the  male  save  for  a  slight  difference  in  the  position  of  the 
openings  of  the  ureters  into  the  bladder,  but  the  reproductive  organs 
are  naturally  entirely  different.  The  essential  female  organs  are 
the  ovaries,  small  oval  bodies  about  three-quarters  of  an  inch  long 
attached  to  the  dorsa.'  body  wall  by  folds  of  the  peritoneum  the 
mesometria,  just  benind 
the  kidneys.  On  their 
surface  can  be  seen  semi- 
transparent  rounded  pro- 
jections, the  Graffian  folli- 
cles, which  are  vesicular 
structures  containing  the 
ova,  and  they  vary  in  size 
in  accordance  with  the 
state  of  their  development. 
The  oviducts,  which  are 
the  much  developed  and 
modified  pronephric  ducts, 
have  three  distinct  por- 
tions. The  anterior  end 
takes  the  form  of  a  narrow, 
somewhat  convoluted  tube, 
the  Fallopian  tube,  that 
opens  into  the  coelom  quite 
close  to  the  ovary,  and 
has  its  opening  furnished 
with  a  membranous  funnel 
whose  walls  are  much 
folded.  The  succeeding 
portion  is  greatly  enlarged, 
and  has  thick  muscular 
walls.  This  is  the  uterus, 
wherein  the  eggs  undergo 
their  development  and  the 
embryos  are  retained  until 

they  are  born,  so  that  the  size  of  the  two  uteri  depends  upon  whether 
they  contain  embryos  or  not.  Each  embryo  or  foetus  is  actually  at- 
tached to  the  uterus  by  the  intimate  union  of  an  outgrowth  from  itself 
and  a  special  receptive  thickening  of  the  uterine  wall.  The  highly 
vascular  mass  of  the  tissue  formed  in  this  way  partly  from  the  f oetus 
and  partly  from  the  parent  is  termed  the  placenta,  and  it  is  the  organ 
by  means  of  which  nutriment  and  oxygen  are  conveyed  to  the  foetus 
and  carbon  dioxide  and  nitrogenous  waste  matters  removed.  The 


PS 


FIG.  in. — Diagram  of  female  urogenital 
system  of  Lepus. 


A.,  anus  ;  B.,  urinary  bladder  ;  B.C.,  glands  of  Bartho- 
lini ;  C.,  clitoris  ;  C.S.,  corpus  spongiosum ,  F.,  Fallopian 
tube  ;  K.,  kidney ;  O.,  ovary  ;  Pe.,  perineal  gland  ; 
P.S.,  pelvic  symphysis  ;  R.,  rectum ;  R.G.,  rectal  gland  ; 
U.,  ureter  ;  Ut.,  uterus  ;  T" 
vestibule. 


V.,  vulva.  Va.,  vagina  ;   Ve.f 


324  AN   INTRODUCTION  TO  ZOOLOGY 

uteri  open  by  separate  projecting  apertures,  the  ossa  uteri,  into  the 
succeeding  portion  of  the  genital  tract,  the  vagina,  which  is  a  median 
unpaired  structure  representing  the  fused  distal  ends  of  the  old 
pronephric  ducts.  Uteri  and  Fallopian  tubes  are  attached  to  the 
dorsal  wall  of  the  abdominal  cavity  and  held  in  place  by  a  stout  fold 
of  peritoneum,  termed  the  broad  ligament.  The  vagina  passes 
backwards  to  open  into  the  neck  of  the  bladder,  so  that  from  this 
point  on,  as  in  the  male,  we  have  a  common  urogenital  duct  here 
termed  the  vestibule.  Its  walls  are  very  vascular,  corresponding  to 
the  corpus  spongiosum  of  the  male.  In  the  anterior  median  wall 
is  a  small  rod-like  body,  the  clitoris,  corresponding  with  the  penis  in 
the  male,  and  terminating  externally  in  a  soft  sensitive  spot,  the 
glans  clitoridis.  Internally  two  small  corpora  spongiosa  pass  back 
from  it  to  bifurcate  in  the  abdominal  wall  and  become  attached  to 
the  ischia.  The  actual  external  opening  is  termed  the  vulva,  and  it  is 
guarded  by  two  lips.  No  glands  corresponding  to  the  prostate  of 
the  male  are  present,  but  Cowper's  glands  are  represented  by  two 
areas  containing  the  small  glands  of  Bartholini.  Perineal  and  rectal 
glands  are  present  as  in  the  male,  and  the  urogenital  tract  is  similarly 
related  to  the  rectum. 

Ductless  Glands. 

Before  passing  on  to  consider  the  nervous  system  it  will 
be  as  well  to  glance  quite  briefly  at  the  ductless  glands.  As  in  the 
frog,  we  shall  include  under  this  heading  not  only  the  glands  in  the 
strict  sense  of  the  term,  but  also  the  lymphoid  organs. 

The  Spleen.— This  is  a  dark  red  elongated  body  attached  to  the 
dorsal  side  of  the  cardiac  end  of  the  stomach  by  a  side  fold  of  the 
mesentery,  the  gastro-splenic  omentum.  It  is  the  largest  of  the 
haemolymph  organs,  and  is  enclosed  in  a  quite  distinct  capsule 
composed  of  a  mixture  of  fibrous  and  non-striate  muscular  tissue. 
From  this  capsule  strands  of  tissue,  the  trabeculse,  pass  inwards  to 
ramify  through  the  mass  of  the  organ  which  is  mainly  composed  of  a 
soft  tissue  termed  the  splenic  pulp,  which  contains  several  varieties 
of  characteristically  shaped  cells.  It  is  plentifully  supplied  with 
blood-vessels,  whose  arterioles  are  surrounded  by  denser  lymphoid 
tissue  that  here  and  there  swells  out  to  form  nodules  often  spoken  of 
as  Malpighian  corpuscles. 

The  Tonsils. — These  are  also  composed  of  masses  of  lymphoid 
tissue  covered  over  by  a  stratified  mucous  membrane,  the  surface  of 
which  is  pitted  with  tiny  pores  leading  into  tube-like  recesses  known 
as  the  crypts  of  the  tonsil. 

The  Thymus.— This  organ  is  a  fairly  large,  soft  pinkish  mass 
lying  in  the  anterior  end  of  the  thoracic  cavity  around  the  bases  of 


LEPUS   CUNICULUS  325 

the  main  blood-vessels.  It  is  relatively  much  larger  in  a  young 
animal  and  may  then  even  partly  cover  the  heart.  When  examined 
in  microscopic  sections  it  is  seen  to  be  composed  mainly  of  a  lymphoid 
tissue.  It  appears  to  be  concerned  with  nutrition  and  metabolic 
activities. 

The  Thyroid  Gland.— This,  the  fiist  ol;  the  ductless  glands,  is  a 
soft  dark-coloured  body  situated  at  the  anterior  end  of  the  trachea. 
It  is  composed  of  two  fairly  large  flat  lateral  lobes  connected  across 
the  mid- ventral  line  by  a  thin  strand  of  its  own  tissue  closely  attached 
to  the  trachea  just  below  the  thyroid  cartilage.  It  is  also  subject 
to  considerable  variation  in  size.  It  is  found  to  consist  of  an  inter- 
lacing framework  of  connective  tissue,  which  also  forms  a  capsule 
and  binds  together  a  number  of  spherical  vesicles.  The  walls  of 
these  vesicles  are  composed  of  a  characteristic  cubical  glandular 
epithelium,  and  the  secretion  of  their  cells  is  found  to  fill  the  vesicular 
cavities  with  a  readily  coagulable  fluid,  which  with  its  surrounding 
cells  gives  to  the  section  of  this  body  a  striking  appearance.  It  has 
already  been  noted  that  this  secretion  contains  a  protein  rich  in 
iodine  and  apparently  plays  an  important  role  in  katabolic  activities. 

Near  each  thyroid  is  a  pair  of  small  bodies  termed  the  para- 
thyroids, which  are  not  composed  of  such  typical  glandular  tissue. 
In  addition  to  these  there  are  certain  other  small  accessory  para- 
thyroids in  different  places,  sometimes  included  in  another  body. 
Their  function  is  but  little  understood. 

The  Supra-renal  Bodies. — In  the  rabbit  these  bodies  are  not 
directly  connected  with  the  kidney  as  in  man,  but  lie  a  short  distance 
away  at  the  point  where  the  renal  vein  of  the  same  side  flows  into 
the  post-caval.  They  are  conspicuous  light  yellow-coloured  bodies 
composed  of  two  distinct  varieties  of  tissue,  termed  the  cortex  on 
the  outside  and  the  medulla  within. 

The  Pituitary  Body. — This  body  is  lodged  within  the  sella  turcica 
in  the  floor  of  the  cranium,  and  is  separated  from  the  brain  itself  by 
the  interposition  of  a  tough  double  fold  of  the  dura  mater  save  at 
one  point  where  it  joins  on  to  the  infundibulum  coming  from  the 
floor  of  the  third  ventricle.  When  examined  microscopically  three 
distinct  regions  can  be  distinguished.  They  are  termed  the  pars 
nervalis,  the  pars  intermedium  and  the  pars  glandularis.  The  latter 
is  composed  of  typical  glandular  cells,  and  it  arises  in  the  embryo 
from  an  outgrowth  of  the  dorsal  wall  of  the  stomodreum  or  mouth 
cavity  which  becomes  completely  cut  off  from  the  cavity  in  the 
couise  of  development.  It  produces  an  important  internal  secretion 
whose  function  has  been  discussed  briefly  when  considering  this 
body  in  the  frog. 

The  ovaries  and  testes  also  must  be  regarded  as  ductless  glands, 


326  AN   INTRODUCTION   TO    ZOOLOGY 

for  in  addition  to  their  primary  function,  that  of  producing  the 
germ  cells,  they  also  secrete  substances  which  play  important  parts 
in  the  development  and  activity  of  other  parts  of  the  body.  Among 
other  things  they  affect  the  development  of  what  are  termed  the 
secondary  sexual  characters,  that  is  to  say,  structures  which,  while 
not  actually  of  value  in  reproduction,  serve  as  distinguishing  marks 
of  the  two  sexes.  These  are  very  common  in  birds  where  the  male 
is  often  of  quite  a  different  plumage  from  the  female.  As  an  example 
of  the  action  of  the  gonads  we  may  consider  the  case  of  horns  in 
certain  mammals.  If  the  testes  be  removed  from  the  young  animal 
it  does  not  grow  horns  even  when  of  full  size.  Similar  instances  of 
other  structures  behaving  in  this  way  could  be  cited,  but  this  one  is 
sufficient  to  indicate  the  mode  of  action  of  the  gonads  as  ductless 
glands. 


CHAPTER  XIII 
LEPUS   CUNICULUS—  (continued) 

The  Nervous  System  and  Sense  Organs — The  Mammalian  Brain. 

As  in  the  lower  forms,  we  find  the  whole  of  the  central 
nervous  system  provided  with  a  series  of  protective  coverings,  the 
meninges.  The  outermost  of  these  is  the  dura  mater,  a  tough  fibrous 
membrane  which  lines  the  cavity  of  the  cranium  and  neural  canal. 
This  dips  down  into  the  fissure  between  the  cerebral  hemispheres 
forming  a  sort  of  median  membranous  partition,  the  falx  cerebri  ; 
it  also  dips  down  in  a  similar  way  between  the  hinder  end  of  the 
hemispheres  and  the  cerebellum,  thus  giving  rise  to  a  transverse 
partition,  the  tentorium.  In  some  mammals  one  or  both  of  these 
folds  may  become  ossified,  and  so  give  rise  to  thin  bony  plates 
attached  to  the  inside  of  the  cranium  and  dipping  down  into  the 
fissures  of  the  brain.  Underlying  the  dura  mater  is  a  small  sub-dural 
lymph  space,  and  then  a  delicate  arachnoid  membrane,  which  is 
composed  of  a  close  felting  of  connective  tissue.  Finally,  we  have 
the  innermost  membrane,  the  pia  mater,  also  delicate  and  extremely 
vascular.  This  invests  closely  the  whole  of  the  nervous  matter 
dipping  down  into  all  the  grooves  of  the  brain  and  spinal  cord. 

The  central  nervous  system  is  divisible  into  the  brain  and 
spinal  cord,  and  the  nervous  tissue  of  which  it  is  composed  is  divisible 
into  grey  matter  and  white  matter  as  in  all  vertebrates.  In  the 
brain  the  former  is  on  the  outside,  constituting  the  cortex,  and  the. 
latter  is  inside,  while  in  the  spinal  cord  their  positions  are  reversed. 
The  grey  matter  is  mainly  composed  of  nerve  cells  with  non-medul- 
lated  fibres,  and  the  white  is  formed  entirely  of  fibres  most  of  which 
are  medullated. 

Brain. 

The  brain  of  the  rabbit  is  small  and  somewhat  highly 
specialised,  so  that  for  more  detailed  investigation  it  is  better  to 
take  the  brain  of  a  larger  animal  such  as  the  sheep,  and  only 
consider  the  main  outlines  of  its  structure  in  the  rabbit.  The  same 
main  divisions  of  the  brain  that  we  have  already  seen  in  Scyllium 

327 


328 


AN   INTRODUCTION   TO  ZOOLOGY 


and  Rana  are  also  to  be  found  in  Lepus,  but  the  relative  size  of  the 
paits  is  very  different,  so  that  the  brains  as  a  whole  are  very  dis- 
similar in  external  appearance.  Perhaps  the  most  striking  difference 
is  the  enormous  development  of  the  cerebral  hemispheres  in  Lepus, 
where  they  form  a  large  part  of  the  total  mass  of  the  whole  brain. 
There  is  no  doubt  that  mammals  owe  their  dominant  position  in 


OL 


FL 


Oc 


sc 


FIG.  112. — Brain  of  Lepus.     Dorsal  view. 

A.C.,  anterior  quadrigeminal  body  ;  C.H.,  cerebral  hemisphere  ;  F.,  flocculus  ;  F.L.,  frontal 
lobe  ;  M.,  medulla  ;  Oc.,  occipital  lobe  ;  O.L.,  olfactory  lobe  ;  P.,  paraflocculus  ;  P.C.,  posterior 
quadrigeminal  body  ;  P.L.,  parietal  lobe  ;  S.C.,  spinal  cord  ;  S.F.,  sagittal  fissure  ;  T.L.,  temporal 
lobe  ;  V.,  vermis. 

tke  animal  kingdom  to-day  to  the  large  development  of  this  region 
of  the  brain  and  the  increase  in  intelligence  that  accompanies  it. 

The  front  end  of  the  brain,  the  telencephalon,  is  expanded 
into  the  large  cerebral  hemispheres  which  form  the  anterior  two- 
thirds  of  the  brain  and  may  be  spoken  of  together  under  the  inclusive 
term  cerebrum.  Both  from  the  dorsal  and  lateral  aspects  they 


LEPUS   CUNICULUS  329 

present  the  shape  of  rounded  triangles  with  their  blunt  apices 
directed  forward.  Internally  they  are  hollow,  containing  the  lateral 
ventricles  which  communicate  posteriorly  through  the  foramina  of 
Munro  with  the  third  ventricle.  Externally  their  convexly  rounded 
surfaces  are 'markedly  shallow  grooves,  the  sulci,  one  of  which, 
lying  at  the  side,  is  rather  more  marked  than  the  remainder  and 
divides  the  hemisphere  into  an  anterior  frontal  lobe  and  a  posterior 
parietal  lobe,  from  which  a  ventral  descending  area,  the  temporal 
lobe,  is  marked  off  by  another  distinct  sulcus.  The  two  hemi- 
spheres are  separated  in  the  mid  dorsal  line  by  the  deep  sagittal 
fissure,  in  which  is  the  falx  cerebri,  and  if  they  are  pressed  apart  it 
will  be  seen  that  a  little  way  down  their  posterior  ends  are  held 
together  by  a  broad  transverse  band  of  tissue,  the  corpus  callosum, 
which  joins  the  nervous  centres  of  the  two  sides  of  this  part  of  the 
brain.  This  band  of  fibres  is  peculiar  to  the  higher  mammals,  not 
being  found  outside  that  class,  and  it  is  even  absent  in  the  two 
lowest  orders  of  the  Mammalia,  viz.  the  Monoteremes  and  the 
Marsupials.  On  the  anterior  ventral  surface  of  the  telencephalon 
are  two  club-shaped  bodies,  the  olfactory  lobes,  whose  swollen 
extremities  project  beyond  the  front  end  of  the  cerebral  hemispheres. 
The  first  pair  of  cranial  nerves,  the  olfactory,  leave  their  anterior 
surfaces  as  a  series  of  fibres. 

The  next  portion  of  the  brain,  the  thalamencephalon,  contains 
the  third  ventricle,  and  is  roofed  by  the  anterior  choroid  plexus, 
but  it  is  not  visible  from  the  dorsal  side,  since  it  is  entirely  overlain 
by  the  posterior  ends  of  the  hemispheres.  The  epipbysis  cerebri  or 
pineal  body  coming  from  its  postero-dorsal  surface  can  sometimes 
be  made  out  between  the  divergent  posterior  lobes  of  the  cerebrum. 
On  the  ventral  surface  of  this  region  of  the  brain  we  find  the  optic 
chiasma,  the  infundibulum  and  the  pituitary  body  or  hypophysis 
cerebri.  In  spite  of  its  small  size  and  inconspicuousness  we  shall 
find  when  we  come  to  consider  the  details  of  its  structure  and 
relationship  that  it  is  a  most  important  part  of  the  brain. 

The  mesencephalon  or  mid  brain  is  also  largely  hidden  by  the 
hemispheres,  but  can  be  seen  easily  if  their  hinder  ends  are  pushed 
apart.  On  the  dorsal  side  are  two  pairs  of  rounded  bodies,  the 
corpora  guadrigemina  or  optic  lobes,  homologous  with  the  corpora 
bigemina  of  the  dogfish  or  frog,  but  subdivided.  The  ventro- 
lateral  part  of  this  region  is  constituted  by  the  crura  cerebri,  two 
smooth  bands  of  fibres  passing,  backwards  and  the  hollow  within  it 
is  the  iter  a  tertio  ad  quart um  ventriculum. 

The  cerebellum  is  the  well-developed  dorsal  region  of  the  meten- 
cephalon.  It  forms  a  transversely  elongated  mass  consisting  of  a 
median  lobe,  the  vermis,  and  two  lateral  or  floccular  lobes,  which 


30  AN   INTRODUCTION   TO  ZOOLOGY 

bear  at  their  postro-lateral  corners  two  small,  easily  detached  lobes, 
the  paraflocculi,  situated  within  the  floccular  fossae  of  the  cranium. 
The  whole  of  the  surface  of  the  cerebellum  is  marked  by  close  set, 
almost  parallel,  and  for  the  most  part  transverse  folds  or  sulci. 
The  ventral  portion  of  the  metencephalon  is  composed  of  a  trans- 


OL 


OT 


FL 


RF 


OX 


XII 


sc 


FIG.  113.  —  Brain  of  Lepus.     Ventral  view. 

C.C.,  crus  cerebri  ;  C.T.,  corpus  trapezoideum  ;  F.,  flocculus  ;  F.L.,  frontal  lobe  ;  H.L.,  hippo- 
campal  lobule  ;  I.,  infundibulum  ;  M.,  medulla  ;  O.L.,  olfactory  lobe  ;  O.T.,  olfactory  tract  ; 
O.X.,  optic  chiasma  ;  P.,  paraflocculus  ;  Pi.,  pituitary  body  ;  P.V.,  pons  Varolii  ;  R.F.,  rhinal 
fissure  ;  S.C.,  spinal  cord  ;  T.L.,  temporal  lobe  ;  V.P.,  ventral  pyramids  ;  II.-XIL,  roots  of  spinal 


versely  running  broad  band  of  fibres,  the  pons  Varolii,  which  connects 
the  two  sides  of  the  cerebellum  across  the  mid  ventral  line. 

The  myelencephalon  is  composed  of  a  somewhat  flattened  mass, 
the  medulla  oblongata,  whose  roof  is  formed  by  the  posterior  choroid 
plexus  which,  however,  is  hidden  beneath  the  enlarged  cerebellum. 
On  its  ventral  surface  we  can  recognise  two  narrow  longitudinal 


LEPUS  CUNICULUS  331 

bands,  the  ventral  pyramids,  lying  one  on  each  side  of  the  shallow 
median  ventral  fissure.  Outside  these  at  the  front  end,  that  is, 
immediately  behind  the  pons,  are  to  be  distinguished  a  pair  of  areas 
termed  the  corpora  trapezoidea.  Posteriorly  the  medulla  passes  on 
into  the  spinal  cord  without  any  sharp  line  of  demarcation. 

There  is  but  one  cavity  common  to  both  the  metencephalon  and 
myelencephalon,  and  that  is  the  fourth  ventricle. 

Cranial  Nerves. 

Twelve  large  pairs  of  nerves  leave  the  cranium  in  the  rabbit, 
and  they  are  commonly  termed  the  cranial  nerves,  but,  strictly 
speaking,  one  of  them,  the  eleventh  or  spinal  accessory,  is  largely 
composed  of  fibres  coming  from  the  spinal  cord.  The  points  of 
origin  of  these  nerves,  while  they  can  be  made  out  in  the  rabbit,  are 
much  better  studied  in  a  larger  brain,  and  they  will,  therefore,  be 
considered  later,  although  it  should  be  borne  in  mind  that  they  arise 
in  the  same  relative  position  in  both  rabbit  and  sheep,  and,  indeed, 
the  first  ten  are  constant  throughout  the  chordate  series  from  dog- 
fish to  man  himself.  It  should  be  borne  in  mind  that  although  we 
speak  of  ten  cranial  nerves  in  lower  vertebrates  and  twelve  in  the 
higher  forms,  there  is  present  in  all  of  them  from  lowest  to  highest 
another  nerve  right  at  the  anterior  end.  This  is  termed  the  nervus 
terminals,  and  it  arises  from  the  front  median  portion  of  the  telence- 
phalon  and  passes  forward  in  the  neighbourhood  of  the  olfactory 
lobe  to  be  distributed  in  the  region  of  the  internasal  septum.  Its 
exact  functional  significance  has  not  yet  been  ascertained,  though 
it  is  present  in  all  forms.  Again,  we  find  in  all  vertebrates  a  second 
small  nerve,  the  vomero-nasalis  or  septalis,  which  arises  not  far 
from  the  other,  but  developmentally  just  behind  the  olfactory. 
This,  too,  is  distributed  in  the  neighbourhood  of  the  nasal  septum, 
and  its  function  is  not  yet  understood.  So  that  to  be  strictly 
accurate  we  should  say  there  are  twelve  cranial  nerves  in  the  lower 
Chordates  such  as  Rana  and  Scy Ilium,  and  fourteen  in  the  higher 
forms  like  Lepus. 

We  may  now  consider  briefly  the  distribution  of  these 
nerves  in  the  rabbit. 

The  nervus  terminalis  is  a  small  nerve,  comprising  only  a  few 
fibres,  passing  out  on  the  mesial  wall  of  the  cerebral  hemisphere  in 
the  sagittal  fissure.  It  runs  through  the  cribriform  plate  with  the 
fibres  of  the  olfactory  nerve,  and  spreads  out  to  help  to  form  a 
plexus,  in  company  with  the  nervus  septalis,  in  the  neighbourhood 
of  the  nasal  septum  and  Jacobson's  organ. 

The  olfactory  nerve,  composed  of  numerous  branches,  ramifies 
all  over  the  olfactory  membrane  in  the  nasal  chamber  and  covering 


332  AN   INTRODUCTION_LTO  ZOOLOGY 

the  turbinal  bones.  It  is  purely  sensory  in  function  and  subserves 
the  sense  of  smell.  Its  fibres  pass  in  numerous  bundles  through  the 
cribriform  plate  to  enter  the  olfactory  bulb. 

The  vomero-nasal  or  septal  nerve  is  a  small  nerve  whose  fibres 
also  pass  through  the  cribriform  plate  and,  as  just  noted,  form  a 
plexus  with  those  of  the  riervus  terminalis. 

The  optic  nerve  enters  the  orbit  and  runs  straight  to  the  eye. 
It  penetrates  the  sclerotic  coat,  the  choroid  coat  and  the  retina,  and 
at  the  blind  spot  spreads  out  radially  to  be  distributed  over  the 
internal  surface  of  the  retina.  It  is  a  purely  sensory  nerve  and  sub- 
serves the  function  of  sight. 

The  oculomotor  nerve  enters  the  orbit  through  the  foramen 
lacerum  anterius.  It  is  a  motor  nerve,  and  breaks  up  to  be  dis- 
tributed to  the  internal,  superior  and  inferior  recti  and  the  inferior 
oblique  muscles  of  the  eye-ball. 

The  pathetic  or  trochlear  nerve  is  also  a  motor  nerve  accompany- 
ing the  third  into  the  orbit,  but  passing  across  its  inner  wall  to  the 
superior  oblique  muscle. 

The  trigeminal  nerve  is  a  very  large  mixed  nerve  containing  both 
sensory  and  motor  fibres.  A  short  distance  from  its  origin  it  swells 
out  to  form  the  large  Gasserian  ganglion,  from  which  its  three  main 
trunks  are  given  off,  viz. :  the  ophthalmic,  the  maxillary  and  the 
mandibular.  The  first  two  of  these  leave  the  skull  by  the  foramen 
lacerum  anterius,  while  the  last  leaves  through  the  anterior  portion 
of  the  foramen  lacerium  medium,  that  is,  the  portion  that  in  many 
mammals  is  a  separate  aperture,  the  foramen  ovale. 

The  ophthalmic  trunk  is  a  large  nerve  which  crosses  the  mesial 
border  of  the  orbit  and,  after  giving  off  twigs  to  the  tissues  in  the 
region  of  the  eyelid  and  lachrymal  region,  reaches  the  front  end  of 
the  dorso-mesial  wall  of  the  orbit.  Here  it  splits  into  two  branches, 
the  nervus  frontalis  and  the  nervus  naso-ciliaris.  The  frontalis 
goes  through  the  notch  at  the  front  end  of  the  supra-orbital  process 
of  the  frontal  bone  to  the  skin  and  subcutaneous  region  of  the  upper 
eyelid.  The  naso-ciliaris  passes  through  the  internal  orbital  foramen 
to  the  nasal  region. 

The  second  trunk,  the  maxillary,  passes  along  the  inner  border  of 
the  orbit  and  the  main  part  of  it  goes  through  the  infra- orbital  canal 
as  the  infra-orbital  nerve,  passing  to  the  front  end  of  the  upper  lip 
and  the  snout,  and  also  giving  branches  to  the  anterior  teeth. 
Within  the  orbit  it  gives  off  at  the  hinder  end  a  subcutaneous  branch 
going  outwards  to  the  skin  ;  a  sphenopalatine  branch,  which  after  a 
short  distance  enlarges  to  form  the  sphenopalatine  ganglion  and  the 
posterior  dental  nerve  going  to  the  hindermost  teeth.  From  the 
sphenopalatine  ganglion  are  given  off  several  naso-sphenopalatine 


LEPUS   CUNICULUS  333 

nerves  which  pass  through  the  anterior  palatine  foramen  and  so 
reach  the  nasal  region.  The  ganglion  also  gives  off  palatine  nerves, 
of  which  the  anterior  palatine  goes  through  the  posterior  palatine 
foramen  to  supply  the  mucous  membrane  of  the  hard  palate,  while 
the  posterior  palatine  branches  supply  the  soft  palate. 

The  mandibular  or  inferior  maxillary  nerve  leaves  the  cranium, 
as  noted,  through  the  foramen  lacerum  medium,  and  almost  at  once 
gives  off  twigs  supplying  the  pterygoid,  masseter  and  temporal 
muscles  of  the  head  and  the  adjacent  tissue?.  After  a  short  course 
it  divides  into  two  branches,  the  lingual  and  the  mandibular.  The 
lingual  nerve  passes  downwards,  and  below  the  level  of  the  tympanic 
cavity  receives  an  anastomosing  branch  from  the, seventh  nerve. 
It  then  passes  forwards  and  inwards  to  supply  the  tissues  of  the 
tongue.  The  mandibular  nerve  passes  a  little  more  posteriorly  and, 
further,  ventrally  to  the  inner  surface  of  the  ramus  of  the  mandible 
into  the  substance  of  which  it  penetrates  through  the  inferior  dental 
foramen,  thus  reaching  the  teeth.  Its  terminal  branches  issue  on 
the  outside  of  the  anterior  end  of  the  ramus  through  the  mental 
foramen  as  the  nervus  mentalis  to  supply  the  tissue  of  this  region. 

The  abducens  is  quite  small,  and  leaves  the  cranium  through  the 
foramen  lacerum  anterius  in  company  with  the  first  two  branches 
of  the  trigeminal,  and  passes  postero-laterally  a  short  distance  across 
the  orbit  to  supply  the  external  rectus  muscle. 

The  f acialis  is  another  large  mixed  nerve  whose  roots  unite  to  form 
the  geniculate  ganglion,  and  the  greater  portion  of  it  leaves  the 
cranial  cavity  by  the  internal  auditory  meatus,  and  the  skull  by 
the  stylo-mast  oid  foramen.  From  the  ganglion  three  main  branches 
come  off.  The  first  is  the  chorda  tympani,  the  branch  which,  as  we 
have  seen,  anastomoses  with  the  lingual  branch  of  the  trigeminal. 
It  passes  posteriorly  to  the  tympanic  cavity.  The  second  is  the 
ramus  palatinus,  termed  also  in  mammals  the  nervus  petrosus 
superficialis  major,  which  passes  forwards  above  the  tympanic 
cavity  to  the  palatine  region  and  sends  a  branch  to  anastomose  with 
the  maxillary  branch  of  the  trigeminal  at  the  sphenopalatine  gang- 
lion. The  main  hyomandibular  branch  of  the  nerve  passes  behind 
the  tympanic  cavity  and  is  distributed  generally  to  the  tissues  of  the 
dorsal  and  ventral  sides  of  the  hinder  end  of  the  head,  the  external 
mandibular  region  and  to  the  hyoid  region. 

The  auditory  nerve  also  leaves  the  cranial  cavity  by  the  meatus 
auditorius  internus,  and  at  the  end  of  this  canal  divides  into  a 
cochlear  branch  distributed  mainly  to  the  cochlea,  and  a  vestibular 
branch  serving  the  majority  of  the  remaining  part  of  the  membranous 
labyrinth. 

The  glosso-pharyngeal  nerve  leaves  the  skull  through  the  foramen 


334  AN   INTRODUCTION  TO  ZOOLOGY 

lacerum  posterius  in  company  with  the  tenth  and  eleventh  nerves 
and  the  internal  jugular  vein.  It  passes  downwards  between  the 
jugular  vein  and  the  internal  carotid  artery  and  then  turns  forward 
to  spread  out  in  the  tongue,  tonsils  and  pharyngeal  region. 

As  we  have  seen,  the  vagus  nerve  leaves  the  skull  through  the 
foramen  lacerum  posterius,  and  it  swells  out  shortly  after  to  form  a 
distinct  vagus  ganglion.  It  descends  the  neck  external  to  the 
common  carotid  artery  into  the  thoracic  cavity,  and  then  through 
this  alongside  the  oesophagus  into  the  abdomen,  where  in  proximity 
to  the  stomach  it  breaks  up.  Just  beyond  the  ganglion  at  the 
anterior  end  of  the  neck  it  gives  off  the  anterior  laryngeal  nerve,  a 
small  trunk  running  inwards  and  forwards  dorsal  to  the  carotid 
artery,  to  be  distributed  to  the  larynx  and  the  crico-thyroid  muscle. 
A  second  branch,  the  depressor  nerve,  arises  generally  from  the 
preceding,  but  it  may  come  directly  from  the  main  trunk  at  or  near 
the  same  point.  This  passes  back  along  the  neck  mesial  to  the 
main  vagus  trunk  and  dorsal  to  the  common  carotid  artery  to  go 
to  the  heart.  It  receives  its  name  from  the  fact  that  stimu- 
lating it  produces  a  depression  of  the  beating  of  the  heart. 
The  last  noticeable  branch  from  the  vagus  is  the  posterior  or 
recurrent  laryngeal  nerve,  which  is  differently  disposed  on  the  two 
sides.  On  the  right  it  leaves  the  vagus  just  in  front  of  the  sub- 
clavian  artery,  then  loops  around  this  vessel  and  finally  takes  up  a 
position  at  the  side  of  the  trachea  and  passes  forward  along  it  to 
supply  most  of  the  laryngeal  muscles.  On  the  left  side  it  comes  off 
further  down,  behind  the  sub-clavian  artery  and  actually  in  the 
thoracic  cavity.  It  loops  around  the  arch  of  the  aorta  and  so 
reaches  its  position  by  the  side  of  the  trachea.  This  curious  looping 
of  the  nerves  is  comprehensible  when  we  consider  the  development 
of  this  region.  Originally  the  heart  is  situated  much  more  anteriorly, 
and  its  carotid  and  systemic  trunks,  representing  the  third  and 
fourth  of  the  afferent  branches  of  a  fish,  e.g.  Scyllium,  are  morpho- 
logically in  front  of  the  point  of  origin  of  the  larynx.  The  nerves 
then  run  behind  these  arteries.  In  the  course  of  development  the 
heart,  and  with  it  the  arteries,  shifts  backwards  and  consequently 
the  nerve  becomes  pulled  out  into  a  loop. 

While  traversing  the  thorax  the  main  trunk  of  the  vagus  gives  off 
branches  to  the  lungs,  oesophagus  and  heart,  the  last  named  forming 
a  plexus  around  the  roots  of  the  aorta  and  pulmonary  artery. 

The  spinal  accessory  nerve  is  a  short  nerve  also  leaving  the 
cranium  by  the  foramen  lacerum  posterius.  It  passes  almost 
vertically  downwards  lateral  to  the  vagus,  and  is  distributed  to  the 
sterno-mastoid  and  other  adjacent  muscles.  A  short  distance 
along  it  gives  off  a  ramus  internus  which  joins  the  vagus. 


LEPUS   CUNICULUS 


335 


The  hypoglossal  nerve  is  a  stout  trunk  that  comes  out  of  the 
skull  by  the  condylar  foramen  and  runs  backwards  lateral  to  the 
preceding  nerves  and  slightly  inwards.  When  it  reaches  a  point 
near  the  place  where  the  common  carotid  artery  splits  into  its 
internal  and  external  branches  it  divides  into  two.  The  anterior 
nerve  crosses  the  internal  carotid  artery  ventrally  and  runs  forward 
parallel  to  and  outside  the  external  carotid,  and  also  to  the  posterior 
cornu  of  the  hyoid  bone  to  the  base  of  the  tongue,  which  it  serves 
together  with  the  muscles  of  that  region.  The  posterior  branch 
passes  backwards,  and  after  a  short  distance  crosses  the  common 
carotid  ventrally  and  breaks  up  into  branches  supplying  the  muscles 
of  the  hyoid  and  thyroid  region.  Some  of  these  nerves  receive  fibres 
also  from  the  first  and  second  spinal  nerves. 

Spinal  Cord. 

The  spinal  cord  passes  back    without  visible  line  of  de- 
marcation from  the  hinder   end  of  the   medulla  oblongata  as  a 


FIG.  114. — Roots  of  a  spinal  nerve  issuing  from  the  cord. — From  Quain. 
A,  from  before  ;   B,  from  the  side  ;  C,  from  above  ;  D,  the  roots  separated. 

i  ventral  fissure  ;  2,  dorsal  fissure  ;  3  and  4,  lateral  grooves  of  the  cord  ;  5  ventral  root 
6  posterior  root  ;  6',  dorsal  ganglion ;  7,  the  united  or  compound  nerve  ;  7  ,  the  dorsal 
branch.  In  A  one  ventral  root  is  divided  and  turned  upwards. 

long  rod,  at  first  somewhat  oval,  but  soon  nearly  round,  the  whole 
length  of  the  neural  canal.  The  side  walls  of  the  posterior  end  of  the 
roof  of  the  fourth  ventricle  meet  together  in  the  mid-dorsal  line, 


336 


AN   INTRODUCTION  TO  ZOOLOGY 


and  their  junction  is  marked  by  the  formation  of  a  groove.  This 
continues  on  down  the  cord  as  a  narrow  but  quite  deep  cleft,  the 
dorsal  fissure.  On  the  ventral  side  is  another  groove,  the  ventral 
fissure,  not  quite  so  deep  and  somewhat  wider  than  the  dorsal  one. 
As  previously  noted,  the  same  meninges  that  we  find  in  the  brain 
surround  the  spinal  cord,  but  the  arachnoid  is  divided  into  two 
layers,  separated  by  quite  a  large  sub-arachnoid  space,  which 
communicate  on  the  dorsal  side  by  a  septum. 

The  canalis  centralis  is  very  much  reduced,  and  its  lumen  appears 
as  a  tiny  space  situated  nearer  the  end  of  the  dorsal  fissure  than  the 
ventral.  It  is  lined  by  ependymal  epithelium  continuous  with  that 
of  the  brain  cavities.  In  transverse  section  the  cord  is  seen  to  be 
composed  of  a  characteristic  H-shaped  arrangement  of  grey  matter 
surrounded  by  white  matter.  The  canalis  centralis  lies  in  the 
middle  of  the  transverse  bar  of  the  H,  and  has  above  it  a  band  of 
transverse  fibres,  the  dorsal  commissure,  and  below  it  a  wider  but 
similar  band,  the  ventral  commissure. 

Spinal  Nerves. 

Throughout  the  whole  of  its  length  the  spinal  cord  gives 
off  the  paired  segmental  spinal  nerves,  leaving  the  vertebral  column 

by  the  intervertebral  fora- 
mina. Each  arises  as  in 
all  vertebrates  by  two 
roots,  a  dorsal  root  related 
to  the  dorsal  horn  of  the 
grey  matter,  and  a  ventral 
root  issuing  from  the  ven- 
tral horn.  The  dorsal  root 
bears,  a  short  distance 
from  its  origin,  the  dorsal 
root  ganglion,  almost  im- 
mediately beyond  which 
the  two  roots  unite  to 
form  a  common  trunk. 
The  common  nerve  so 
constituted  leaves  the 
vertebral  column  and 

gives  off  a  small  ramus  dorsalis  passing  to  the  muscles  and  skin  of 
the  dorsal  body  region  and  tiny  strands  to  the  sympathetic  chain. 
The  main  part  goes  on  as  the  ramus  ventralis  to  be  distributed  to 
the  more  ventrally  situated  portions  of  the  body. 

The  nature  of  the  two  roots  of  the  spinal  nerves  is  a  matter  of 
some  interest.  If  the  common  trunk  after  the  point  of  fusion  be 


FIG.  115. — Illustrating  the  functions  of  the 
roots  of  the  spinal  nerves. — From  Fur- 
neaux 

a.,  ventral  root ;  p.,  dorsal  root. 

Divided  at  a. — Irritated  at  i  :  no  result.  Irritated 
at  2  :  contraction  of  muscles  supplied  with  fibres  from 
the  root. 

Divided  at  £. — Irritated  at  3  :  no  result.  Irritated 
at  4 :  pain  produced. 


LEPUS  CUNICULUS  337 

cut  and  the  ends  stimulated,  it  will  be  found  :  (i)  that  from  the 
distal  end  a  message  is  conveyed  to  the  muscles  showing  that  it 
contains  motor  or  efferent  fibres  conducting  impulses  away  from  the 
central  nervous  system,  and  (2)  that  from  the  proximal  end  a 
message  or  sensation  is  carried  to  the  brain,  indicating  that  some  of 
its  fibres  are  afferent  or  sensory.  The  whole  nerve  is  therefore  mixed, 
containing  both  motor  and  sensory  fibres. 

If  the  dorsal  root  be  cut  between  the  ganglion  and  the  cord,  and 
stimulated,  it  will  be  found  :  (i)  that  from  the  proximal  end  a  sen- 
sory message  is  conveyed  to  the  brain,  and  (2)  that  from  the  distal 
end    no    results    can    be 
obtained.        The     dorsal 
root,  therefore,  is  entirely 
a  sensory  root  and,  like 
all  nerve  fibres,  its  con- 
stituents   can    convey    a 
message  in  one  direction 

n  v<  FIG.  116. — Illustrating  the  functions  of  the 

If   the  ventral  root  be  spinal  nerves. — From  Furneaux. 

severed  before   its   poinf         Divided  at  a._Irritated  at  t :  pain.   Irritated  at  2 : 

of     Union     We     Shall     See  :        muscular  contraction. 

(i)  that  stimulation  of  the 

proximal  end  produces  no  response,  so  that  it  contains  no  sensory 
or  afferent  fibres,  but  (2)  that  stimulation  of  the  distal  end  results 
in  movement.  The  ventral  root  is,  therefore,  a  purely  motor  nerve. 
The  distribution  of  the  spinal  nerves  calls  for  little  notice,  and 
we  need  only  deal  with  certain  small  points.  The  third  spinal 
nerve  gives  off  a  large  branch  termed  the  great  auricular  nerve,  that 
runs  up  the  postero-lateral  border  of  the  pinna  supplying  the 
neighbouring  tissues.  The  fourth  spinal  nerve  gives  off  a  fairly 
large  branch  that  runs  backwards  below  the  roots  of  the  others 
receiving  a  tributary  from  the  fifth  nerve,  and  sometimes  from  the 
sixth  also.  The  combined  trunk  passes  into  the  thorax  as  the 
phrenic  nerve  to  be  distributed  over  the  diaphragm.  In  certain 
regions,  noticeably  the  axillary  and  lumbar,  there  is  a  certain  amount 
of  intercrossing  of  fibres  from  adjacent  nerves  constituting  the 
brachial  and  lumbar  plexuses  respectively. 

Sympathetic  Nervous  System. 

The  sympathetic  nervous  system  is  constituted  in  funda- 
mentally the  same  way  as  in  the  frog,  but  it  is  somewhat  more 
specialised,  and  the  ganglionation  of  the  two  lateral  trunks  lying 
beneath  the  vertebral  column  is  not  quite  so  regular.  As  we  have 
seen,  each  spinal  nerve  on  emerging  from  the  vertebral  column 

z 


338  AN   INTRODUCTION  TO  ZOOLOGY 

sends  off  a  small  twig  to  join  the  common  trunk.  The  main  trunk 
on  each  side  is  connected  anteriorly  with  the  sphenopalatine  ganglion 
and  communicates  with  the  intra-cranial  portions  of  the  trigeminal, 
glosso-pharyngeal  and  vagus.  Extra-cranially  it  is  connected  also 
with  the  hypoglossal.  At  the  anterior  end  of  the  neck  it  lies  close 
to  the  trachea,  slightly  dorsal  and  mesial  to  the  carotid  artery,  and 
there  enlarges  to  form  an  anterior  cervical  ganglion,  partly  in  front  of 
the  vagus  ganglion.  It  passes  backwards,  and  at  the  hinder  end 
of  the  neck  just  in  front  of  the  sub-clavian  artery  it  enlarges  to  form 
a  median  cervical  ganglion.  The  chain  is  connected  with  a  plexus 
of  sympathetic  fibres  in  the  heart,  and  then  passes  back  into  the 
abdominal  cavity,  where  it  is  related  to  three  median  ganglionic 
enlargements.  The  first  of  these  is  the  coeliac  ganglion,  lying 
between  the  coeliac  and  anterior  mesenteric  arteries.  The  second 
is  the  anterior  mesenteric  ganglion,  situated  just  behind  the  similarly 
named  artery.  Fibres  run  back  from  this  to  a  much  smaller 
posterior  mesentric  ganglion  lying  in  the  mesentery  just  in  front  of 
the  artery  of  the  same  name.  These  three  ganglions,  particularly 
the  front  two,  are  sometimes  referred  to  as  the  solar  plexus. 

The  fibres  from  the  sympathetic  chain  are  distributed  to  all 
the  viscera  and  regulate  the  activity  of  the  non-striate  muscles  so 
that  the  system  is  sometimes  spoken  of  as  the  involuntary  nervous 
system. 

Sense  Organs. 

Little  remains  to  be  noted  in  regard  to  the  sense  organs, 
since  they  have  already  been  dealt  with  sufficiently  in  other  forms, 
and,  indeed,  the  description  of  the  eye  given  was  based  mainly 
upon  that  of  a  mammal. 

The  olfactory  organ  is  very  well  developed  and  occupies  the 
large  anterior  nasal  chamber.  Although  a  large  space  it  is  almost 
completely  filled  with  the  complicated  turbinal  bones,  over  all  of 
which  spreads  the  typical  olfactory  neuro-epithelium. 

The  details  of  the  structure  of  the  eye  call  for  no  further  ex- 
pansion, but  it  will  be  seen  from  the  position  that  it  occupies,  well 
sunk  in  the  deep  orbit  at  the  side  of  the  skull,  that  each  eye  looks 
out  to  its  own  side,  and  that  there  is  practically  no  overlapping  of 
the  two  fields  of  vision  as  in  our  own  case. 

It  is  in  the  ear  that  we  meet  with  the  greatest  advance  over  the 
conditions  in  Rana.  In  the  first  place  the  tympanic  membrane  is 
no  longer  upon  the  external  surface  of  the  skull,  but  has  sunk  down 
a  considerable  way,  and  the  wide  canal  leading  from  it  to  the 
outside,  i.e.  the  external  auditory  meatus,  is  enclosed  in  the  tympanic 
bone.  On  .the  outside  of  the  skull,  in  order  to  compensate  for  the  loss 


LEPUS   CUNICULUS  339 

of  accessibility,  a  trumpet-shaped  structure,  the  external  ear  or 
pinna,  is  developed.  This  collects  the  sound  waves  and  conducts 
them  through  the  meatus  to  the  tympanic  membrane.  A  columella 
auris  is  not  present,  but  its  place  is  taken  functionally  by  a  chain 
of  auditory  ossicles,  the  malleus,  the  incus,  the  os  orbiculare  and  the 
stapes,  by  whose  agency  the  vibrations  of  the  tympanum  are  trans- 
ferred to  the  fenestra  ovalis  and  so  to  the  perilymph  surrounding  the 
membranous  labyrinth. 

The  internal  ear  itself  is  also  much  modified.  We  find,  as  before, 
a  vestibule  relatively  small  and  divided  into  a  sacculus  and  utriculus. 
From  it  come  off  the  three  semicircular  canals  which,  while  differing 
somewhat  from  those  in  the  lower  forms,  are  essentially  the  same 
in  structure  and  function.  A  small  duct  us  endolymphaticus  is  also 
present,  but  it  only  runs  a  short  distance  and  then  swells  out  into  a 
small  blind  enlargement,  the  saccus  endolymphaticus.  The  most 
noticeable  alteration  is  in  the  cochlea.  In  the  frog  this  is  a  com- 
paratively small  projection  from  the  wall  of  the  sacculus,  while  in 
the  rabbit  it  has  developed  into  a  very  complicated  structure  much 
larger  than  the  vestibule  itself.  The  cochlea  is  coiled  upon  itself 
so  as  to  resemble  somewhat  a  snail  shell,  and  its  internal  cavity  is 
divided  up  into  three  separate  compartments  by  longitudinally 
running  partitions.  It  is  not  proposed  to  enter  into  a  detailed  dis- 
cussion of  the  histology  or  anatomy  of  this  structure,  but  it  should 
be  noted  that  it  is  probably  to  be  regarded  as  a  highly  specialised 
organ  for  the  appreciation  of  musical  sounds. 

The  organs  of  touch  and  taste  are  very  similar  to  those  described 
previously,  save  that  they,  too,  appear  to  reach  a  higher  degree  of 
development,  and  have  a  characteristic  distribution,  as  we  have  seen 
in  the  case  of  the  latter  on  the  tongue. 

This  then  concludes  the  account  of  the  rabbit,  which  has  been 
taken  as  a  type  of  the  class  Mammalia,  which  is  the  highest  class  of 
animals  now  living  or,  so  far  as  we  know,  has  ever  lived,  and  cul- 
minates in  man  himself.  They  are  characterised  above  all  by  the 
great  development  of  their  brain,  and  in  this,  too,  man  outstrips  all 
other  forms.  So  that  before  leaving  them  it  will  be  as  well  to  con- 
sider this  organ  in  slightly  more  detail  in  a  larger  form  than  Lepus. 

The  Brain  of  a  Mammal — the  Sheep. 

For  the  more  detailed  study  of  the  mammalian  brain  we 
may  take  that  of  the  sheep.  It  has  the  advantage  of  being  of  a 
suitable  size  and  readily  procurable,  also,  in  so  far  as  we  shall  treat 
of  it,  the  structures  are  fairly  typically  arranged.  For  the  purposes 
of  dissection  it  is  best  to  use  a  brain  that  has  been  hardened  for  some 
time  previously. 


340  AN   INTRODUCTION  TO  ZOOLOGY 

The  meninges  are  readily  studied  in  the  sheep's  brain,  and  with 
a  little  care  they  may  be  removed  from  the  cranium  intact  with  the 
"brain.  The  falx  cerebri  is  easily  recognisable,  although  it  only 
penetrates  the  sagittal  fissure  a  very  short  distance  save  at  its 
posterior  end,  where  it  joins  the  tentorium,  which  is  well  developed. 
In  the  thickness  of  these  two  folds  lie  venous  sinuses  which  convey 
the  blood  away  from  the  brain.  They  are  termed  the  sinus  sagittalis 
and  the  sinus  transversus  respectively.  Turning  now  to  the  brain 
itself  we  find  it  to  be  a  large  ovoid  structure  of  which  the  anterior 
three-quarters  of  the  dorsal  portion  is  constituted  by  the  cerebrum. 

Fore-Brain. 

The  large  hemispheres  are  highly  specialised  outgrowths 
from  the  dorsal  region  of  the  telencephalon,  and  they  extend  so  far 
back  that  they  completely  hide  the  thalamencephalon  and  mesence- 
phalon.  They  are  separated  in  the  middle  line  by  a  well-marked 
sagittal  fissure,  and  their  well-rounded  surfaces  are  covered  with 
very  distinct  grooves  or  sulci  dividing  them  off  into  a  series  of  well- 
marked  broad  ridges,  the  convolutions  or  gyri.  Certain  of  these 
sulci  are  of  importance,  since  they  serve  to  mark  off  the  surface  of 
the  brain  into  more  or  less  well-defined  areas.  The  first  of  these 
is  the  fissura  cruciata,  which  starts  as  a  deep  groove  in  the  mid- 
dorsal  line  and  passes  transversely,  following  a  slightly  curved  course 
and  separates  an  anterior  frontal  lobe  from  a  larger  posterior 
parietal  lobe.  The  second  is  the  fissura  suprasylvia,  which  arises  in  a 
Y-shaped  union  of  two  sulci  just  behind  the  fissura  cruciata  about 
half-way  out,  and  runs  at  first  backwards  and  then  outwaids  and 
downwards  over  the  side  of  the  hemisphere  ;  in  its  posterior  two- 
thirds  it  marks  the  boundary  between  the  parietal  and  the  descend- 
ing temporal  lobe.  The  third  of  them  is  the  fissura  rhinalis,  which, 
although  visible  from  the  side,  is  best  seen  from  the  ventral  aspect, 
as  it  starts  just  lateral  to  the  olfactory  lobe  and  passes  backwards 
with  a  slightly  bowed  course  to  the  hinder  end  of  the  hemisphere. 
The  area  on  the  inside  of  this  is  sometimes  termed  the  pyriform  lobe, 
and  its  hinder  end  distinguished  as  the  hippocampal  lobule.  All  the 
various  grooves  and  ridges  are  formed  by  the  folding  of  the  wall  of 
the  cerebellum,  and  they  have  the  result  of  giving  a  greatly  increased 
surface.  As  has  been  noted  before,  the  grey  matter  constituting 
the  cortex  is  the  seat  of  the  actual  nerve  cells,  and  so  by  this  means 
a  much  larger  area  is  provided  and  so  many  more  cells  can  be 
accommodated. 

If  the  sagittal  fissure  be  opened  on  the  dorsal  side,  or  if  the  median 
edges  of  the  hemispheres  be  sliced  off,  the  corpus  callosum  will  be 
revealed.  It  is  a  transverse  band  of  fibres  occupying  about  two- 


LEPUS   CUNICULUS  341 

thirds  of  the  total  length  of  the  cerebrum  whose. hemispheres  project 
both  in  front  and  behind  it.  At  the  hinder  end  of  the  corpus  will  be 
seen  the  small  pineal  body,  arising  from  the  posterior  dorsal  end  of 


FC 


FL 


TL 


SA 


M 


FIG.  117. — Brain  of  sheep,  dorsal  aspect,  adapted  from  Burkholder. 

F.,  fissure  Ipngitudinalis  ;  F.C.,  fissura  cruciaia  :  F.L.,  frontal  lobe  ;  F.S.,  fissura  suprasylvia  ; 
H.,  hemisphsrium  cerebelli  ;  M.,  medulla  ;  P.,  paraflocculus  ;  P.L.,  parietal  lobe  ;  S.A..  spinal 
accessory  nerve  ;  T.L.,  temporal  lobe  ;  V.,  vermis. 

the  telencephalon  and  behind  and  lateral  to  it,  the  two  anterior 
quadrigeminal  bodies  belonging  to  the  mid-brain.  The  posterior 
quadrigeminal  bodies  lie  just  behind  the  anterior  pair,  but  they  are 
much  smaller  and  hidden  by  the  larger  ones. 


342 


AN   INTRODUCTION   TO  ZOOLOGY 


That  portion  of  the  cerebrum  lying  within  the  limits  of 
the  rhinal  fissures  may  be  regarded  as  the  ventral  surface  of  the 
telencephalon,  and  calls  for  notice  owing  to  the  presence  thereon  of 


ou 


FO 


XII 


sc 


FIG.  1 1 8. — Brain  of  sheep,  ventral  aspect,  adapted  from  Burkholder. 

C.C.,  crus  cerebri ;  Ce.,  cerebellum  ;  C.M.,  corpus  mammilare  ;  C.T.,  corpus  trapezoideum  ; 
E.O.,  external  olfactory  root  ;  F.S.,  sagittal  fissure  ;  H.L.,  hippocampal  lobe  ;  M.,  medulla  : 
M.O.,  median  olfactory  root ;  O.C.,  optic  chiasma  ;  O.L.,  olfactory  lobe  ;  Op.T.,  optic  tract  ; 
O.T.,  olfactory  tubercle  ;  P.L.,  pyriform  lobe  :  P.P.,  posterior  perforated  spot ;  P.V.,  pons  Varolii ; 
R.F.,  rhinal  fissure  ;  S.C.,  spinal  cord  ;  T.C.,  tuber  cinereum  ;  V.P.,  ventral  pyramid  ;  II.-XII., 
roots  of  cranial  nerves. 

certain  important  structures.  Right  at  the  front  end,  and  pro- 
jecting slightly  beyond  the  hemispheres,  are  two  obliquely  directed 
olfactory  lobes,  completely  separated  by  the  sagittal  fissure  which 


LEPUS   CUNICULUS  343 

passes  round  the  front  end  of  the  corpus  callosum  and  back  for  some 
distance  on  the  ventral  surface.  Each  lobe  sends  back  three  roots 
to  the  front  end  of  the  pyriform  lobe.  Of  these  two,  a  distinct 
lateral  root  passing  backwards  and  outwards,  and  a  less  distinct 
median  root  running  backwards  and  inwards  to  disappear  in  the 
sagittal  fissure,  are  discernible  externally,  while  the  third  or  inter- 
mediate root  is  internal.  Just  behind  the  limit  of  the  median  roots 
where  they  pass  into  the  sagittal  fissure  is  a  small  oval  area,  the 
olfactory  tubercle.  Behind  this  is  an  area  limited  externally  by 
the  inner  margin  of  the  lateral  root,  mesially  by  the  sagittal  fissure 
and  anteriorly  by  the  olfactory  tubercle.  This  is  known  as  the 
anterior  perforated  spot,  because  of  the  numerous  branches  arising 
from  the  anterior  cerebral  artery  that  here  perforate  the  surface  of 
the  brain  to  supply  its  internal  tissues.  Posteriorly  this  area  and 
the  sagittal  fissure  are  terminated  by  the  prominent  optic  chiasma, 
from  which  arise  the  large  optic  nerves  one  on  each  side.  The 
nerves  are  represented  only  by  short  stumps,  as  they  have  to  be  cut 
in  removing  the  brain  from  the  cranium.  The  chiasma  is  formed  by 
two  large  curved  bands  of  fibres,  the  optic  tracts,  which  come  from 
the  base  of  the  anterior  quadrigeminal  bodies  and  pass  ventrally 
and  slightly  forwards  to  unite  in  the  middle  line. 

Immediately  behind  the  middle  of  the  optic  chiasma  is  a  low 
median  elevation,  the  tuber  cinereum,  lying  on  the  floor  of  the  third 
ventricle,  the  cavity  of  which  is  continued  downwards  as  a  short, 
somewhat  conical  canal,  the  infundibulum,  into  the  tuber.  When 
the  brain  is  in  situ,  the  fairly  large  oval  pituitary  body  or  hypophysis 
cerebri,  which  lies  in  the  sella  turcica,  is  almost  completely 
separated  from  the  brain  by  a  double  fold  of  the  dura  mater.  It  is 
attached  to  the  tuber,  and  the  infundibular  cavity  passes  on  into 
it.  Generally  in  removing  the  brain  from  the  cranium  the  pituitary 
body  is  left  behind  so  that  its  position  is  marked  by  a  perforation 
in  the  tuber.  Behind  the  tuber  is  a  second  low  eminence,  the  corpus 
mammillare,  which  marks  the  hinder  end  of  the  floor  of  the  third 
ventricle  and  is  divided  into  two  corpora  by  a  median  groove  in 
some  animals.  If  the  pituitary  body  remains  attached  to  the  brain 
it  completely  hides  the  corpus  mammilare. 

This,  then,  completes  the  structures  visible  on  the  outside 
of  the  fore-brain,  and  before  passing  on  to  the  mid-brain  it  will  be 
as  well  to  study  the  internal  anatomy  of  the  prosencephalon.  As  it 
is  a  hollow  structure  we  may  start  with  the  cavities.  The  lateral 
ventricles  are  a  pair  of  symmetrically  arranged  hollows,  one  in  each 
hemisphere,  and  as  they  arise  as  antero-lateral  outgrowths  of  the 
fore-brain  vesicle  of  the  embryo  they  are  lined  by  the  ependyma, 
the  characteristic  epithelium  that  lines  all  the  brain  cavities  and  the 


344 


AN  INTRODUCTION  TO  ZOOLOGY 


OL 


C  H 


Sp.C. 


Cu 


FIG.  119. — Brain  of  sheep,  median  longitudinal  section.     Transversely 
running  fibres  indicated  diagrammatically  in  wide  cross  hatching. 

A.C.,  anterior  commissure  ;  A.M.,  anterior  medullary  velum,  valve  of  Vieussens  ;  A. P.,  anterior 
pillar  of  fornix  ;  A.Q.,  anterior  quadrigeminal  body  ;  A.V.,  arbor  vitae  ;  Cb.,  cerebellum  ;  C.C., 
corpus  callosum ;  C.Ce.,  canalis  centralis  ;  C.H.,  cerebral  hemisphere  ;  C.Ha.,  corpus  habenulare  ; 
C.M.,  corpus  mammillare  ;  Cr.,  crus  cerebri ;  F.,  body  of  fornix  ;  P.M.,  foramen  of  Munro  ;  G.C., 
genu  of  corpus  callosum  ;  G.Co.,  grey  commissure  ;  H.C.,  hippocampal  commissure  ;  I.,  iiifundi- 
bulum  ;  It.,  iter  ;  L.T.,  lamina  terminalis  ;  M.,  medulla;  O.C.,  optic  chiasma  ;  O.L.,  olfactory  lobe  ; 
P. B.,  pineal  body  ;  P.C.,  posterior  commissure  ;  P.M.,  posterior  medullary  velum  ;  P.O.,  pre-optic 
recess  ;  P.Q.,  posterior  quadrigeminal  body  ;  P.V.,  pons  Varolii .;  S.C.,  splenium  of  corpus  callosum  ; 
S.Co.,  superior  commissure  ;  S.L.,  septum  lucidum  ;  Sp.C.,  spinal  cord  ;  V.I.,  velum  interpositum  ; 
III.  third  and  IV.  fourth  ventricles. 


LEPUS  CUNICULUS  345 

canalis  centralis  of  the  spinal  cord,  and  which  is  derived  from  the 
inner  cells  of  the  original  neural  tube.  Each  lateral  ventricle  lies 
below  the  corpus  callosum  whose  ventral  fibres  form  its  roof,  and  it 
is  a  fairly  narrow  irregularly  shaped  cleft  with  very  well-marked 
anterior  and  descending  horns.  The  anterior  horn  passes  forwards 
and  downwards,  being  somewhat  T-shaped  in  transverse  section, 
gradually  becoming  reduced  in  size  until  finally  as  a  small  slit  it 
passes  up  the  olfactory  stalk  and  enlarges  slightly  to  form  the 
ventricle  of  the  olfactory  bulb.  The  descending  or  inferior  lateral 
horn  of  the  ventricle  runs  backwards,  outwards  and  downwards 
into  the  hinder  region  of  the  hemisphere,  i.e.  the  temporal  lobe. 
The  region  where  the  two  horns  join  is  sometimes  termed  the  body 
of  the  lateral  ventricle,  and  it  opens  into  the  third  ventricle  by  a 
narrow  vertical  aperture,  the  foramen  interventriculare  or  foramen 
of  Munro. 

The  mesial  wall  of  the  anterior  cornu  below  the  corpus  callosum 
is  formed  by  a  delicate  semi-transparent  membrane  which  comes  to 
lie  so  close  to  its  fellow  in  the  middle  line  that  the  two  join  and  form 
what  is  known  as  the  septum  lucidum,  which  is  limited  on  the  ventral 
side  by  a  thickened  mass  of  tissue,  the  fornix.  In  man  there  is  a 
cleft-like  cavity  within  the  septum.  The  ventro-lateral  wall  of  this 
cornu  is  composed  of  a  thick  important  mass  of  nervous  tissue 
known  as  the  corpus  striatum,  from  the  striated  appearance  it 
presents  in  transverse  section.  The  lateral  and  dorso-lateral  walls 
of  the  anterior  cornu  is  composed  of  the  grey  matter  of  the  cerebral 
cortex  and  also  of  white  matter  arranged  in  a  characteristic  radiating 
manner  to  form  the  corona  radiata,  which  is  the  lateral  continuation 
of  the  corpus  callosum. 

The  mesial,  posterior  and  ventro-lateral  walls  of  the  descending 
cornu  are  formed  by  a  large  ganglionic  mass,  the  hippocampus,  and 
attached  to  this  is  the  posterior  pillar  of  the  fornix.  Its  lateral 
and  dorso-lateral  walls  are  composed  of  the  white  matter  underlying 
the  cerebral  cortex.  In  front  of  the  pillar  of  the  fornix  a  narrow 
strip  of  the  wall  of  the  ventricle  becomes  very  thin,  and  it  is  con- 
tinuous through  the  interventricular  foramen  with  the  epithelium 
of  the  roof  of  the  third  ventricle,  immediately  above  which  the 
highly  vascular  pia  mater  forms  the  anterior  choroid  plexus.  This 
thin  epithelium  with  its  pia  mater  is  deeply  invaginated  into  the 
ventricle  as  a  marked  folded  band  of  tissue,  and  so  constitutes  the 
choroid  plexus  of  the  descending  horn.  In  removing  the  meninges 
this  portion  of  the  pia  mater,  together  with  the  epithelium  with  which 
it  is  bound  up,  is  usually  torn  away,  so  leaving  what  appears  to  be  a 
curved  slit-like  opening,  sometimes  spoken  of  as  the  "  choroidal 
fissure,"  leading  from  the  ventricle  to  the  outside.  It  should  be 


346 


AN   INTRODUCTION  TO  ZOOLOGY 


borne  in  mind,  however,  that  it  is  not  a  true  fissure,  and  simply 
represents  an  artificial  slit  made  by  tearing  away  that  part  of  the 
ventricular  wall. 

The  corpus  striatum  is  composed  of  two  ganglionic  masses 
of  grey  matter,  an  inner  and  an  outer.  The  inner  portion  is  termed 
the  nucleus  caudatus,  and  it  projects  far  into  the  floor  of  the  an- 
terior cornu  of  the  ventricle  as  a  pear-shaped  body  with  the  larger 
end  in  fiont  and  the  hinder  smaller  end  passing  backwards  into 
the  descending  horn.  The  outer,  somewhat  smaller  part  of  the 
corpus  is  the  nucleus  lenticularis,  and  the  two  nuclei  are  separated 


CR 


CC 


EC 


>CS 


1C 


FIG.  1 20. — Transverse  section  of  cerebral  hemispheres  in  region 
of  corpus  striatum. 

C.,  claustrum  ;  C.C.,  corpus  callosum  ;  Ce.,  cerebral  cortex  ;  C.R.,  corona  radiata  ;  C.S.,  corpus 
striatum  ;  B.C.,  external  capsule  ;  I.C.,  internal  capsule  ;  L.F.,  longitudinal  fissure  ;  L.V.,  lateral 
ventricle  ;  N.C.,  nucleus  caudatus  ;  N.L.,  nucleus  lenticularis  ;  S.L.,  septum  lucidum. 

by  a  band  of  white  matter,  the  internal  capsule,  which  passes 
upwards  around  the  side  of  the  lateral  ventricle  into  the  corona 
radiata.  A  similar  band  of  white  matter,  the  external  capsule,  runs 
up  on  the  outside  of  the  lenticular  nucleus  also  to  join  the  corona. 
Along  the  outer  border  of  the  external  capsule  is  a  narrow 
band  of  grey  matter,  the  claustrum.  The  corpus  callosum,  as 
we  have  seen,  is  a  broad  sheet  of  transversely  running^white  fibres 
passing  from  one  hemisphere  to  the  other.  It  continues  on  into  the 
corona,  and  thence  its  fibres  are  distributed  in  radiating  strands  to 
all  parts  of  the  cerebral  cortex.  At  the  front  end  it  bends  sharply 


LEPUS  CUNICULUS  347 

ventrally  and  the  bend  is  termed  the  genu,  and  then  passes  ventro- 
posteriorly  for  a  short  distance  as  the  somewhat  pointed  rostrum. 
The  hinder  end  also  bends  round,  but  only  for  a  short  distance, 
enlarging  to  form  a  blunt  mass,  the  spleniuni.  On  the  lower  mesial 
wall  of  the  ventricle,  separated  from  the  corpus  callosum  by  the 
septum  lucidum,  is  the  fornix.  This  is  an  elongated  triangular 
band  of  fibres  lying  in  the  middle  line.  Anteriorly  it  passes  down- 
wards and  bifurcates  to  give  rise  to  two  small  but  distinct  rounded 
cords  of  white  tissue  which  pass  downwards  and  backwards  in  the 
walls  of  the  third  ventricle  to  terminate  in  the  corpus  mammilare. 
These  are  known  as  the  eolumnse  fornicis  or  anterior  pillars  of  the 
fornix.  At  the  hinder  end  it  reaches  back  to  the  splenium  of  the 
corpus  callosum,  and  there  bifurcates  into  two  coids,  the  posterior 
pillars  of  the  fornix  or  cruree  fornicis  posterior,  which  pass  along  the 
front  or  concave  margins  of  the  hippocampi  as  the  fimbriae  hippo- 
campi. The  main  mass  of  the  fornix  is  composed  of  fibres  running 
in  a  longitudinal  direction,  but  at  the  hinder  end,  connected  with 
the  posterior  pillars,  there  is  a  band  of  transversely  running  fibres 
which  serve  to  form  a  means  of  communication  between  the  two 
hippocampi.  These  constitute  the  hippocampal  commissure  or 
psalterium. 

The  hippocampus  itself  is  a  large  crescent-shaped  mass  of  grey 
matter  which  starts  as  a  narrow  band  on  each,  side  just  behind  the 
psalterium.  It  increases  rapidly  in  size  and  passes  ventrally, 
first  posteriorly,  then  laterally  and  finally  anteriorly,  reaching  its 
maximum  size  in  its  transverse  or  lateral  portion  and  narrowing 
again  as  it  runs  forwards.  Thus  it  takes  part  in  the  formation  of 
the  mesial,  posterior  and  latero- ventral  wall  of  the  descending  cornu. 
As  already  noted,  the  fibres  of  the  posterior  horn  of  the  fornix  are 
closely  attached  to  its  anterior  concave  border  as  the  fimbriae. 

Before  leaving  the  cerebral  hemispheres  we  must  consider 
how  they  are  connected  together.  The  nerve  cells  of  the  cortex  of 
the  two  sides  are  put  in  communication  with  one  another  by  three 
different  transversely  running  bands  of  fibres  which  are  called  the 
cerebral  commissures.  Two  of  these  we  have  already  noted,  viz. 
the  corpus  callosum  and  the  posterior  region  of  the  fornix.  The 
third  is  a  small  compact  band  of  fibres  known  as  the  anterior  com- 
missure, situated  at  the  front  end  of  the  third  ventricle.  In  saggittal 
section  it  appears  as  a  circular  area  just  below  the  anterior  end  of 
the  fornix,  whose  anterior  pillars  pass  one  on  each  side  of  it.  If  this 
cord  be  traced  by  scraping  away  the  ventrally  lying  grey  matter  it 
will  be  seen  to  be  horse-shoe  shaped,  its  transverse  portion  lying  just 
above  and  partly  in  front  of  the  optic  chiasma.  and  its  two  ends  pass- 
ing outwards  and  forwards  to  terminate  in  the  olfactory  bulbs.  A 


348 


AN   INTRODUCTION   TO  ZOOLOGY 


few  of  its  fibres  also  appear  to  run  to  the  frontal  and  temporal  lobes. 
These  three  commissures  are  generally  regarded  as  arising  from  the 
lamina  terminalis  or  median  anterior  wall  of  the  embryonic  fore- 
brain.  If  this  be  so,  then  the  septum  lucidum  must  be  regarded 
as  the  attenuated  portion  of  the  lamina  lying  between  the  corpus 
callosum  and  the  fornix.  All  these  structures  are  somewhat  dis- 
placed from  their  original  position  owing  to  the  enormous  develop- 
ment of  the  outgrowths  from  the  antero-lateral  parts  of  the  fore- 
brain  to  form  the  cerebral  hemispheres. 


AC 


CM 


FIG.  121. — Diagram  to  show  relation  of  fornix  and  hippocampus, 
the  structures  being  represented  as  pulled  out  laterally. 

A.,  anterior  pillar  of  fornix  ;  A.C.,  anterior  commissure  ;  C.M.,  corpus  mammillare  ;  F.,  body  of 
fornix  ;  H.,  hippocampus  ;  H.C.,  hippocampal  commissure,  psalterium  ;  P.,  posterior  pillar  of 
fornix,  fimbria. 

It  will  be  seen  then  that  we  can  regard  the  hemisphere  as 
composed  of  an  anterior  basal  mass,  the  corpus  striatum,  joined  on 
to  the  main  stem  of  the  whole  brain  and  a  wall  that  passes  from  this 
outwards,  backwards,  upwards  and  inwards  to  enclose  the  ventricle, 
and  so  forming  a  sort  of  fold  termed  the  mantle  or  pallium.  In  this 
pallium  two  functionally  distinct  regions  can  be  distinguished. 
The  first  is  the  ventro-lateral  portion,  limited  externally  by  the 
rhinal  fissure  and  hippocampal  fissure,  and  comprising  the  olfactory 
bulbs,  the  pyriform  lobes,  olfactory  tubercles,  hippocampi  and  the 
fornix.  All  of  this  is  concerned  in  the  main  with  olfactory  sensations, 


LEPUS   CUNICULUS  349 

and  perhaps  also  with  taste.  This  portion  is  designated  the  olfactory 
pallium  or  archi-pallium.  In  lower  vertebrates,  for  example 
Scyllium  and  Rana,  the  archi-pallium  forms  practically  the  entire 
bulk  of  the  fore- brain.  As  we  have  seen  in  the  mammal,  that  is 
the  higher  mammal,  the  remaining  part  of  the  pallium,  lateral,  dorsal 
and  dorsal-mesial  in  position  is  even  larger  than  the  other.  Since 
it  is  the  part  that  has  been  added  to  the  old  olfactory  pallium  during 
the  course  of  evolution,  and  is  particularly  characteristic  of  the 
latest  group,  the  Mammalia,  it  is  termed  the  neo-pallium.  It  is 
very  highly  developed  and  concerned  with  visual,  auditory  and 
general  body  sensations,  and  the  voluntary  actions  connected 
therewith.  It  is  also  the  seat  of  the  higher  mental  processes,  and  in 
man  is  the  seat  of  the  mind.  These  various  activities  are  not 
diffused  generally  through  the  neo-pallial  cortex,  but  are  more  or 
less  definitely  localised  in  certain  definite  regions,  the  so-called 
sensory  areas.  For  example,  along  the  dorsal  region  of  the  hemi- 
sphere from  before  backwards,  we  find  areas  for  the  larynx,  face, 
arm,  trunk,  leg,  tail  and  anus.  .  On  the  lateral  surface  are  distinct 
areas  concerned  with  mastication,  oculo-motor  and  auris  centres, 
and  in  man  there  is  a  speech  centre  and  other  regions  concerned  with 
association  and  correlation.  To  this  highly-specialised  neo-pallial 
cortex  the  mammal  owes  its  great  mental  powers,  which,  as  has 
been  pointed  out,  are  in  the  main  responsible  for  its  dominating 
position,  and  this  development  and  differentiation  reach  their 
highest  point  in  man  himself. 

The  third  ventricle  is  a  deep  vertical  cleft  lying  in  the 
thalamencephalon,  and  so  being  the  modified  embryonic  fore-brain 
vesicle.  It  is  bounded  at  the  front  end  by  the  anterior  pillars  of  the 
fornix  and  the  lamina  terminalis,  including  the  anterior  commissure 
and  the  posterior  portion  of  the  septum  lucidum.  On  each  side  of 
the  latter  are  the  foramina  inter  vent  ricularia,  putting  the  third 
ventricle  in  communication  with  the  lateral  ventricles.  Posteriorly 
it  is  much  reduced  and  continued  backwards  as  the  iter.  The  side 
walls  are  furnished  by  the  optic  thalami.  Its  floor  is  formed  by  the 
optic  chiasma,  the  tuber  cinereum,  the  corpus  mammiUare  and  the 
part  of  grey  matter  of  the  posterior  perforated  spot.  The  roof  is 
very  thin,  being  composed  of  ependymal  epithelium,  and  imme- 
diately above  it  a  double  fold  of  vascular  pia  mater  runs  forward 
some  distance  under  the  hinder  ends  of  the  hemispheres  as  the 
velum  interpositum.  This  fold  of  the  pia  mater  pushes  the  roof 
down  on  the  two  sides  to  form  the  anteiior  choroid  plexuses,  which 
thus  project  into  the  ventricular  cavity.  Anteriorly  these  plexuses 
become  continuous  through  the  interventricular  foramina  with 
those  of  the  lateral  ventricles.  At  each  side  of  the  middle  line,  the 


350  AN  INTRODUCTION  TO  ZOOLOGY 

hinder  end  of  the  roof  and  top  part  of  the  side  wall  of  the  ventricle 
are  thickened  to  form  the  trigonum  habenula  or  habenular  ganglia. 
Between  these  arises  the  pineal  body  or  epiphysis  cerebri,  a  median 
unpaired  outgrowth  consisting  of  a  flattened  stalk  and  an  enlarged 
terminal  portion.  The  cavity  of  the  third  ventricle  projects  a  short 
distance  into  the  stalk  as  the  pineal  recess,  and  so  divides  it  into 
anterior  and  posterior  laminae.  In  the  anterior  lamina  runs  a 
transverse  band  of  fibres  which  connect  together  the  two  habenular 
ganglia,  and  is  known  as  the  habenular  or  superior  commissure.  At 
the  hinder  end  of  the  posterior  lamina  where  it  passes  on  into  a  band 
of  tissue  lying  between  the  anterior  corpora  quadrigemina  is  another 
transverse  band,  the  posterior  commissure,  which  is  functionally 
related  to  the  optic  centres,  but  anatomatically  considered  as  mark- 
ing the  posterior  limit  of  the  third  ventricle  and,  consequently,  the 
thalamencephalon.  The  pineal  body  itself  is  a  small  reddish  pear- 
shaped  mass  whose  functional  significance  is  not  clearly  understood, 
and  it  should  really  be  considered  as  forming  with  the  structures 
immediately  surrounding  it  one  pineal  complex.  In  the  primitive 
vertebrate  this  complex  has  two  potentialities,  one  glandular  and 
one  related  to  vision,  so  that  in  some  of  the  lower  vertebrates,  e.g. 
certain  Lizards  and  the  Lampreys,  we  find  it  represented  by  a  pair 
of  eyelike  structures,  the  pineal  eyes.  In  other  vertebrates  such  as 
the  mammals,  this  visual  potentiality  is  suppressed,  and  the 
glandular  function  becomes  more  marked  so  that  in  the  adult  rabbit 
and  sheep,  for  example,  we  find  that  the  single  pineal  body  has 
apparently  a  glandular  significance. 

The  optic  thalami  are  two  large  masses  of  grey  matter  forming  the 
main  part  of  the  walls  of  the  thalamencephalon.  They  run  forward 
to  the  corpora  striata,  from  which,  however,  they  are  separated  by  a 
slight  groove,  and  at  the  hinder  end  each  terminates  in  a  swelling, 
the  pulvinar.  They  are  so  thick  that  they  almost  obliterate  the 
cavity  of  the  ventricle,  and,  indeed,  in  the  centre  they  actually  meet 
and  adhere  in  the  middle  line  over  an  area  known  as  the  massa 
intermedia.  This  is  sometimes  referred  to  as  the  "  middle  or  grey 
commissure,"  an  erroneous  term,  since  there  is  no  crossing  of  fibres 
at  this  point,  but  merely  an  adhesion.  Just  behind  and  slightly 
lateral  to  the  pulvinar  are  two  ganglionic  masses,  the  internal  and 
external  geniculate  bodies.  The  optic  chiasma  is  formed  by  a  large 
band  of  fibres  from  each  side  running  together  in  the  middle  line, 
some  of  them  pass  out  away  from  the  brain  as  the  optic  nerve  of  the 
same  side,  while  others  cross  over  and  leave  the  brain  with  the  optic 
nerve  of  the  other  side.  The  optic  tract,  as  this  broad  band  of  fibres 
is  called,  passes  externally  over  the  lateral  surface  of  the  thalamus 
and  terminates  in  connection  with  the  pulvinar,  the  external 


LEPUS   CUNICULUS  351 

geniculate  body  and  the  anterior  quadrigeminal  bodies.  Certain 
parts  of  the  lamina  terminalis,  namely,  the  anterior  commissure, 
the  septum  lucidum,  the  fornix  and  the  corpus  callosum,  we  have 
already  considered  ;  but  there  still  remains  a  small,  almost  unaltered 
portion  of  it  at  the  anterior  end  of  the  ventricle.  This  runs  from  the 
anterior  commissure  downwards  and  backwards  to  the  optic  chiasma, 
and  to  this  the  term  lamina  terminalis  in  the  limited  sense  may  be 
applied.  The  chiasma  projects  into  the  floor  of  the  ventricle,  leaving 
a  small  pre-optie  recess  in  front  of  it,  whose  floor  is  formed  by  the 
lamina.  The  remaining  structures  in  the  floor  of  the  third  ven- 
tricle have  already  been  considered. 

Mid -Brain. 

The  mid-brain  occupies  but  a  small  part  of  the  total  brain 
mass  in  the  adult  and  its  cavity,  the  iter  a  tertio  ad  quartum  ventri- 
culum  or  aqueduct  of  Sylvius,  is  reduced  to  a  short  narrow  passage 
which  is  roofed  by  the  quadrigeminal  bodies  and  has  its  side  walls 
formed  by  the  crura  cerebri.  The  anterior  quadrigeminal  bodies  or 
colliculi  superiores  are  fairly  large  oval  masses,  and  much  larger 
than  the  posterior.  Each  gives  off  two  white  bands  of  fibres,  an 
anterior  brachium,  which  passes  forwards  and  downwards  behind 
the  pulvinar  into  the  optic  tract,  and  a  posterior  brachium,  which 
passes  down  to  disappear  under  the  internal  geniculate  body.  The 
posterior  quadrigeminal  bodies  or  colliculi  inferiores  are  small 
structures  underlying  the  former,  and  each  gives  off  a  small  brachium 
which  can  be  traced  forward  to  the  posterior  part  of  the  corona 
radiata. 

The  crura  or  pedunculi  cerebri  are  two  large  masses  running 
backwards  from  under  the  hemispheres  and  coming  out  between  the 
hippocampal  lobules.,  They  pass  backwards  from  the  optic 
chiasma,  visible  on  the  ventral  surface  as  two  wide  pillars,  and  leaving 
between  their  anterior  ends  the  posterior  perforated  spot,  and  they 
disappear  beneath  the  pons.  As  noted,  they  are  extremely  thick, 
and  constitute  the  side  walls  and  floor  of  the  iter. 

Hind-Brain, 

The  visible  dorsal  surface  of  the  hind-brain  is  composed 
entirely  of  the  cerebellum.  In  the  middle  line  this  is  constituted 
by  a  median  longitudinal  portion  termed  the  vermis,  the  surface  of 
which  is  marked  by  a  number  of  close-set  transverse  sulci.  At  its 
lateral  border  is  a  longitudinal  fissure  separating  it  from  the  lateral 
lobe  of  the  cerebellum,  or  hemisphserium  cerebelli.  This  in  its  turn 
has  attached  to  its  ventro-lateral  border  a  small  lateral  lobe,  the 


352  AN   INTRODUCTION  TO  ZOOLOGY 

flocculus,  traversed  only  by  five  or  six  sulci.  Between  the  flocculus 
and  the  hemisphgerium  is  a  lateral  lobe,  the  paraflocculus,  bent  upon 
itself  and  covering  the  dorsal  surface  of  the  flocculus. 

The  ant ero- ventral  surface  is  marked  by  a  broad,  slightly  raised 
transverse  band  of  tissue,  the  pons  Varolii,  whose  sides  pass  round 
latero-dorsally  to  join  the  cerebellum.  The  two  structures,  pons 
and  cerebellum,  form  the  major  part  of  the  metencephalon.  Just 
behind  the  pons  lies  another  less  conspicuous  transverse  area,  the 
corpus  trapezoideum,  which  is  marked  in  the  middle  line  by  a 
shallow  longitudinal  groove,  the  ventral  sulcus,  that  passes  back 
to  become  continuous  with  the  ventral  fissure  of  the  spinal  cord. 
It  also  bears  two  median  longitudinal  bands,  the  pyramids,  one 
on  each  side  of  the  furrow.  Behind  this  again  these  fairly  narrow 
pyiamidal  tracts  pass  backwards,  and  are  obliquely  truncated  at 
their  posterior  extremities,  this  region  being  termed  the  decussa- 
tion  of  the  pyramids,  since  at  this  place  fibres  from  one  side  of 
the  brain  pass  over  to  the  opposite  side  of  the  spinal  cord.  From 
this  region  onwards  the  floor  of  the  medulla  oblongata  or  myelence- 
phalon  passes 'backwards  without  noticeable  line  of  demarcation 
into  the  spinal  cord. 

The  fourth  ventricle,  the  cavity  of  the  hind-brain,  is  not  visible 
externally,  since  it  is  covered  by  the  cerebellum.  It  is  a  long 
triangular  depression  on  the  dorsal  side  of  the  hind- brain,  continuous 
with  the  iter  in  front  and  the  canalis  centralis  of  the  spinal  cord 
behind.  The  ventricular  roof  is  very  thin,  being  formed  of  ependyma 
and  the  overlying  pia  mater.  It  is  divided  into  two  moieties  by  the 
cerebellum.  The  front  part,  known  as  the  anterior  medullary 
velum  or  valve  of  Vieussens,  runs  back  from  the  posterior  colliculi 
under  the  anterior  end  of  the  cerebellum,  with  which  it  becomes 
continuous.  It  contains  a  layer  of  nervous  matter,  thin  posteriorly, 
but  thickening  in  front.  The  hinder  portion  of  the  ventricle  is 
roofed  by  the  posterior  medullary  velum  or  choroid  plexus  of  the  fourth 
ventricle.  This  is  mainly  non-nervous  and  highly  vascular,  and  it 
passes  forwards  under  the  end  of  the  cerebellum,  with  which  it 
becomes  continuous. 

The  cerebellum  is  joined  to  the  rest  of  the  brain  on  each 
side  by  three  large  bands  of  white  tissue,  which  help  to  form  the 
dorso-lateral  walls  of  the  ventricle.  The  most  anterior  is  the 
brachium  conjunctivum  or  anterior  cerebellar  peduncle,  which  runs 
downwards  and  forwards  beneath  the  corpora  quadrigemina,  and 
between  the  two  brachia  stretches  the  anterior  medullary  velum, 
which  becomes  fairly  thick  at  its  front  margin.  The  middle  cere- 
bellar peduncle  or  brachium  pontis  is  a  stout  band  of  white  fibres 
passing  ventrally  to  form  the  main  mass  of  the  pons,  which  thus 


LEPUS   CUNICULUS  353 

constitutes  a  connection  between  the  two  hemispheres  of  the  cere- 
bellum. The  posterior  or  inferior  peduncle  is  constituted  by  fibres 
running  to  the  restiform  bodies.  The  cerebellum  itself  is  deeply 
cut  into  by  the  sulci,  and  the  grey  matter  on  its  periphery  is  thrown 
into  a  number  of  subsidiary  folds  with  the  result  that  the  white 
matter  presents  a  very  characteristic  tree-like  appearance  in  sagittal 
section,  and  is  for  that  reason  termed  the  arbor  vitae.  The  side  walls 
of  the  fourth  ventricle  are  composed  to  a  considerable  extent  of  two 
large  masses,  the  areee  acusticse,  to  which  the  auditory  nerves  are 
related.  Just  behind  this  and  dorso-lateral  to  it  are  two  transverse 
masses,  the  corpora  restiformia.  The  floor  of  the  ventricle  is  mainly 
composed  of  a  thick  mass  of  grey  matter,  the  medulla  oblongata, 
continuous  behind  with  the  spinal  cord  and  containing  a  number  of 
important  ganglionic  masses  related  to  the  nerves  arising  from  it. 
The  positions  of  some  of  these  are  indicated  by  slight  internal 
elevations. 

Origins  of  Spinal  Nerves. 

Now  that  the  general  anatomy  of  the  brain  has  been  studied 
it  is  easy  to  pass  on  to  consider  the  points  of  origin  of  the  cranial 
nerves. 

The  nervus  terminalis  is  a  small  nerve  with  which  apparently 
sympathetic  nerves  are  associated.  It  originates  in  the  ventro- 
mesial  wall  of  the  hemisphere  ventral  and  lateral  to  the  olfactory 
tract  and  quite  close  to  the  lamina  terminalis. 

The  first  nerve,  or  olfactorius,  is  not  a  single  structure,  but  is 
represented  by  numerous  small  nerve  bundles  coming  in  from  the 
olfactory  membrane  through  perforations  in  the  cribriform  plate 
to  enter  the  olfactory  bulb.  From  a  small  elevation,  the  accessory 
olfactory  bulb,  on  the  mesial  side  of  the  bulb,  arise  a  group  of 
fibres  constituting  the  small  naso-vomerme  nerve  or  nervus 
septalis. 

As  we  have  seen,  the  second  nerve,  or  opticus,  coming  from  the 
eye,  pierces  the  ventro-lateral  cranial  wall  at  the  optic  foramen  and 
runs  inwards  and  backwards  to  join  the  ventral  side  of  the  optic 
chiasma  which  it  helps  to  form. 

The  third  nerve,  the  oculomotorius,  is  medium-sized  and  arises 
on  each  side  by  three  or  four  roots,  leaving  the  crura  cerebri  near  the 
middle  line  about  half-way  between  the  corpus  mammillare  and  the 
front  border  of  the  pons. 

The  fourth  nerve,  the  patheticus  or  trochlearis,  is  quite  small,  and 
the  only  one  arising  from  the  dorsal  surface  of  the  brain.  It  springs 
from  the  dorso-lateral  part  of  the  thickened  anterior  margin  of  the 
anterior  medullary  velum  just  behind  the  colliculus  inferior.  Thence 

2  A 


354  AN   INTRODUCTION  TO  ZOOLOGY 

it  passes  latero-ventrally,  and  shows  from  the  ventral  surface  just 
in  front  of  and  to  the  side  of  the  pons. 

The  fifth  nerve,  the  trigeminus,  is  very  large  and  stout,  and 
comes  from  the  ventro-lateral  aspect  of  the  pons  below  the  cerebellum. 
It  arises  by  two  roots  close  together,  an  external  sensory  root  and  an 
internal  smaller  motor  root. 

The  sixth  nerve,  the  abducens,  is  quite  a  small  nerve  springing 
from  the  corpus  trapezoideum  at  the  lateral  border  of  the  pyramid. 

The  seventh  nerve,  the  facialis,  is  of  moderate  size,  and  emerges 
from  the  ventro-lateral  border  of  the  corpus  trapezoideum  slightly 
behind  the  level  of  the  fifth  nerve. 

The  eighth  nerve,  the  auditorius,  is  about  the  same  size  as  the 
preceding,  and  arises  just  external  to  it.  The  main  trunk  almost 
at  once  breaks  into  two  branches,  the  cochlear  and  the  vestibular, 
which  pass  parallel  with  the  seventh  to  the  meatus  auditorius 
internus  and  are  distributed  to  the  correspondingly  named  part 
of  the  labyrinth. 

The  ninth,  or  glosso-pharyngeus  nerve,  has  three  roots,  and  the 
tenth,  or  vagus,  arises  by  four  roots.  These  all  arise  behind  one 
another  on  the  latero- ventral  border  of  the  front  end  of  the  medulla, 
slightly  medial  to  the  seventh  and  eighth,  and  the  foremost  root 
comes  off  just  behind  the  corpus  trapezoideum. 

The  eleventh,  or  nervus  accessorius,  arises  by  about  ten  thin 
roots,  and  the  most  posterior  of  them  is  well  down  the  spinal  cord 
at  about  the  level  of  the  fifth  spinal  nerve.  The  posterior  end  of  it 
runs  forward  along  the  side  of  the  anterior  end  of  the  spinal  cord, 
entering  the  cranium  through  the  foramen  magnum.  It  leaves  the 
cranium  through  the  foramen  lacerum  posterium  in  company  with 
the  ninth  and  tenth  nerves. 

The  twelfth  nerve,  the  hypoglossus,  takes  origin  from  eight  or 
nine  fine  roots  from  the  hinder  end  of  the  medulla,  nearer  the  middle 
line  than  the  ninth  or  tenth  and  slightly  behind  the  latter.  The 
foremost  roots  come  from  the  lateral  border  of  the  pyramids,  and 
consequently  certain  of  them  are  on  the  same  level  as  some  of  those 
of  the  accessories,  but  mesial  to  them. 

Thus  it  will  be  seen  that  although  in  conformity  with  long- 
established  custom  we  still  speak  of  twelve  cranial  nerves  in  mammals, 
there  are  in  reality  fourteen,  the  two  extra  ones  being  the  nervus 
terminalis  and  the  nervus  vomeronasalis  or  septalis,  as  noted 
previously. 


CHAPTER  XIV 
HISTOLOGY  AND    CYTOLOGY 

IN  the  foregoing  pages  we  have  seen  that  the  body  of  a  higher 
animal  consists  of  a  supporting  skeleton,  the  flesh,  and  a  large 
number  of  organs  arranged  in  systems  ;  we  cam  even  speak  of  the 
skeletal  and  muscular  systems  regarding  the  individual  parts  as 
organs.  Generally  speaking,  each  of  these  organs  is  of  a  character- 
istic shape,  and  always,  when  examined  in  greater  detail,  is  seen  to 
be  composed  of  a  definite  tissue,  as  it  is  termed,  or  a  combination  of 
tissues.  When  we  proceed  still  further  we  find  that  many  of  these 
tissues  are  composed  of  small  units,  termed  cells,  and  when  we  take 
into  account  their  development  we  find  that  all  parts  of  the  body  are 
composed  of  cells  or  of  structures  formed  by  or  from  cells.  This 
statement  of  the  composition  of  the  body  of  the  higher  animals  is 
sometimes  termed  the  "  cell  theory,"  and  while  it  is  possible  to 
indicate  certain  exceptions,  it  may  be  taken  to  be  in  the  main  a  true 
one.  We  thus  have  two  special  branches  of  minute  anatomy,  and 
also  of  physiology,  since  we  can  also  discuss  them  from  the  point 
of  view  of  function,  namely,  "  Histology,"  the  study  of  the  tissues, 
and  "  Cytology,"  the  study  of  the  cell.  Certain  general  points 
regarding  each  of  these  -need  considering. 

Histology. 

The  actual  details  of  the  histology  of  a  number  of  different 
tissues  have  already  been  dealt  with  in  treating  the  different  animal 
types,  and  it  now  only  remains  to  indicate  the  way  in  which  these 
tissues  may  be  classified  for  the  purposes  of  study.  In  the  first 
place  we  may  regard  them  from  the  point  of  view  of  similarity  of 
structure  and  function,  and  we  find  that  they  can  be  separated  into 
four  main  groups :  (i)  Epithelial,  (2)  Connective,  (3)  Muscular,  and 
(4)  Nervous  tissue.  In  addition  to  these  are  the  two  body  fluids, 
the  blood  and  the  lymph,  which,  since  they  contain  cells,  may  be 
conveniently  treated  with  the  tissues,  and,  indeed,  are  regarded  by 
some  authorities  as  being  highly  specialised  connective  tissue. 

An  epithelium  is  a  layer  of  cells  covering  free  surface  ;  it  need  not 

355 


356  AN   INTRODUCTION  TO  ZOOLOGY 

be  an  external  surface,  for  it  can  line  an  internal  cavity.  Epithelia 
may  be  divided  in  the  following  way. 

Simple  epithelia  consist  of  but  one  layer  of  cells,  and  are  sub- 
divided according  to  the  shape  of  the  cells  into  :  Squamous,  con- 
sisting of  flat  cells  ;  Cubical,  in  which  the  cells  are  approximately 
cubical  in  shape  ;  and  Columnar,  with  cells  much  longer  than  they 
are  wide. 

Compound  epithelia  have  their  cells  arranged  in  a  number  of 
superimposed  layers,  and  are  subdivided  into  :  Stratified,  in  which 
the  cells  are  arranged  in  a'series  of  layers,  the  innermost  cubical  or 
polygonal,  and  the  outermost  quite  flat.  This  is  the  most  common 
and  typical,  and  Transitional,  consisting  of  two  or  three  layers,  all 
of  more  or  less  polygonal  cells,  and  not  becoming  flattened  on  the 
outside.  This  latter  variety  is  found  in  the  bladder  and  the  cornea 
of  the  eye. 

It  is  the  epithelia,  particularly  the  simple  ones,  that  provide 
most  of  the  secretory  tissues  of  the  glands,  and,  consequently,  we 
find  that  almost  any  type  of  epithelium  may  be  glandular  in  nature. 
They  also  form  the  essential  part  of  the  sense  organs,  where  they 
are  termed  neuro-epithelia.  A  further  modification  is  to  be  found 
in  the  pharyngeal  region  of  Rana,  and  in  the  trachea  of  Lepus, 
where  the  cells  are  provided  with  an  external  covering  of  cilia 
constituting  a  ciliated  epithelium. 

As  a  rule,  we  find  that  the  glandular  epithelium  does  not  remain 
upon  the  surface,  but  becomes  invaginated  to  form  a  gland.  This 
may  be  a  simple  ingrowth,  and  so  form  a  tubular  gland,  such  as  we 
find  in  the  mucosa  of  the  stomach,  or,  if  its  deeper  end  or  fundus  is 
much  enlarged,  it  produces  a  flask  or  saccular  gland,  similar  to  those 
in  the  skin  of  Rana.  On  the  other  hand,  the  invagination  may 
become  complexly  branched  and,  if  its  branches  remain  tubular, 
form  a  compound  tubular  gland,  as  in  the  kidney,  or,  if  the  ends  of 
the  branches  dilate,  a  compound  saccular  or  racemose  gland,  as  in 
the  pancreas  and  salivary  glands. 

The  connective  tissues,  as  we  have  seen,  are  characterised  by 
possessing  a  fluid  matrix  in  which  their  cells  float,  and  through 
which  pass  typically  white  and  yellow  fibres.  Any  one  of  these 
elements  may  be  modified,  developed  to  a  marked  degree,  or,  on 
the  other  hand,  suppressed.  In  certain  cases  also,  as  in  adipose  tissue, 
bone,  etc.,  other  substances  may  be  laid  down  in  it.  The  various 
kinds  of  tissue  included  under  the  heading  of  connective  tissues  are  : 
Areolar  or  Sub-cutaneous,  Fibrous,  Elastic,  Adipose  and  Lymphoid 
tissues,  and  Bone  and  Cartilage.  By  some  authorities  Blood  and 
Lymph  are  also  regarded  as  being  highly  specialised  varieties  of 
connective  tissue. 


HISTOLOGY  AND  CYTOLOGY  357 

The  muscular  tissues,  as  we  have  seen,  are  of  two  kinds,  exhibiting 
a  fundamental  difference.  In  the  first  place  we  have  the  varieties 
in  which  the  cellular  structure  is  retained  as  in  the  involuntary  and 
cardiac  muscle.  In  the  second  place  there  is  the  voluntary  muscular 
tissue  which,  while  commencing  in  the  embryo  as  strands  of  meso- 
dermal  cells,  ends  as  long  complicated  fibres  without  any  trace  of 
cellular  structure,  but  in  the  formation  of  each  of  which  a  number  of 
cells  took  part.  All  muscular  tissue  is  characterised  by  exhibiting 
the  power  of  contractility  to  a  marked  degree. 

The  nervous  tissue  is  composed  of  highly  modified  and  very 
characteristic  cellular  elements,  the  neurons,  and  much  of  the  total 
bulk  of  the  nervous  tissue  in  the  body  is  constituted  by  the  processes 
of  these  cells  and  the  protective  medullary  sheath  they  have 
developed  around  them. 

There  is,  however,  another  and  entirely  different  way  of 
classifying  the  body  tissues  in  animals,  and  that  is  according  to  their 
derivation.  As  we  shall  see  later,  at  a  very  early  stage  in  the 
development  of  the  embryo  of  all  higher  forms  we  can  recognise  three 
layers  of  cells,  namely,  an  ectoderm,  covering  the  outer  surface  ;  an 
entoderm,  forming  the  lining  of  the  main  part  of  the  gut  from 
pharynx  to  rectum  ;  and  a  mesoderm,  which  lies  between  these  two 
layers.  Within  the  mesoderm  a  cavity,  the  ccelom,  appears,  so  that 
it  becomes  divided  into  two  layers,  an  outer  part  helping  to  form  the 
body  wall,  and  an  inner  part  taking  part  in  the  formation  of  the  gut 
wall,  but  both,  of  course,  are  just  parts  of  one  and  the  same  layer. 
Through  the  presence  of  the  ccelom,  however,  it  is  possible  to 
recognise  quite  early  two  parts  of  the  mesoderm  that  have  slightly 
different  fates.  The  layer  lining  the  ccelom  is  by  its  very  position 
an  epithelium,  and  is  spoken  of  as  the  mesothelium  ;  while  the 
remaining  part  between  this  and  either  the  ectoderm  or  entoderm 
is  in  the  embryo  a  sort  of  non-specialised  padding  tissue,  and  so 
termed  the  mesenchyme.  The  result  is  then,  that  we  can  divide  up 
the  tissues  of  the  adult  into  three  main  groups,  according  to  the 
embryonic  layer  from  which  they  are  derived,  and  one  of  these,  that 
of  the  mesoderm,  falls  into  two  sub-groups : — See  over. 

This  classification,  while  not  of  much  assistance  in  practical 
work  in  histology,  since  it  is  not  based  on  structural  similarity,  is 
of  considerable  importance  in  comparative  work,  and  also  in  a 
consideration  of  the  nature  of  the  organ  and  of  the  diseased  or 
pathological  conditions  to  which  it  may  be  subject. 


358 


AN   INTRODUCTION  TO  ZOOLOGY 


CLASSIFICATION  OF  THE  TISSUES. 


A.    ECTODERMAL. 

1 .  Epidermis  : 

(a)  Epidermal  append- 
ages,   hair,    scales, 
etc. 

(b)  Skin  glands  (sweat, 
sebaceous,    mam- 
mary, lachrymal). 

2.  Epithelium  of : 

(a)  Conjunctiva. 

(b)  Olfactory    chamber 
(neuro-epithelium). 

(c)  Auditory          organ 
(neuro-epithelium). 

(d)  Oral     cavity,     oral 
organ,    enamel    or- 
gan, salivary  glands, 
part  of  hypophysis. 

(e)  Anus,  rectal  glands. 
(/)  Chorion. 

Amnion. 
Lens. 

3.  Nervous  tissue. 

(a)  Cells       of       brain, 
spinal      cord      and 
sympathetic 
ganglia. 

(b)  Retina  of  eye. 

(c)  Ependymal   epithe- 
lium. 

(d)  Part  of  hypophysis. 

(e)  Pineal  body. 


B.  MESODERMAL. 

1 .  Mesothelium : 

(a)  Epithelium  of  peri- 
cardium,       perito- 
neum, pleura,  uro- 
genital  organs   and 
kidney. 

(b)  Striated  muscles. 

2 .  Mesenchyme : 

(a)  Connective  tissue, 
non-striate  muscle, 
adipose  tissue,  and 
pigment  cells. 

(6)  Supporting  tissue, 
cartilage,  bone,  liga- 
ment, bone  marrow. 

(c)  Spleen. 

(d)  Blood,  blood- 
vessels,      endothe- 
lium. 

(e)  Eye-parts       except 
lens,     retina,     and 
conjunctiva. 

(/)  Framework  of 

gonads.       ,..  • 


C.  ENTODERMAL. 

1.  Notochord. 

2.  Epithelium  of : 

(a)  Digestive        canal, 
glands  (gastric,  in- 
testinal,   liver    and 
pancreas),        allan- 
tois. 

(b)  Pharynx,       Eusta- 
chian  tubes,  tonsils, 
thymus,  thyroid. 

(c)  Respiratory     tract, 
larynx,        trachea, 
lungs. 


Cytology. 

As  we  have  seen,  when  we  study  the  details  of  any  tissue  we  are 
brought  down  to  the  cell  as  the  ultimate  particle  in  almost  all  cases. 
The  cells  are  the  fundamental  vital  units,  and,  indeed,  in  some  cases, 
e.g.  the  corpuscles  of  the  blood  and  lymph,  appear  to  be  almost 
separate  organisms,  only  dependent  on  the  body  to  a  certain  extent. 
Just  as  the  cell  c*an  be  regarded  as  the  structural  unit,  so  it  must  also 
be  looked  upon  as  the  physiological  unit,  for  all  questions  of  function 
ultimately  can  become  questions  of  the  physical  and  chemical 
changes  occurring  in  the  cell.  In  the  case  of  the  lowest  animals, 
the  Protozoa,  the  cell  is  in  all  senses  the  living  unit,  for  each  cell 
constitutes  a  separate  individual  capable  of  manifesting  all  the  vital 
phenomena.  As  we  ascend  the  animal  scale,  however,  we  find,  in 
the  cell  aggregates  we  call  the  Metazoa,  that  the  capabilities  of 
individual  cells  become  limited,  and  we  have  a  division  of  labour 
brought  about  whereby  certain  functions  are  confined  to  certain 


HISTOLOGY  AND  CYTOLOGY  359 

* 

groups  of  cells.  Pari  passu  with  this  gradual  division  of  function 
as  we  ascend  the  animal  scale  we  find  also  a  gradual  differentia- 
tion of  structure  whereby  the  cells  originally  equipotential,  or 
approximately  so,  and  of  similar  structure,  become  totally  unlike 
and  particularly  fitted  for  different  purposes.  In  the  course  of  our 
examination  of  the  higher  types  we  have  noted  numerous  illustra- 
tions of  the  differential  specialisation  of  structure,  and  we  have  seen, 
for  example,  how  nerve  cells,  bone  cells,  glandular  cells,  muscles,  and 
so  on,  are  quite  different  from  one  another,  and  each  adapted  to 
perform  its  own  work.  However  much  they  have  been  changed, 
however,  with  few  exceptions,  they  still  remain  cells,  and  we  may 
now  consider  the  structure  of  an  idealised  cell  from  which  they  can 
all  be  derived.  Naturally  this  will  be  far  more  like  the  primitive 
Protozoon  than  the  cell  of  any  tissue  of  a  higher  form  with  the 
exception  of  the  white  blood  corpuscles  and  the  lymph  cells,  which 
are  not  much  specialised. 

The  cell  consists  of  a  more  or  less  independent  mass  of 
protoplasm  within  which  is  a  denser  body,  the  nucleus,  so  that  we 
are  able  to  distinguish  a  cell  body  composed  of  cytoplasm,  and  the 
inner  part  the  nucleus,  the  material  of  which  is  often  termed  nucleo- 
plasm.  Under  the  low 'powers  of  the  microscope  this  protoplasm 
appears  as  a  granular,  semi-transparent,  greyish-white  mass  of  very 
viscous  substance,  and  under  higher  magnification  it  is  seen  to  present 
a  very  characteristic  reticulate  appearance,  which  has  been  inter- 
preted in  different  ways  by  various  authorities.  It  was  supposed 
that  protoplasm  is  composed  of  a  sort  of  sponge-like  interlacing 
network  of  solid  fibres,  termed  the  spongioplasm,  the  interstices  of 
which  are  filled  with  a  fluid  substance,  the  cell  sap  or  hyaloplasm. 
There  are  certain  objections  from  the  physical  aspect  to  this  way  of 
looking  at  things,  and  while  the  terms  have  been  retained  here  as 
convenient  for  descriptive  purposes,  it  is  not  intended  to  imply  that 
the  theory  with  which  they  are  associated  is  the  correct  one  ;  indeed, 
it  has  been  almost  completely  given  up.  The  most  generally  accepted 
explanation  is  that  protoplasm  possesses  an  emulsion  or  foam-like 
structure  resulting  from  the  mixture  of  substances  of  different 
surface  tensions,  and  that  the  so-called  fibres  composing  the  spongio- 
plasm are  probably  produced  by  the  different  optical  properties  of 
the  constituents,  and  that  this  appearance  is  retained  permanently 
when  the  protoplasm  is  killed.  It  is  possible  that  actual  condition 
varies  with  the  different  stages  of  physiological  activity  of  the  cell. 
When  dealing  with  so  small,  delicate  and  transparent  an  object  as 
the  living  cell  it  is  extremely  difficult  to  make  out  its  finer  structure 
accurately  without  staining  it  in  some  way  to  emphasise  its  com- 
ponents. This  procedure  usually  necessitates  killing  the  cell  in  some 


AN   INTRODUCTION  TO   ZOOLOGY 


way,  so  that  actually  our  knowledge  of  most  of  its  details  is  obtained 
from  a  study  of  the  dead  cell.  One  of  the  first  objects  of  Cytology, 
therefore,  is  to  devise  ways  of  killing  the  cell  in  such  a  manner  that 
its  structure  remains  as  much  like  the  living  cell  as  possible.  This 
process  is  termed  fixation,  and  a  number  of  different  solutions  have 
been  discovered  for  fixing  cells  and  tissues.  Each  of  these  acts 
most  favourably  with  certain  kinds  of  cells,  or  shows  up  certain 


FIG.  122. — Diagram  of  a  typical  cell. — From  Kellicott. 

«.,  aster  c.,  centrosome  (centriole)  ;  ch.,  chromatin  ;  cr.,  chromidia  ;  cs.,  centrosphere  ;  d., 
deutoplasmic  granules  ;  en.,  endoplasm  ;  ex.,  exoplasm  (cortical  plasm)  ;  hy.,  hyaloplasm  ;  k., 
karyosome  ;  /.,  lining  network  ;  m,,  cell  membrane  ;  n.,  nucleus  ;  nm.,  nuclear  membrane  ; 
o.,  nucleolus  ;  p.,  plastids  ;  sp.,  spongioplasm  ;  v.,  fluid  vacuoles  (metaplasm). 

structures  in  the  cell,  or  is  specially  adapted  for  use  when  it  is  to  be 
followed  by  certain  staining  substances  ;  and  although  some  have 
a  much  wider  range  of  action  than  others,  not  one  is  the  best  for  use 
under  all  or  any  circumstances.  It  is  beyond  the  scope  of  an 
elementary  book  to  go  further  into  this  important  matter,  but  it 
should  be  borne  in  mind  that,  in  the  description  that  follows,  various 
methods  of  treatment  are  necessary  in  order  to  render  conspicuous 
all  the  structures  considered. 


HISTOLOGY  AND  CYTOLOGY  361 

Under  the  high  powers  of  the  microscope  it  will  be  seen  that 
protoplasm,  in  addition  to  showing  the  structure  just  noted,  always 
presents  a  granular  appearance.  This  is  due  to  its  containing  a 
number  of  tiny  particles  that  in  fixed  cells  appear  as  solid  granules, 
and  are  of  several  different  kinds.  In  the  first  place  there  are  always 
present  an  enormous  number  of  minute  granules,  the  microsomes, 
which  are  distributed  along  the  spongioplasm  fibres,  and  appear  to  be 
an  essential  constituent  of  the  protoplasm.  The  cells  of  the  lower 
animals,  particularly  the  Protozoa,  often  have  granules  of  an  easily 
staining  material — chromatin,  which  is  really,  as  we  shall  see  later,  a 
constituent  of  the  nucleus  that  has  wandered  out  into  the  cytoplasm. 
These  chromidia,  as  they  are  termed,  are  not  common  in  the  cells  of 
the  higher  animals.  Then,  too,  we  have  a  number  of  slightly  larger 
granules  not  necessarily  related  to  the  spongioplasm  that  are  the 
result  of  the  chemical  activity  of  the  cell,  and  may  represent  food 
material,  which  is  stored  as  a  reserve  or,  as  the  result  of  the  anabolic 
changes,  is  on  the  way  to  being  transformed  into  protoplasm  ;  or, 
on  the  other  hand,  they  are  the  katabolic  products  of  the  cell  going 
to  form  its  secretions  or  waste  matter.  These  are  included  in  the 
general  term  metaplasmic  granules.  In  many  cells,  more  particu- 
larly the  germ  cells,  are  yet  other  groups  of  somewhat  larger  bodies 
that  may  be  of  a  spherical  shape,  when  they  are  termed  chrondo- 
somes,  or  of  an  elongated  or  rod  shape,  termed  mitochondria.  They 
can  easily  be  seen  after  certain  methods  of  fixing  and  staining,  but 
disappear  after  other  fixing  fluids.  Different  varieties  of  these 
bodies  have  been  made  out,  and  the  terminology  employed  to 
describe  them  is  not  uniform,  and  sometimes  contradictory  ;  but 
for  elementary  purposes  it  is  sufficient  to  note  their  presence,  and 
that  they  can  perhaps  be  included  under  the  terms  given.  Their 
exact  significance  has  not  yet  been  ascertained,  but  it  has  been 
shown  that  they  play  an  important  part  in  the  activities  of  the  cell, 
and  when  the  cell  divides  they  appear  to  be  more  or  less  carefully 
distributed  to  the  daughter  cells.  In  many  cells  we  find  certain 
spaces  filled  with  a  clear  fluid  and  termed  vacuoles.  In  the  case  of 
free-living  cells  these  often  represent  the  digestive  or  excretory 
apparatus,  as  we  have  seen  in  the  case  of  Amceba  and  Paramcecium. 
Certain  cells  also  contain  cytoplasmic  specialisations,  to  which  we 
can  give  the  general  name  of  plastids.  They  are  most  frequently 
met  with  in  plant  cells,  and  are  the  centres  at  which  some  substance 
is  collected,  e.g.  the  chloroplasts  containing  the  chlorophyll,  or  the 
amyloplasts  where  the  starch  is  aggregated. 

There  is  also,  in  the  cytoplasm,  another  compound  structure 
situated  quite  close  to  the  nucleus,  and,  indeed,  the  evidence  pro- 
vided bv  certain  Protozoa  seems  to  show  that  primitively  it  was 


362  AN   INTRODUCTION  TO  ZOOLOGY 

actually  a  part  of  the  nucleus  itself,  within  which  it  is  to  be  found 
in  some  forms.  In  the  cells  of  higher  animals,  however,  it  lies 
outside  the  nucleus,  and  so  can  be  considered  as  a  cytoplasmic 
inclusion.  It  takes  the  form  of  a  tiny  spherical  granule,  termed  the 
centriole  or  centrosome,  that  stains  very  intensely  with  certain  dyes 
and  so  stands  out  quite  clearly.  Often  in  higher  forms  it  is  not 
single,  but  a  pair  of  granules  side  by  side,  and  it  is  then  termed  the 
diplosome.  Strangely  enough  this  granule,  so  constant  in  animal 
cells  and,  as  we  shall  see  later,  playing  such  a  notable  part  in  the 
process  of  cell  division,  has  not  been  shown  to  be  present  in  plant 
cells,  and  so  its  presence  constitutes  one  of  the  most  striking  and,  up 
to  now,  inexplicable  differences  between  the  cells  of  animals  and 
plants.  It  is  surrounded  by  a  small  sphere  of  very  clear,  apparently 
structureless  protoplasm,  spoken  of  as  archoplasm,  while  the  sphere 
itself  is  termed  the  centrosphere  or  attraction  sphere.  This,  then, 
completes  the  list  of  the  main  structures  to  be  found  in  the  cytoplasm. 

The  nucleus  in  the  higher  animals  is  in  the  form  of  a  sphere  or  a 
more  or  less  elongated  ovoid,  but  in  the  Protozoa  it  may  be  of  a 
very  irregular  shape,  and  in  some  cases  divided  up  into  two  parts 
of  different  functions.  The  nucleus,  as  a  whole,  plays  a  very  im- 
portant part  in  the  activities  of  the  cell,  and  appears  to  be  the 
controlling  centre  of  the  cell.  Not  only  does  it  seem  responsible 
for  the  chemical  changes  in  the  cytoplasm,  particularly  those  leading 
to  assimilation,  but  it  also  takes  the  initiative  in  the  processes  leading 
up  to  cell  division. 

The  most  important  constituent  of  the  nucleus  is  a  material 
termed  chromatin,  which  is  generally  distributed  throughout  the 
nucleus  in  the  form  of  granules  and  threads.  In  lower  animals 
some  of  it  may  be  extranuclear  in  position.  It  receives  its  name 
from  the  fact  that  it  very  readily  takes  up  certain  basic  dyes,  such, 
for  example,  as  hsematoxylin.  Its  full  significance  will  be  more 
readily  appreciated  when  we  come  to  consider  the  part  it  plays  in 
division  and  in  the  activities  of  the  germ  cells.  Chemically, 
chromatin  is  noticeable  because  it  contains  nucleic  acid,  an  organic 
acid  rich  in  phosphorus.  Another  important  part  of  the  nucleus 
is  the  linin  or  achromatin,  so  called  because  of  the  difficulty  with 
which  it  can  be  induced  to  take  up  stains.  This  is  in  the  form  of  a 
fine  interlacing  network  of  fine  fibres  with  which  the  chromatin  is 
closely  associated.  The  nucleus  is  separated  off  from  the  cytoplasm 
by  a  very  thin,  homogeneous  nuclear  membrane,  which  also  seems  to 
be  made  of  the  same  achromatinic  substance  and  is  difficult  to  stain. 

Two  other  substances  are  present  in  the  nucleus  which  also 
stain  readily  and  very  similarly  to  chromatin.  The  first  of  these  is 
volutin,  which  with  special  treatment  stains  slightly  differently  from 


HISTOLOGY  AND  CYTOLOGY  363 

chromatin,  and  is  regarded  as  forming  a  reserve  from  which  the 
chromatin  is  recuperated  as  it  is  used  up.  The  other  is  termed 
plastin,  and  this  is  harder  to  differentiate  by  means  of  staining,  but 
it  behaves  differently  during  division.  Within  the  nucleus  is 
usually  one  or  several  distinct  rounded  bodies  that  may  be  of  two 
different  kinds.  The  nucleolus  is  a  rounded  mass  of  plastin  and 
takes  no  part  in  division.  The  karyosome  appears  to  consist  of  a 
plastin  basis  with  which  is  incorporated  a  greater  or  less  amount  of 
true  chromatin,  and  so  it  takes  part  in  division. 

Lastly,  we  have  the  fluid  portion  of  the  nucleus  which  appears 
to  be  somewhat  more  liquid  than  that  of  the  general  cytoplasm,  and 
is  termed  the  nuclear  sap  or  enchylema. 

Having  considered  the  structure  of  the  cell  and  the  nucleus, 
in  what  is  termed  the  resting  condition,  we  can  examine  the  manner 
in  which  it  divides.  It  has  been  noted  that  reproduction  in  the 
Protozoa  is  brought  about  by  cell  division,  and  that  the  multi- 
cellular  condition  of  the  Metazoa  results  from  the  same  process, 
only  the  daughter  cells  as  they  are  produced  do  not  separate,  but 
remain  together  to  form  layers.  Two  distinct  kinds  of  cell  division 
occur,  but  both  of  them  agree  in  that  it  is  the  nucleus  that  initiates 
the  activities,  and  that  the  cytoplasm  follows.  The  first  method  is 
the  simplest,  and  is  known  as  direct  division  or  amitosis,  in  order  to 
contrast  it  with  the  second  variety,  which  is  termed  indirect  division, 
mitosis  or  karyokinesis. 

The  first  indication  of  direct  division  is  the  elongation  of  the 
nucleus,  which  is  followed  by  a  lengthening  of  the  cell  in  the  same 
plane.  The  nucleus  then  becomes  dumbbell-shaped  and  finally 
breaks  into  two,  one  part  going  to  each  end  of  the  cell.  Shortly 
after  this  a  constriction  appears  in  the  middle  of  the  cell,  which 
gradually  deepens  and  finally  divides  it  into  two,  each  of  which 
contains  a  nucleus.  Although  a  simple  form  of  division,  it  is  a 
comparatively  rare  one.  It  occurs,  as  we  have  seen,  in  the  macro- 
nucleus  of  Paramcecium,  possibly  in  some  forms  of  Amoeba  and  in 
the  higher  animals  in  certain  pathological  conditions,  such  as 
cancerous  growths. 

Indirect  or  mitotic  division  is  more  complicated,  and  its  details 
vary  slightly  in  different  animals  ;  but  there  is  a  sufficient  general 
resemblance  to  enable  us,  for  descriptive  purposes,  to  divide  the 
process  into  four  more  or  less  distinct  stages,  namely,  the  prophase, 
the  metaphase,  the  anaphase  and  the  telophase. 

The  first  indication  of  the  approaching  division  is  given  by  the 
centrosome,  which,  if  single,  divides  into  two  granules  that  move 
apart,  or,  if  double  already,  they  start  to  separate.  When  they  have 
moved  about  a  short  distance  each  is  surrounded  by  a  series  of 


364  AN   INTRODUCTION  TO  ZOOLOGY 

radiating  lines,  the  astral  rays,  and  is  termed  an  aster.  During  this 
time  the  chromatin  of  the  nucleus  becomes  more  in  amount  and 
stains  more  readily,  and  it  takes  up  its  position  on  the  linin  threads 
apparently  as  a  series  of  granules.  The  karyosome  breaks  up  and 
contributes  its  chromatin  to  the  general  supply,  while  the  nucleolus 
is  apparently  passed  out  into  the  cytoplasm,  where  it  disappears, 
taking  no  part  in  the  subsequent  changes.  All  the  chromatin 
becomes  arranged  in  a  long  convoluted  and  seemingly  continuous 
thread,  termed  the  skein  or  spireme.  The  asters  separate  more 
widely,  and  as  they  do  so  their  adjoining  fibres  unite  to  form  a  long 
spindle-shaped  arrangement  termed  the  spindle.  The  centrosomes 
pass  on  until  they  come  to  lie  at  opposite  poles  of  the  nucleus, 
through  which  the  spindle  fibres  are  enabled  to  pass  because  of  the 
disappearance  of  the  nuclear  membrane.  Meanwhile,  the  spireme 
becomes  broken  up  into  a  number  of  definite  independent  pieces, 
usually  about  equal  in  length,  termed  the  chromosomes,  which 
finally  take  up  a  fairly  symmetrical  position  in  the  middle  of  the 
spindle,  with  their  length  at  right  angles  to  the  line  joining  the 
centrosomes,  and  form  what  is  termed  the  equatorial  plate.  Thus  is 
produced  a  very  characteristic  arrangement ;  there  is  a  centrosome 
at  each  end  with  its  radiating  astral  rays,  and,  joining  them,  the 
spindle,  now  strongly  marked  and  quite  wide  in  its  middle  region. 
Across  the  widest  part  is  arranged  the  equatorial  plate  of  chromo- 
somes, which  are  situated  in  the  peripheral  region  of  the  spindle. 
The  outermost  spindle  fibres  seem  to  be  attached  to  the  chromo- 
somes, and  so  form,  as  it  were,  an  outer  zone,  sometimes  described 
as  the  mantle  fibres,  surrounding  the  inner  central  core  of  the  spindle, 
the  fibres  of  which  pass  from  one  end  to  the  other.  This  completes 
the  typical  prophase,  and  the  resulting  figure  is  a  very  striking  one 
and  known  as  the  amphiaster  or  achromatic  figure. 

While  a  certain  amount  of  variation  is  met  with,  for  example, 
the  entire  spindle  may  be  well  formed  outside  the  nucleus,  and  later, 
as  it  were,  break  across  it ;  yet  we  find  in  general  a  fairly  close 
similarity  in  the  prophase,  as  it  is  found  in  practically  all  species  of 
higher  animals.  Among  some  of  the  Protozoa  we  can  see  certain 
marked  differences,  so  much  so  that  they  suggest  that  the  process  of 
mitosis  was  evolved  in  that  phylum,  and  that  certain  of  its  members 
have  not  yet  developed  to  the  stage  when  its  mechanism  has  been 
perfected,  and  in  simple  cases  the  whole  figure  may  arise  within  the 
nuclear  membrane.  In  the  normal  divisions  of  all  the  cells  of  the 
body  and  its  various  tissues,  in  both  the  embryonic  and  adult  stages, 
there  is  produced  an  amphiaster.  As  has  been  pointed  out,  it 
appears  as  if  actual  fibres  are  produced,  some  of  which  are  attached 
to  the  chromosomes,  and  others  not,  and  it  may  be  significant  that 


HISTOLOGY  AND  CYTOLOGY 


365 


their  configuration  in  the  amphiaster  resembles  quite  closely  the 
disposition  of  the  lines  of  force  in  an  electro-magnetic  field.     The 


,  )f  lift  fill!  jf 


«  I 


actual  nature  of  these  threads,  e.g.  whether  they  are  threads  or  an 
arrangement  of  the  nucleoplasmic  structure,  and  so  on,  is  not  quite 


366  AN   INTRODUCTION  TO   ZOOLOGY 

clear,  and  there  are  conflicting  theories  put  forward  as  to  the  forces 
that  are  responsible  for  the  phenomenon. 

The  chromosomes  themselves  take  on  a  variety  of  forms,  spheres, 
short  rods,  long  rods,  V-shapes,  Y-shapes,  and  so  on,  and  different- 
shaped  chromosomes  may  occur  in  one  and  the  same  nucleus, 
though  each  species  or  tissue  usually  retains  the  same  arrangement 
in  its  mitoses.  The  next  step  in  the  division,  i.e.  the  metaphase,  is 
that  each  individual  chromosome  splits  longitudinally,  that  is,  in  a 
direction  at  right  angles  to  the  long  axis  of  the  spindle.  Half  of  each 
chromosome,  therefore,  is  apparently  attached  to  the  mantle  fibres 
of  one  aster,  and  the  other  half  to  the  fibres  of  the  other  aster.  This 
constitutes  the  essential  stage  in  mitosis,  and  to  which  the  prophase 
can  be  considered  as  preparatory.  In  the  latter  we  find  that  the 
chromatin  is  collected  up,  sorted  out  into  definite  pieces,  the 
chromosomes,  and  these  are  so  arranged  that  their  division  and 
distribution  can  be  carried  out  as  accurately  as  possible.  The 
remaining  two  phases  can  be  regarded  as,  in  a  way,  reconstruction 
and  finishing  stages,  serving  to  secure  the  proper  distribution  of  the 
chromatin.  In  some  instances  there  is,  as  it  were,  a  pushing  forward 
of  the  splitting  process,  even  as  far  as  the  spireme  stage,  so  that  from 
that  point  on  the  chromatin  and  chromosomes  when  formed  are 
double.  Strictly  speaking,  the  term ' '  metaphase ' '  can  only  be  applied 
when  the  chromosomes  split  after  being  arranged  in  the  equatorial 
plane,  but  it  will  be  seen  that,  whatever  method  may  be  adopted,  it 
does  not  alter  the  fundamental  phenomenon  of  the  sorting  and 
division  of  the  chromatin  into  equal  and  similar  portions. 

In  the  anaphase  the  halves  of  each  chromosome  move  towards  the 
opposite  poles  of  the  amphiaster.  It  appears  as  if  their  role  in  this 
is  entirely  a  passive  one,  and  that  they  are  drawn  apart  by  the 
contraction  of  the  mantle  fibres.  The  groups  of  chromosomes  are 
carried  right  to  the  pole,  and  so  are  ready  for  the  last  stage.  As  the 
groups  separate  there  will  be  left  in  the  middle  region  of  the  spindle, 
the  central  fibres,  which  are  now  termed  the  interzonal  fibres. 
These  are  further  added  to  by  other  fibres  which  appear  between  the 
diverging  chromosomes  in  the  position  formerly  occupied  by  the 
mantle  fibres.  The  origin  of  these  secondary  fibres  is  not  clear,  but 
they  are  also  included  in  the  term  "  interzonal  fibres."  In  plant  cells 
frequently  the  interzonal  fibres  thicken  in  the  middle  to  form  what 
is  known  as  the  mid-body  or  cell  plate,  and  this  appears  to  take 
part  in  the  formation  of  the  cell  wall  separating  the  daughter  cells. 
During  this  phase  there  is  usually  a  definite  constriction  in  the 
cytoplasm  of  animal  cells,  foreshadowing  its  division  into  two  in  a 
plane  passing  through  the  middle  of  the  interzonal  fibres. 

The  concluding  stages,  the  telophase,  lead  to  the  transformation  of 


HISTOLOGY  AND  CYTOLOGY  367 

the  -chromosome  groups  at  the  two  poles  into  the  condition  of  the 
resting  nucleus,  with  its  membrane  and  reticulum  restored.  This 
may  be  regarded  as  being  brought  about  by  a  process  that  is  practi- 
cally the  reverse  of  the  changes  occurring  in  the  prophase.  It  is 
subject  to  considerable  variation,  and  in  the  case  of  rapidly  dividing 
cells,  such  as  we  find  in  developing  animals  or  tissues,  may  be 
practically  omitted.  The  centrosome  of  each  nucleus  may  divide 
into  two  during  the  anaphase,  and  all  that  is  necessary  is  for  the 
chromosome  group  at  the  end  of  the  anaphase  to  rotate  through  an 
angle  of  90°  so  as  to  constitute  a  typical  amphiaster  practically 
at  once.  The  stages  of  the  telophase  are  passed  through  if  the  cell 
returns  to  a  normal  non-dividing  condition,  and  the  above  variations 
may  be  regarded  simply  as  short  cuts  to  enable  cell  division  to 
proceed  at  a  rapid  rate.  While  the  nuclear  reconstruction  is  pro- 
ceeding, or  very  sfiortly  after  it  is  completed,  the  cytoplasm  of  the 
mother  cell  is  cut  into  two  and  two  daughter  cells  are  produced. 

Certain  points  in  connection  with  mitosis  need  to  be 
emphasised.  In  the  first  place  we  see  that  it  is  an  elaborate 
mechanism  which  has  as  its  result  the  careful  collecting  up  of  the 
chromatin  into  a  number  of  definite  bodies,  the  chromosomes  ;  each 
of  these  is  split  into  two,  and  a  half  goes  to  each  daughter  cell. 
Thus  not  merely  is  the  chromatin  content  of  the  nucleus  halved  in 
amount,  but  it  is  first  sorted  out  and  halves  of  the  individual  units 
distributed  to  the  new  cells,  suggesting  that  the  halving  is  also 
qualitative.  It  would  appear  from  the  universality  of  the  occurrence 
of  mitosis  and  the  consistency  of  its  result,  that  it  is  necessary  for 
this  material  to  be  carefully  dealt  with,  and  so  apparently  the  chro- 
matin itself  must  be  of  great  importance  to  the  subsequent  activities 
of  the  resulting  cells.  All  other  cytological  evidence  tends  to  confirm 
this  conception  of  the  functional  value  of  chromatin. 

Numerous  observations  on  many  species  of  animals  have  also 
revealed  the  fact  that  in  any  one  species  there  is  a  remarkable 
constancy  in  the  number  and  variety  of  chromosomes  produced. 
Thus  for  each  species  there  is  a  fixed  number  of  chromosomes, 
and  they  are  arranged  in  a  fairly  constant  manner  in  all  the  cell 
divisions  of  the  body,  no  matter  what  organ  or  tissue  they  are  going 
to  form.  This  constant  number  is  known  as  the  somatic  or  diploid 
number. 

It  will  be  noted  that  this  constant  number  refers  to  the 
cells  of  the  body,  and  the  description  of  mitosis  given  is  similarly 
applicable  to  the  somatic  cells.  A  modified  form  of  such  cell 
division  is  encountered  in  the  history  of  the  germ  cells,  both  male 
and  female,  in  practically  all  cases.  As  a  result  of  this  modification 
we  find  that  the  mature  germ  cells  contain  only  half  the  number  of 


368  AN   INTRODUCTION  TO  ZOOLOGY 

chromosomes  that  is  characteristic  of  the  body  cells,  and  this  is 
termed  the  reduced  or  haploid  number.  The  exceptions  are  found 
in  those  animals  that  produce  germ  cells  that  can  undergo  develop- 
ment without  fertilisation  ;  a  form  of  reproduction  termed  partho- 
genesis.  Thus  the  number  of  chromosomes  in  a  germ  cell,  i.e.  the 
haploid  number,  can  be  represented  in  a  general  way  by  the  constant 
n,  and  the  number  in  the  body  cells,  or  diploid  number,  will  be 
therefore  2n*  The  processes  undergone  in  the  production  of  the 
germ  cells  differ  slightly  in  the  two  sexes,  and  need  to  be  treated 
separately,  although,  as  will  be  seen,  they  are  fundamentally  the 
same.  Cells  destined  to  give  rise  only  to  the  germ  cells  appear  at  a 
very  early  stage  in  the  development  of  animals,  including  the 
higher  forms  ;  these  are  known  as  the  primitive  germ  cells.  They 
take  up  a  position  in  the  tissue  that,  with  their  descendants,  will  give 
rise  to  the  gonads,  and,  after  a  certain  period  of  •multiplication,  are 
termed  the  spermatogonia  in  the  male,  and  the  oogonia  in  the 
female. 

The  process  in  the  male  resulting  in  the  production  of 
the  male  gametes  is  termed  spermatogenesis.  The  spermatogonia, 
together  with  certain  other  nutritive  or  nurse  cells,  form  the  lining 
of  the  seminiferous  tubules  of  the  testes.  They  have  been  repro- 
duced from  the  primitive  germ  cells  by  a  series  of  ordinary  mitotic 
divisions,  and  while  in  the  tubule  they  grow  until  they  attain  a 
certain  size,  when  they  again  divide  into  two  by  mitosis.  One 
daughter  cell,  the  outer,  persists  as  a  parent  spermatogonium,  which 
grows  to  regain  its  original  size  and  then  divides  again,  and  so  on. 
The  other,  and  inner  cell,  forms  a  sperm-mother  cell  or  primary 
spermatocyte.  This  cell  also  grows  slightly  and  then  divides  to 
form  the  secondary  spermatocytes  ;  but  this  division  is  not  an 
ordinary  mitotic  one,  and  its  main  points  need  to  be  considered. 

The  number  of  chromosomes  entering  this  cell  is,  of  course,  the 
diploid  or  somatic  number.  As  the  spireme  appears  it  passes  into 
an  unusual  condition,  for  its  fibres  all  mass  together  at  one  pole  of 
the  nucleus  near  the  centrosomes  to  form  a  dense  clump,  such  as  is 
not  encountered  in  ordinary  division,  and  this  condition  is  termed 
the  contraction  phase  or  synizesis.  After  a  time  this  chromatin 
knot  begins  to  unravel,  and  almost  as  it  does  so  it  segments  into 
chromosomes  ;  but  the  number  of  chromosomes  produced  in  this 
way  is  only  half  the- ordinary,  i.e.  it  is  the  haploid  number.  Closer 
examination  shows  that  each  apparent  chromosome  is  really  a  pair 
of  chromosomes  in  close  apposition,  but  they  are  not  completely 

*  There  are  certain  exceptions  to  this  also  in  the  case  of  animals  that 
possess  an  extra  chromosome  in  one  sex,  but  this  is  a  point  into  which  it  is  not 
necessary  to  enter. 


HISTOLOGY  AND  CYTOLOGY 


369 


fused,  and  a  narrow  longitudinal  slit  between  them  can  be  made  out. 
This  coming  together  of  the  chromatin  threads  just  before  the 
formation  of  the  chromosomes  is  termed  synapsis,  and  it  constitutes 
a  fundamental  difference  between  this  type  of  cell  division  and 


C  H  cT 

FIG.  124. — Spermatogenesis. — From  Bourne. 

A ,  a  spermatocyte  preparing  for  the  first  or  meiotic  division  ;  a  large  karyosomc  is  present  and 
the  rest  of  the  chromatin  is  scattered  in  the  form  of  granules  on  the  achromatic  network  of  the 
nucleus  ;  on  the  top  of  the  nucleus  is  the  centrosphere  with  two  centrpsomes.  B,  spireme  with 
a  single  row  of  chromatin  granules  ;  the  loops  of  the  spireme  show  a  distinct  polarity.  C,  early 
stage  of  synapsis,  the  chromatin  granules  and  the  spireme  ribbon  are  divided  longitudinally.  D, 
twelve  bivalent  chromosomes  of  various  shape  becoming  arranged  round  the  equator  of  the  spindle. 
E,  metaphase  of  the  meiotic  division  ;  each  bivalent  chromosome  is  being  divided  into  its  two 
components,  one  of  which  is  being  drawn  to  each  pole  of  the  spindle.  F,  nuclear  telophase  and  first 
division  of  the  spermatocyte.  G,  two  secondary  spermatocytes  preparing  for  division  ;  the 
upper  figure  showing  an  earlier  stage  of  mitosis  than  the  lower  ;  the  nucleus  of  each  spermatocyte 
contains  twelve  univalent  longitudinally-split  chromosomes.  H,  the  upper  spermatocyte  shows 
the  metaphase,  the  lower  the  early  anaphase  of  the  first' post -meiotic  (homceotype)  division.  /,  the 
two  secondary  spermatocytes  have  divided  to  form  four  spermatids  ;  the  nuclei  are  shown  in  differ- 
ent stages  passing  from  the  late  anaphase  to  the  resting  condition.  Note  that  the  figures  G,  H, 
J  are  drawn  on  a  somewhat  smaller  scale  than  the  rest. 

ordinary  mitosis,  and,  as  we  shall  see  later,  one  that  is  of  great 
importance  subsequently.  Generally,  as  these  double  or  bivalent 
chromosomes  move  to  take  up  their  position  on  the  equatorial  plate, 
each  of  them  splits  longitudinally  and  at  right  angles  to  the  original 

2  B 


370  AN   INTRODUCTION  TO  ZOOLOGY 

cleft.  Thus  each  apparent  chromosome  consists  of  four  similar 
pieces  lying  closely  side  by  side,  and  in  this  condition  it  may  be 
termed  a  tetrad.  The  division  now  proceeds  as  in  mitosis,  with  the 
result  that  each  daughter  chromosome,  formed  by  the  separation  of 
the  tetrad  along  the  line  of  the  second  split,  comes  to  consist  of  a 
bivalent  chromosome  of  two  pieces  side  by  side.  If  not  already 
present  the  second  split  appears  at  this  stage,  and  in  either  case  the 
anaphase  is  entered  upon.  Each  one  of  the  pair  in  the  bivalent 
daughter  chromosomes  is  a  half  of  the  corresponding  member  of  the 
pair  formed  during  synapsis. 

When  this  division  is  watched,  therefore,  it  will  be  seen  that  each 
daughter  cell,  or  secondary  spermatocyte,  has  apparently  half  the 
number  of  chromosomes  that  went  into  the  original  primary 
spermatocyte.  To  distinguish  this  type  of  division  from  mitosis  it 
is  spoken  of^as  Meiosis  or  reducing  division.  (In  older  terminology 
ordinary  mitosis  was  said  to  be  homceotype,  since  each  daughter  cell 
had  the  same  number  of  chromosomes  as  the  parent  cell,  while 
meiosis  was  designated  heterotype,  since  parents  and  daughters  did 
not  have  the  same  number  of  chromosomes.)  The  subsequent 
division  of  the  secondary  spermatocyte  starts  in  a  normal  way,  and 
the  bivalent  chromosomes  take  up  a  position  on  the  equatorial  plate. 
Each  chromosome  is  already  split,  and  does  not  do  so  again,  so  that 
a  half  goes  to  each  daughter  cell  or  spermatid.  There  is  no  reduction 
in  the  apparent  number  of  chromosomes  (in  old  terminology  the 
division  is  homceotype),  although  the  chromosomes  have  become 
univalent. 

Thus  from  each  original  primary  spermatocyte  we  have  produced 
four  daughter  cells,  each  of  which  contains  half  the  number  of 
chromosomes  that  went  into  the  primary  cell.  Moreover,  the 
spermatids  only  receive  half  'of  one  member  of  the  bivalent  chromo- 
somes formed  during  synapsis,  and  not,  as  might  be  supposed,  a 
quarter  of  both.  The  spermatids,  containing  the  haploid  number, 
undergo  no  further  divisions,  but,  as  the  result  of  an  alteration  of 
structure,  become  transformed  into  spermatozoa. 

While  the  spermatozoa  of  the  higher  animals  exhibit  a 
variety  of  shapes  and  sizes  they  are  composed  of  essentially  the  same 
parts,  and  the  description  that  follows^  is  based  mainly  on  that  of 
man  or  one  of  the  higher  mammals.  Under  moderate  powers  of  the 
microscope  a  spermatozoon  appears  to  consist  of  three  portions,  a 
small  head,  a  smaller  middle  piece  or  neck,  and  a  long  fine  vibratile 
tail.  Further  investigations  shows  that  it  really  possesses  a  fairly 
complicated  structure,  all  the  parts  of  which  are  derived  from  pre- 
existing portions  of  the  spermatid.  The  head  appears  to  be  mainly 
composed  of  the  chromatin  of  the  nucleus  of  the  spermatid,  and  it  is 


HISTOLOGY  AND  CYTOLOGY 


provided  at  its  anterior  end  with  a  covering,  or  cap,  derived  from  the 
centrosphere.  This  cap  serves  as  an 
organ  for  perforating  the  egg,  and  is 
differently  shaped  in  various  species. 
The  neck  is  a  very  small  inconspicuous 
portion,  but  important,  since  it  contains 
granules  derived  from  the  centrosome  of 
the  spermatid,  and  from  which  the 
centrosomes  of  the  fertilised  ovum  are 
presumed  to  be  derived.  The  tail  con- 
sists of  an  axial  filament  surrounded  for 
the  greater  part  of  its  length  by  a  cyto- 
plasmic  sheath,  but  three  distinct  parts 
can  be  recognised  in  it.  Firstly,  next  to 
the  neck  is  a  moderate-sized  connecting 
piece,  also  containing  granules  derived 
from  the  centrosome  and  other  struc- 
tures ;  secondly  comes  a  long  main  piece 
with  just  the  axis  and  its  sheath;  and 
finally  an  end  piece,  consisting  of  the 
axial  filament,  only  without  its  sheath. 

The  process  of  egg  formation, 
or  obgenesis,  in  the  female,  while  it 
differs  from  spermatogenesis  in  certain 
points,  is  nevertheless  fundamentally  the 
same  as  far  as  the  divisions  are  con- 
cerned. It  has  not  been  worked  out  in 
so  many  forms  as  has  spermatogenesis, 
since  the  ovum,  being  loaded  with  yolk 
in  the  lower  forms,  and  being  inaccessible 
in  the  higher  animals,  presents  greater 
technical  difficulties  in  investigation. 
Where  it  has  been  studied  it  has  been 
found  to  correspond  with  sperm  forma- 
tion in  nuclear  detail. 

The  primitive  germ  cells  undergo 
mitotic  divisions  until  they  are  quite 
numerous,  and  then  follows  a  period  of 
growth  at  the  erM  of  which  they  are 
oogonia.  Each  oogonium  divides  to 
produce  the  primary  obcytes,  or  ovarian 
ova,  which  are  provided  with  the  diploid 
number  of  chromosomes  by  a  mitotic 
division .  These  then  grow  until  in  most  forms  they  are  considerably 


ABC 

FIG.  125. — Spermatozoa. 
—From  Kellicott. 


A,  B,  two  views  of  a  human 
sperm  cell. — After  Retzius.  X 
2000.  C,  diagram  of  the  structure 
of  a  generalised  type  of  flagellate 
spermatozoon. — After  Meves. 

a.,  annulus ;  ac,  anterior  cen- 
trosome ;  af,  axial  filament  ;  c., 
cenlrosomes  (end  knobs)  ;  e.,  pro- 
toplasmic envelope  ;  h.,  head  ; 
m.,  middle  piece  ;  mi.,  mitochon- 
dria ;  n.,  nucleus  ;  ne.,  neck  ;  p., 
perforatorium  (acrosome)  ;  PC, 
posterior  centrosome  ;  s.,  spiral 
filament  ;  t.,  tail  piece  ;  tf,  ter- 
minal filament. 


372 


AN   INTRODUCTION  TO  ZOOLOGY 


larger  than  the  spermatocytes,  and,  indeed,  in  animals  with  heavily 
yolked  eggs,  may  become  hundreds  of  times  larger.  They  then 
enter  into  a  mitotic  division ;  the  spireme  concentrates  in 
synizesis,  and  from  the  chromatin  mass  reappear  threads  that 


Spermalogenes- 


Oogenesis 


Sperotitogoma    < 


Primary 
Spermaio'cytes 


Multiplication  of  Epithelial 
Cells  ( Spermat ogonia  and 
Oogonia)  in  [he  Gonad 
(Testis  or  Ovary)  by  ordinary 
Somacic   Mitosis 


Synapsis  or  Pairing   xs! 
of  the  Chromosomes  V' 

1 1 

Reducing  Division       I  \ 
(Meiosis  )  ;  5 


I        r        i       \ 
Male    Gametes 


J4>  Secondary  Oocytts 
(4f,  and  Polar  Bodies 


•^S  Mature  0r* 
&  Polar  Bodies 


&     © 

"Conjugation  of / 

^eniiizatiotf      Femai 

\ of 'Ovum/  y 

(^)  2>y^  OP  Fertilized  Ovuio 


Mature 


Segmentation  of 
the  Zygote  by 
Ordinary  Somatic 
Mitosis 


(The  figures  indicate  the  numbers 
of  Chromosomes  present) 


FIG.  126. — Diagram  of  gametogenesis. — From  Dendy. 

are  double  as  the  result  of  synapsis,  and  they  segment  into 
chromosomes.  These  chromosomes  are  bivalent,  and  so  we  have 
only  the  haploid  number  present.  An  amphiaster  is  formed, 
and  the  chromosomes  become  arranged  upon  the  equatorial  plate, 
but  instead  of  this  taking  place  in  the  middle  of  the  cell,  it  is  right 


HISTOLOGY  AND  CYTOLOGY  373 

out  near  the  periphery.  A  splitting  of  the  chromosomes  occurs, 
and  the  anaphase  is  entered  upon.  With  the  consequent  division 
of  the  cytoplasm  comes  a  marked  difference  from  sperm  formation. 
Instead  of  the  cell  dividing  equally  it  splits  into  one  large  cell,  and 
one  very  small  one  that  contains  little  more  than  a  haploid  group  of 
bivalent  chromosomes.  These  two  daughter  cells  may  be  termed 
the  secondary  ooeytes,  but,  on  account  of  their  striking  difference 
both  in  size  and  fate,  the  large  cell  is  spoken  of  as  the  ovum,  and 
the  small  one  as  the  polar  body.  In  many  animals  no  further 
changes  are  undergone  while  the  egg  remains  in  the  ovary,  and  it  is 
not  until  after  it  is  shed  that  the  next  division  occurs.  This  division 
is  an  ordinary  and  not  a  reducing  one,  and,  as  far  as  the  distribution 
of  the  cytoplasm  is  concerned,  is  similar  to  the  one  preceding  it. 
That  is  to  say,  the  result  is  a  large  cell,  now  termed  the  mature  ovum, 
and  another,  or  second,  polar  body.  The  first  polar  body  may  also 
divide  into  two,  and  although  it  does  not  always  do  so,  it  may  be 
considered  that  it  should  divide  in  typical  and  primitive  cases. 
Thus  from  the  primary  oocyte  there  are  produced  four  cells  which, 
however,  are  not  equal  in  size  and  potentialities,  as  are  the  sperma- 
tids,  but  consist  of  one  large  mature  ovum  capable  of  development  if 
fertilised,  and  three  minute  polar  bodies  which  cannot  develop  and 
have,  as  it  were,  sacrificed  their  cytoplasm  to  produce  one  large  ovum. 
The  relation  between  the  two  processes  of  germ  cell  forma- 
tion, or,  as  they  are  collectively  termed,  the  maturation  o!  the  gametes 
in  the  male  and  female,  can  be  briefly  set  out  in  the  following  way : — 
They  are  alike  in — 

(a)  Their  nuclei  contain  the  haploid  number  of  chromosomes 
the  result  of  a  meiotic  and  one  subsequent  division. 

(b)  Their  chromosomes  are  alike  in  form,  size  and  with  few 
exceptions  in  number. 

(c)  Generally  they  can  develop  only  after  union. 
They  differ  in — 

Spermatozoon.  Ovum. 

(a)  Little  cytoplasm.  Much  cytoplasm. 

(b)  No  food  or  yolk.  Always    contains     yolk    and 

sometimes  very  large  amount. 

(c)  Actively  motile.  Non-motile. 

(d)  Centrosome  present.  Centrosome  absent. 

(e)  One  of  four  similar  cells         One   of   four   dissimilar   cells 
derived  from  the  spermatocyte     derived   from    the    oocyte,    the 
all  being  functional.  other  three  non-functional. 

(f)  Usually   completely    ma-         Usually  formed  in  ovary,  but 
tured  and  formed  in  testis.  not    matured    until    after    dis- 
charged. 


374  AN  INTRODUCTION  TO  ZOOLOGY 

The  ova  of  different  species  vary  in  the  amount  and 
distribution  of  food  material  they  contain,  and  also  in  the  way  in 
which  they  may  be  protected  by  the  development  of  enclosing 
membranes.  While  in  the  ovary  the  egg  is  contained  within  a 
follicle  which  may  fit  closely  round  it,  or  may  enlarge  and  become 
vesicular  as  in  the  Mammal,  when  the  egg  with  a  certain  number  of 
cells  surrounding  it  lie  to  one  side.  While  in  the  ovary  two  enclosing 
egg  membranes  may  arise.  Firstly,  there  is  an  envelope  actually 
secreted  by  the  ovum  itself  ;  this  is  termed  the  primary  egg  mem- 
brane, or  vitelline  membrane,  and  is  usually  very  thin  and  structure- 
less, but  may  be  thicker  and  perforated  by  minute  radial  pores, 
when  it  is  designated  the  zona  radiata.  In  some  cases  it  may  even 
consist  of  an  internal  zona  and  an  external  clear  membrane. 

A  secondary  or  follicular  membrane  may  also  be  developed,  but 
this  is  a  product  of  the  follicular  cells,  and  not  of  the  ovum  itself. 
In  many  animals  it  is  hard  to  distinguish  how  much  of  the  envelope 
around  the  ovum  in  the  ovary  is  primary  and  how  much  is  secondary. 
Generally,  in  the  Insects  and  Bony  Fishes,  the  follicular  membrane 
is  quite  distinct,  and  termed  the  "  chorion."  Often  these  two 
membranes  are  perforated  at  one  point  by  a  tiny  canal,  the  micropyle, 
which  allows  the  sperms  to  reach  the  ovum. 

When  the  ovum  is  ready  it  escapes  from  the  ovary  by  the  actual 
rupture  of  the  follicle,  a  process  that  is  termed  ovulation.  It  makes 
its  way  to  the  oviducal  funnel,  and  as  it  passes  down  the  oviduct  it 
may  have  deposited  around  it,  as  the  result  of  the  activity  of  the 
oviducal  glands,  one  or  more  tertiary  egg  membranes.  These  may 
consist  of  a  jelly-like  mucilaginous  substance  as  in  Rana,  or  an 
albuminous  layer  as  in  ScylUum  and  the  fowl,  and  in  the  case  of 
these  two  animals,  the  dogfish  has,  in  addition,  a  chitinous  case, 
while  the  fowl  has  a  calcareous  shell.  The  provision  of  any  or  all 
of  these  membranes  and  their  nature  varies  to  meet  the  requirements 
of  the  conditions  under  which  the  egg  is  laid  and  develops. 

Finally,  the  ovum  itself  may  be  provided  with  a  large  or  small 
amount  of  food  material,  the  deutoplasm,  usually  in  the  form  of 
tiny  granules,  the  yolk  spheres.  In  certain  animals,  e.g.  Amphioxus 
and  the  higher  mammals,  this  supply  is  quite  small  and  fairly  evenly 
distributed  throughout  the  cytoplasm  of  the  ovum,  and  such  an  egg 
is  termed  homolecithal.  On  the  other  hand,  and  more  commonly, 
there  is  a  large  amount  of  yolk  present,  and  as  it  is  unevenly  dis- 
tributed the  ova  are  termed  heterolecithal.  In  some  forms,  like 
Insects  and  Crustacea,  it  is  concentrated  towards  the  central  part 
of  the  egg,  leaving  the  peripheral  protoplasm  more  or  less  free,  a 
condition  known  as  centrolecithal.  The  dogfish  and  the  fowl  have 
eggs  with  an  enormous  amount  of  yolk  that  is  concentrated  all  at 


HISTOLOGY  AND  CYTOLOGY  375 

one  side  or  pole,  and  this  condition  is  termed  telolecithal.  Indeed, 
such  a  large  amount  of  yolk  is  present  that  it  results  in  the  active 
cytoplasm  of  the  ovum  being  confined  to  a  small  area,  the  germinal 
disc,  which  contains  the  nucleus  or  germinal  vesicle,  as  it  is  frequently 
termed,  and  is  situated  at  one  point  of  the  surface.  The  question 
of  the  amount  and  distribution  of  the  yolk  is  not  an  important 
matter  from  the  point  of  view  of  the  fundamental  structure  of  the 
egg,  since  it  is  all  non-active  food  material,  and  so  a  question  of 
detail.  On  the  other  hand,  it  is  of  extreme  importance  to  the 
changes  occurring  after  fertilisation,  since  it  determines  to  a  large 
extent  the  way  the  early  development  can  proceed. 

With  this  general  description  of  the  germ  cell  we  can  leave  the 
study  of  Cytology  and  pass  on  to  consider  what  happens  when  the 
germ  cells  unite  in  certain  species  of  Chordata. 


CHAPTER    XV 
EMBRYOLOGY 

Fertilisation,  Segmentation  and  Germ  Layer  formation. 

THE  study  of  Embryology,  or  the  development  of  animals,  may 
well  commence  with  the  act  that  marks  its  inception,  namely, 
fertilisation,  by  which  is  understood  the  fusion  of  the  nucleus  of  a 
male  gamete  or  spermatozoon  with  that  of  a  female  gamete  or  ovum, 
i.e.  the  constitution  of  a  new  individual.  The  ovum,  as  we  have 
seen,  is  a  large  immobile  cell  often  of  great  size,  owing  to  its  contained 
yolk.  It  always  contains  a  nucleus,  termed  the  female  pro-nucleus. 
The  sperm  is  little  more  than  a  male  pro-nucleus  with  an  accompany- 
ing centrosome  and  a  tail,  whereby  it  is  able  to  move  more  freely. 
By  some  means  or  other,  in  those  animals  where  fertilisation  is 
internal  by  an  act  termed  copulation,  the  spermatozoa  are  brought 
into  proximity  with  the  ovum.  As  a  rule,  numerous  sperms  sur- 
round one  ovum,  and  by  means  of  their  perforating  caps,  aided  by 
the  active  movements  of  the  tail,  start  to  bore  their  way  into  the 
egg,  which  sometimes  puts  out  a  small  receptive  process  to  meet 
one  of  them.  Typically  only  one  sperm  penetrates  the  egg,  whose 
membranes  then  appear  to  undergo  a  rapid  physical  or  chemical 
change  that  prohibits  the  entry  of  further  sperms.  If  several 
should  enter  the  ovum,  as  sometimes  happens  in  large  eggs,  only  one 
of  them  is  functionally  active,  and  the  others  simply  degenerate. 
When  the  head  has  successfully  entered  the  ovum,  the  tail  is  shed, 
leaving  only  the  head  and  neck,  which  are,  therefore,  to  be  regarded 
as  the  essential  parts.  Once  inside,  the  head  enlarges  to  form  a 
typical  nucleus  with  a  chromatin  network,  and  while  so  doing  it 
rotates  in  such  a  manner  that  the  centrosome  formed  from  the 
granules  in  the  neck,  is  pointed  in  the  direction  of  the  female  pro- 
nucleus.  An  aster  arises  around  the  centrosome,  and  this,  together 
with  the  male  pro-nucleus,  moves  towards  the  egg  nucleus.  While 
this  is  proceeding  the  chromatin  granules  of  both  pro-nuclei  organise 
to  form  a  typical  spireme,  and  one  of  two  courses  may  be  followed. 
The  more  common  way  is  for  the  centrosome  and  aster  to  divide 
and  the  two  poles  to  move  apart,  forming  a  spindle  between  them. 

376 


EMBRYOLOGY  377 

The  two  nuclei  lose  their  membranes,  and  the  spiremes  segment 
to  form  the  chromosomes,  which  arrange  themselves  on  the  equa- 
torial line  of  the  spindle,  sometimes  even  remaining  in  two  distinct 
groups.  Each  of  the  pro-nuclei  contains  the  haploid  number  of 
chromosomes,  so  that  when  the  two  come  together  in  this  manner 
the  typical  diploid,  or  somatic  number,  is  restored  in  the  combined 
groups.  It  will  be  seen  then,  that  if  there  were  no  reduction  in  the 
course  of  the  production  of  the  gametes,  each  fertilisation  would 
result  in  the  doubling  of  the  chromosome  number,  a  proceeding  that 
could  not  possibly  go  on.  From  this  point  on  the  division  proceeds 
as  in  typical  mitosis.  That  is  to  say,  the  metaphase,  anaphase  and 
telophase  supervene,  resulting  in  the  formation  of  two  cells.  The 
single  ovum  has  thus  been  fertilised  and  has  divided  into  two  by 
one  continuous  process,  and  this  division  is  termed  cleavage  or 
segmentation,  and  the  resulting  cells,  the  first  cleavage  cells  or 
blastomeres.  It  will  be  seen  then,  that  there  is  no  loss  of  chromo- 
some identity  in  the  above  phenomena,  and,  particularly  in  those 
cases  where  the  chromosome  groups  do  not  mix,  it  is  clear  that  each 
daughter  cell  has  its  chromosome  complement  made  up,  half  of 
maternal  and  half  of  paternal  chromosomes.  The  next  division,  or 
second  cleavage,  takes  place  almost  at  once,  and  results  in  the  for- 
mation of  the  first  four  cleavage  cells  or  blastomeres.  In  these,  too, 
it  can  sometimes  be  clearly  seen  that  half  the  chromosomes  are 
descended  from  each  parental  germ  cell,  an  important  point  in 
considering  the  question  of  inheritance. 

The  other  method  of  fertilisation,  while  less  common,  is  in 
some  respects  more  primitive  and  not  so  abbreviated.  An  amphi- 
aster  is  formed  as  before,  but  slightly  to  one  side,  while  the  two 
nuclei  lose  their  membranes,  and  their  chromatin  granules,  or 
spiremes  if  they  have  been  formed,  mix  up  indistinguishably.  This 
results  in  the  formation  of  one  chromatin  reticulum  or  spireme, 
which  may  be  termed  the  cleavage  or  fertilisation  nucleus.  The 
amphiaster,  sometimes  not  formed  until  this  time,  now  takes  up  a 
central  position,  and  the  spireme  segments  to  form  the  diploid 
number  of  chromosomes,  which  become  arranged  to  form  an 
equatorial  plate.  Even  in  this  type  there  is  some  evidence  to  show 
that  the  paternal  and  maternal  chromatin  retains  its  individuality, 
and  the  presumption  is  strong,  that  the  cleavage  cells  receive  an 
equal  share  from  both  male  and  female  parent.  After  the  formation 
of  the  amphiaster  the  blastomeres  are  formed  as  in  ordinary  division . 

In  both  types  of  fertilisation  it  is  clear  that  the  fertilised 
ovum  starts  its  career  with  a  chromatin  content  derived  in  equal 
parts  from  both  parents.  If,  as  we  have  reason  to  believe,  the 
chromosomes  constitute  the  bearers  of  parental  characters,  then  it 


378  AN   INTRODUCTION  TO   ZOOLOGY 

will  be  seen  that,  in  spite  of  the  difference  in  size  between  the  two 
germ  cells,  the  total  inheritance  of  the  new  organism  is  derived 
equally  from  both  male  and  female  parent.  This  too  has  to  be 
borne  in  mind  when  considering  the  phenomena  of  heredity.  The 
sperm,  in  addition,  introduced  some  stimulus,  perhaps  closely  bound 
up  with  the  centrosome,  that  is  required  to  initiate  development. 
It  can  be  shown  that  this  stimulus  is  nothing  to  do  with  the  actual 
presence  of  the  chromosomes  themselves  by  experiments  known  as 
"  artificial  fertilisation/'  The  eggs  of  some  marine  animals  can 
be  treated  by  the  addition  of  certain  soluble  metallic  salts  to  the 
sea-water  in  which  they  are  contained,  and  in  this  way  it  has  been 
found  to  be  possible  to  supply  the  stimulus  necessary  to  cause  some 
eggs  to  segment.  In  certain  cases,  even,  it  has  been  found  possible 
to  produce  advanced  embryos  in  this  way.  But  it  is  necessary  to 
supply  a  stimulus,  apparently  a  mechanical  or  chemical  one,  that  is 
not  ordinarily  present  in  the  environment  of  the  egg  in  order  to 
obtain  this  result,  and  under  normal  conditions  this  stimulus  is 
introduced  by  the  sperm. 

As  has  been  noted  previously,  certain  animals  ordinarily  produce 
eggs  that  undergo  development  without  fertilisation..  This  is 
termed  parthogenesis,  and  so  the  experiments  just  described  are, 
on  the  whole,  more  accurately  termed  artificial  parthenogenesis. 
When  parthenogenesis  occurs  normally  it  is  brought  about  by  an 
entirely  different  means.  It  has  been  observed  that  during  the 
production  of  the  second  polar  body  such  eggs  behave  differently 
from  those  of  other  animals,  and  either  the  second  polar  body  is  not 
formed  at  all  or,  if  it  is,  then,  before  its  cytoplasm  separates  from  the 
parent  mass,  its  nucleus  returns  and  reunites  with  that  of  the  ovum. 
So  that  in  these  cases  the  second  polar  body,  as  it  were,  takes  on  the 
function  of  the  sperm  with  the  twofold  result  that  the  diploid  number 
of  chromosomes  is  restored  and  segmentation  commences. 

Amphioxus. 

In  order  to  get  a  clear  idea  of  the  main  features  of  the 
embryology  of  the  CHORDATA  it  is  necessary  that  they  should  be 
considered  briefly  in  several  types,  and  we  take  first  of  all  Amphioxus 
lanceolatus,  where  certain  of  them  are  presented  in  a  simple  con- 
dition. Amphioxus  itself  is  a  small  somewhat  fish-shaped  form, 
growing  to  a  length  of  about  two  inches.  It  is  found  at  certain  spots 
on  the  British  coast,  but  more  commonly  in  the  Mediterranean,  and 
allied  species  are  found  in  the  shallow  seas  of  many  parts  of  the  world. 
It  is  almost  transparent,  and,  although  spending  most  of  its  time 
partly  embedded  in  the  sand,  with  only  its  mouth  and  anterior  end 
protruding,  it  can  swim  and  burrow  into  the  sand  very  rapidly. 


EMBRYOLOGY  379 

Compared  with  such  an  animal  as  Scyllium,  for  example,  it  is  very 
simple  in  structure.  It  has  no  cranium  nor  sense  organs,  nor  is  its 
brain  nearly  so  well  developed.  No  cartilaginous  skeleton  is  present, 
and  even  though  it  has  a  mouth,  this  is  a  circular  orifice  not  provided 
with  any  structures  that  can  be  looked  upon  as  jaws.  No  sign  of 
paired  fins  is  present,  and  all  its  systems  are  much  less  complicated 
with  the  one  exception  of  the  gill  clefts,  which  are  very  numerous. 
The  actual  structure  of  the  adult,  however,  does  not  concern  us  here, 
and  it  is  sufficient  to  note  that  while  it  is  undoubtedly  highly 
specialised  along  certain  lines,  it  is  a  representative  of  one  of  the  most 
primitive  groups  of  Chordates  alive  to-day,  and  so  of  considerable 
interest  in  comparative  anatomy.  Its  embryological  changes  are 
also  specialised,  but,  nevertheless,  they  illustrate  certain  funda- 


jnf 


FIG.  127. — Amphioxus,  general  view. — From  Bourne. 

A,  an.,  anus  ;  aip.,  atriopore  ;  c.,  caudal  fin  ;  ci,  buccal  cirrhi ;  df,  dorsal  fin  ;  e.,  eyespot ; 
fr,  fin-rays  ;  g'.,  g26.,  the  twenty-six  pairs  of  gonadial  pouches  ;  m'.,  the  first,  m36.,  the  thirty- 
sixth,  w52.,  the  fifty-second  myotomes  ;  n.,  neural  tube  ;  nch.,  notochord  ;  vel.,  velum,  in  front  of 
it  are  the  finger-like  processes  of  the  wheel  organ  ;  ves.,  vestibule  ;  «./.,  ventral  fin. 

mental  points  in  a  very  clear  manner,  and  it  is  for  that  reason  we 
treat  of  it  here. 

The  fertilised  ovum  of  Amphioxus  is  a  minute  spherical 
body  about  *io  mm.  in  diameter.  While  the  yolk  that  it  contains 
is  more  abundant  towards  one  pole,  and  so,  strictly  speaking,  it  is 
telolecithal ;  yet  there  is  so  little  present,  and  it  is  not  entirely 
confined  to  one  end,  that  it  is  often  considered  as  homolecithal. 
Shortly  after  fertilisation  the  second  polar  body  is  extruded  at  what 
is  termed  the  animal' pole  of  the  egg,  and  this  is  followed  later  by 
the  first  cleavage.  The  first  division  is  holoblastic,  that  is  to  say,  it 
passes  completely  through  the  ovum,  and  it  takes  place  in  a  plane 
passing  through  the  middle  of  the  two  poles.  Shortly  after,  the 
second  cleavage  takes  place,  at  right  angles  to  the  first,  but  still 
through  the  middle  of  the  poles.  Thus,  after  these  two  meridional 
divisions,  the  egg  comes  to  consist  of  four  equi-sized  cells,  each  being 
half  in  the  animal  and  half  in  the  vegetative  pole.  The  third 
division  takes  place  at  right  angles  to  the  other  two,  and  is  often 
termed  equatorial,  although,  as  a  matter  of  fact,  it  is  nearer  one  pole 


380 


AN   INTRODUCTION  TO  ZOOLOGY 


than  the  other,  and  cuts  off  four  smaller  cells  at  -the  animal  pole 
from  four  larger  ones  at  the  vegetative  pole.     The  eight  cells  are 


XII 


FIG.  128.' — A mphioxus,  early  cleavage.     I.— VII.,  and  XII.,  adapted  from 
Ziegler,  after  Hatchek.     VII.-XL,  adapted  from  Morgan. 

I.,  ovum  unsegmented  ;  II.-VI.,  first  five  cleavages  ;  VI.  and  VII.,  blastula  stage  ;  VIIL,  sec- 
tion of  blastula  just  before  gastrulation  ;  IX.  and  X.,  sections  of  successive  stages  in  gastrulation  ; 
XI.,  section  of  fully  formed  gastrula  ;  XII.,  median  view  of  half  a  late  gastrula  with  neurenteric 
canal  formed. 

A.,  archenteron  ;  B.,  blastoccel ;  B.P.,blastopore  ;  E.,  ectoderm;  En., entoderm  ;  N.E.,  neur- 
enteric canal ;  N.P.,  neural  plate  ;  P.B.,  polar  body. 

turned  into  sixteen  by  the  simultaneous  formation  of  two  meridional 
cleavages,  and  then  into  thirty-two  by  two  latitudinal  divisions. 


EMBRYOLOGY  381 

Very  frequently  certain  irregularities  in  division  have  appeared  by 
this  time,  and  after  it  segmentation  is  not  so  regular,  the  smaller 
cells  at  the  animal  end  divide  more  rapidly  than  the  larger  yolk- 
laden  ones  at  the  other  pole.  While  these  divisions  have  been  in 
progress  a  cavity  has  appeared  in  the  centre  of  the  cells,  and  so  very 
soon  we  have  produced  a  characteristic  hollow  sphere,  the  blastula, 
composed  of  a  single  layer  of  cells  surrounding  a  central  space,  the 
blastoccel  or  segmentation  cavity.  This  is  a  noteworthy  stage,  and 
recalls  the  condition  in  certain  forms  of  colonial  Protozoa,  e.g.  Volvox. 
The  cells  at  the  upper  or  animal  pole  are  smaller  and  contain  less 
yolk  than  those  at  the  lower  or  vegetal  pole. 

The  next  step  is  an  important  one,  since  it  consists  in  the  trans- 
formation of  this  embryo,  as  a  developing  egg  is  termed,  with  but  a 
single  layer  of  cells  into  one  with  two  layers.  This  process  we  term 
gastrulation,  and  it  occurs  in  the  following  way.  The  lower  pole 
becomes  flattened,  and  then  slowly  this  flattened  area  folds  in  and 
finally,  with  the  obliteration  of  the  segmentation  cavity,  lies  next 
to  the  cells  of  the  other  pole.  The  result  is  a  hollow  cup-shaped 
embryo,  the  gastrula,  composed  of  two  layers  of  cells,  an  outer 
covering  of  small  cells  derived  from  the  animal  pole,  and  an  inner 
lining  of  larger  vegetative  cells.  In  this  way  we  have  formed  an 
internal  cavity,  the  primitive  gut  or  archenteron,  which  opens  to  the 
exterior  by  a  wide  circular  aperture,  the  blastopore.  The  cells  lining 
this  cavity  are  termed  the  entoderm  (or  in  older  terminology 
hypoblast),  and  the  external  layer  of  cells  the  ectoderm  (or  epiblast). 
Thus  the  process  of  gastrulation,  resulting  in  the  laying  down  of  the 
two  primary  germ  layers,  as  they  are  termed,  has  led  to  the  formation 
of  a  diploblastic  embryo  which,  in  the  possession  of  these  layers, 
of  a  single  internal  cavity  and  of  a  single  external  opening,  recalls  in 
its  essentials  the  condition  permanent  in  an  adult  Ccelenterate  like 
Hydra.  This  likeness  is  made  more  obvious  when  the  embryo 
lengthens  and  the  blastopore  becomes  considerably  reduced. 

While  this  lengthening  is  in  progress,  a  strip  of  ectodermal 
cells  along  the  middle  of  the  flatter  dorsal  surface  becomes  delimited 
from  the  surrounding  cells  and  sinks  slightly  below  the  level  of  the 
remaining  ectoderm.  This  constitutes  the  neural  or  medullary 
plate,  and  it  extends  from  the  upper  lip  of  the  blastopore  forward 
almost  to  the  anterior  end.  The  cells  bordering  on  this  strip  become 
slightly  raised  up  to  form  the  neural  folds  or  ridges,  and  these 
gradually  grow  together  so  as  to  roof  over  the  neural  plate,  save  for 
a  small  circular  area,  the  neural  pore,  at  the  front  end.  The  process 
of  enclosing  the  plate  at  the  hinder  end  is  also  aided  by  the  upward 
growth  of  the  lower  lip  of  the  blastopore,  so  that  this  aperture 
becomes  closed.  While  the  general  ectoderm  has  in  this  way  been 


382 


AN   INTRODUCTION  TO  ZOOLOGY 


growing  over  it,  the  neural  plate  has  become  curved  downwards 
in  the  middle,  thus  leaving  a  small  space  under  the  superficial 
ectoderm.  Owing  to  the  way  in  which  the  lower  lip  of  the  blastopore 
grows  upwards  it  closes  the  opening  of  the  archenteron  to  the  outside, 
but  leaves  a  small  aperture,  the  neurenteric  canal,  which  puts  the 
archenteric  cavity  in  communication  with  the  space  above  the 
neural  plate.  Eventually  this  plate  becomes  more  and  more  folded 
until,  finally,  its  two  edges  meet  and  fuse  in  the  mid-dorsal  line  and 
so  forms  the  neural  tube,  which  is  the  forerunner  of  the  central 
nervous  system.  It  will  be  seen  then  that  right  from  the  very 
beginning  the  central  nervous  system  is  hollow,  dorsal  in  position, 
and  represents  a  specialised  insinking  of  part  of  the  original  ectoderm, 


jnes. 


net 


FIG.  129. — Amphioxus,  later  cleavage. — From  Bourne,  after  Hatschek. 

N.,  sagittal  section  of  an  embryo  with  three  mesoblastic  somites  ;  O.,  horizontal  section  of  the 
same  embryo  ;  P.,  sagittal  section  of  an  older  embryo  ;  Q.,  sagittal  section  of  an  embryo  with  nine 
pairs  of  mesoblastic  somites  ;  R.,  horizontal  section  of  an  embryo  of  the  same  age  showing  the 
origin  of  the  head-cavities  ;  ach.,  archenteron  ;  mes.,  mesoblastic  somites ;  n.c.,  neural  canal  ; 
nee.,  neurenteric  canal;  np.,  neuropore ;  I.-IX.,  the  several  pairs  of  mesoblastic  somites;  nch., 
notochord. 

all  important  points.     The  neuropore  and  neurenteric  canal  remain 
open  for  some  time,  but  finally  both  close. 

While  the  formation  of  the  neural  tube  has  been  taking 
place  a  series  of  changes  have  occurred  in  the  archenteron.  At  an 
early  stage  it  can  be  seen  that  the  cells  of  the  dorsal  portion  of  the 
entoderm  are  smaller  than  those  on  the  sides  and  floor  of  the  cavity, 
and  these  dorsal  cells  become  arranged  in  three  longitudinal  grooves, 
of  which  the  median  is  first  to  appear.  By  the  time  the  neural  plate 
is  roofed  with  ectoderm  these  grooves  have  become  well  marked, 
and  in  transverse  section  present  the  appearance  of  three  pocket- 
like  outbulgings  from  the  archenteric  cavity.  The  median  of  them 
is  quite  a  narrow  groove.  During  further  growth  its  sides  approxi- 
mate and  their  cells  interlock,  and,  finally,  about  the  same  time  as 


EMBRYOLOGY  383 

the  neural  tube  is  formed,  it  becomes  cut  off  from  the  dorsal  wall 
of  the  archenteron  as  a  solid  rod  of  cells.  This  is  the  notochord  or 
chorda  dorsalis,  and  although  when  first  formed  it  does  not  extend 
to  the  anterior  end  of  the  body  it  does  so  later.  Its  cells  undergo  a 
considerable  amount  of  modification,  becoming  highly  vacuolated 
and  producing  the  characteristic  notochordal  tissue.  The  noto- 
chord, then,  arises  from  the  mid-dorsal  wall  of  the  archenteron,  and 
it  is  looked  upon  as  a  structure  of  such  importance  that  the  posses- 
sion of  a  supporting  rod  of  tissue  derived  in  this  way,  and  exhibiting 
its  striking  structural  characteristics,  is  regarded  as  being  one  of  the 
fundamental  attributes  of  the  Phylum  CHORDATA,  marking  it  off 
from  all  other  Phyla. 

While  the  neural  tube  and  notochord  have  been  forming,  the 
front  part  of  the  lateral  grooves  has  given  rise  to  a  series  of  hollow 
sacs,  the  mesodermal  somites.  These  arise  by  the  grooves  closing 
off  from  the  gut  on  each  side  in  pairs  of  diverticula,  one  behind  the 
other.  This  is  the  first  indication  of  metameric  segmentation, 
another  fundamental  characteristic  of  the  Chordates.  These  out- 
growths, as  we  have  seen,  are  hollow  from  the  beginning,  and  their 
cavities  represent  the  ccelom,  which  since  it  has  been  derived  directly 
from  the  gut  in  this  particular  type,  is  termed  an  enterocoel.  A 
number  of  such  pairs  of  somites  are  formed,  but  with  their  more 
rapid  development  we  find  that  the  posterior  ones  arise  as  solid 
blocks  of  cells  in  which  the  cavity  is  formed  later.  The  first  of  the 
pouches  to  arise  in  the  manner  just  described  is  really  the  third 
pair  of  somites,  there  being  two  other  pairs  situated  more  anteriorly. 
The  first  two  pairs  of  somites  arise  in  a  somewhat  similar  manner, 
but  as  four  independent  sacs  growing  out  from  the  anterior  end  of 
the  dorsal  archenteric  wall  in  front  of  and  not  connected  with  the 
lateral  grooves.  When  the  enteroccelic  pouches  and  the  notochord 
have  been  cut  off  from  the  archenteron,  the  cavity  left  surrounded  by 
entoderm  can  be  spoken  of  as  the  definitive  gut»  the  enteron  or 
mesenteron. 

Turning  now  to  the  enteroccelic  pouches  we  see  from  the  manner 
of  their  formation  that  they  come  to  constitute  a  group  of  cells 
interposed  between  the  ectoderm  and  entoderm,  and  so  they  are 
termed  the  middle  layer  or  mesoderm  (or  mesoblast).  At  the  front 
end  the  mesodermal  somites  of  the  two  sides  are  opposite,  but 
further  back  they  come  to  be  alternate.  The  pouches,  as  soon  as 
they  are  formed,  begin  to  enlarge  by  extending  in  a  ventral  direction 
between  the  entoderm  and  ectoderm,  and  their  walls  differentiate 
into  three  regions.  The  outer  portion  adjoining  the  ectoderm 
becomes  thin  and  is  known  as  the  parietal  or  somatic  mesoderm. 
The  cells  surrounding  the  gut  wall  likewise  thin  out  to  form  the 


AN   INTRODUCTION  TO  ZOOLOGY 


visceral  or  splanchnic  mesoderm.  The  cells  of  the  remaining  part, 
namely,  that  bordering  on  the  notochord  and  neural  canal,  enlarge 
in  the  horizontal  plane  to  form  a  comparatively  thick  plate  to  which 
the  term  myotome  is  applied.  It  is  sometimes  convenient  to 
differentiate  between  the  myotome  and  the  other  two  portions,  i.e. 
the  splanchnic  and  somatic  mesoderm,  and  so  for  these  we  use  the 
inclusive  term  lateral  plate.  The  enlargement  of  the  myotome 
reduces  the  ccelomic  cavity  in  its  vicinity  to  a  small  cleft,  the 


nt. 


FIG.  130. — Amphioxus,  transverse  sections. — From  Bourne, 
after  Hatschek. 

A,  B,  C,  transverse  sections  of  embryos  of  Amphioxus  of  different  ages,  illustrating  the  forma- 
tion of  the  neural  tube,  notochord,  and  mesoblastic  somites,  ack.,  archenteron ;  mes.,  meso- 
blastic  somites;  nch.,  notochord;  np.,  neural  plate;  nt.,  neural  tube.  D,  transverse  section 
through  an  older  embryo  in  which  the  mesoblastic  somites  are  completely  separated  from  the 
archenteron.  mp.,  muscle  plate  ;  cce.,  ccelom  ;  en.,  mesenteron  ;  the  other  lettering  as  above. 
E,  diagrammatic  transverse  section  through  a  larva  with  five  gill-slits,  myc.,  myoccele  ;  spl., 
splanchnocoale.  F,  diagrammatic  transverse  section  of  a  young  Amphioxus  shortly  after  the 
metamorphosis  ;  the  section  is  taken  between  the  atriopore  and  anus.  ao.,  aorta ;  df.,  dorsal 
fin;  scl.,  sclerotome  ;  vf.,  ventral  fin  ;  the  other  lettering  as  above. 

myoecel.  On  the  other  hand  the  remaining  part  of  the  ccelom,  now 
known  as  the  splanchnoccel,  enlarges,  and,  finally,  the  partitions 
between  the  successive  splanchnoccels  break  down,  giving  rise  to 
the  one  large  ccelom  of  the  adult.  While  the  union  of  the  ventrally 
situated  portions  of  the  enteroccelic  cavities  is  taking  place,  a  series 
of  partitions  arise  that  shut  off  the  myoccels  which  do  not  run 
together 


EMBRYOLOGY  385 

We  have  thus  reached  a  stage  of  considerable  importance 
in  which  the  three  primary  germ  layers  have  been  established  :  an 
ectoderm  on  the  outside,  which  also  gave  rise  to  the  neural  tube  ; 
an  entoderm  on  the  inside  destined  to  produce  the  whole  of  the  gut, 
including  the  pharynx  and  a  mesoderm,  which  from  its  commence- 
ment included  a  hollow,  the  coelom,  and  is  differentiated  into 
myotome,  somatic  and  splanchnic  portions.  The  further  history 
of  the  embryo  of  Amphioxus,  while  interesting,  is  fairly  highly 
specialised,  and  relates  to  the  formation  of  the  various  structures 
characteristic  of  the  adult  in  particular,  and  not  so  much  to 
Chordates  in  general.  The  development  so  far  may  be  considered 
as  that  of  a  primitive  Chordate,  whose  egg  contains  but  little  yolk, 
and  whose  early  stages  are  not  complicated  by  the  special  require- 
ments of  the  young  embryo,  and  we  may  now  pass  on  to  see  how  a 
similar  stage  is  reached  in  other  forms. 


Rana. 

The  eggs  of  the  frog  when  laid  are  spherical  bodies  about 
2-3  mm.  in  diameter  and  surrounded  by  a  thin  coat  of  albuminous 
matter  composed  of  the  tertiary  egg  membranes.  As  it  floats  in 
the  water  it  will  be  seen  to  consist  of  an  upper  or  animal  hemisphere, 
dense  black  in  colour  owing  to  the  presence  of  pigment  in  it,  and  a 
lower  or  vegetative  hemisphere,  somewhat  larger  and  white  or  greyish 
white  in  colour.  The  egg  is  provided  with  a  plentiful  supply 
of  yolk  and  is  telolecithal,  that  is  to  say,  the  yolk  is  concentrated 
towards  the  vegetative  pole.  At  the  time  the  egg  is  laid  its  nucleus, 
situated  towards  the  top  of  the  animal  pole,  is  arrested  in  mitosis 
preparatory  to  giving  off  the  second  polar  body  and  so  becoming 
mature.  The  sperms  are  shed  in  the  water  where  fertilisation  occurs 
and  the  entrance  of  the  spermatozoon  apparently  supplies  the 
stimulus  leading  to  the  extrusion  of  the  second  polar  body.  After 
the  egg  has  been  in  water  a  short  time  the  albuminous  envelopes,  of 
which  three  can  be  recognised,  absorb  water  and  swell  up  until  they 
reach  the  size  of  a  small  pea.  During  the  processes  of  formation,  and 
particularly  -during  the  fertilisation  and  maturation  of  the  egg,  its 
contents  are  so  organised  that  the  ovum  as  a  whole  is  not  merely 
divided  into  poles,  but  has  a  definite  bilateral  symmetry  which 
determines- the  direction  of  subsequent  divisions. 

The  first  cleavage  is  holoblastic,.  completely  dividing  the  ovum 
into  two  cells,  and  it  is  finished  about  two  and  a  half  hours  after 
fertilisation.  The  segmentation  is  indicated  externally  by  the 
appearance  of  a  furrow  starting  in  the  animal  pole  and  then  running 
completely  around  the  egg,  but  the  two  cells  remain  close  together, 

2  C 


386 


AN   INTRODUCTION   TO   ZOOLOGY 


each  being  hemispherical  and  do  not  separate  as  much  as  in  Amphi- 
oxus.  The  second  cleavage,  also  holoblastic,  takes  place  in  a  plane 
passing  through  the  centres  of  the  animal  and  vegetal  poles  in  a 
plane  at  right  angles  to  the  first  and  results  in  the  formation  of 
four  blast omeres.  The  third  division,  as  in  Amphioxus,  is  in  a  plane 
at  right  angles  to  the  other  two,  but  although  complete,  is  well  above 
the  equator  and  cuts  off  four  smaller  cells  at  the  animal  pole  from 


ttc. 


yli.c. 


FIG.  131. — Rana,  early  stages  segmentation. — From  Dendy. 


/.,  the  fertilized  ovum  ;  II.-V.,  segmentation  of  the  ovum  ;  VI.,  blastula  ;  VII.,  modified 
blastula,  with  wall  composed  of  more  than  one  layer  of  cells  ;  VIII.,  commencement  of  gastru- 
lation  ;  IX.,  the  modified  gastrula  stage  (VI. -IX.  in  vertical  section),  blc.,  blastoccel  or  segmenta- 
tion cavity  ;  bp.,  blastopore  ;  ent.,  enteron  ;  ep.,  epiblast ;  hyp.,  hypoblast ;  u.l.bp.,  upper  lip 
of  blastopore  ;  yk.c.,  yolk-containing  cells. 


four  much  larger  ones  at  the  vegetative  pole.  The  fourth  cleavage 
is  initiated  by  the  appearance  of  a  pair  of  furrows  bisecting  the 
apical  angles  of  the  cells  of  the  animal  pole  and  it  extends  more 
slowly  down  over  the  four  vegetal  cells,  but  finally  produces  sixteen 
cells.  Somewhere  about  this  stage  the  segmentation  proceeds  less 
regularly  and  takes  place  more  rapidly  at  the  animal  pole,  as  if  it 
were  hindered  at  the  vegetative  pole  by  the  presence  of  too  much 


EMBRYOLOGY  387 

inert  yolk  material.    The  result  is  that  the  number  of  small  dark 
cells  is  greater  than  that  of  the  light-coloured  cells. 

When  the  eight-celled  stage  is  reached,  each  cell  rounds  off 
internally  so  as  to  leave  a  small  internal  space,  the  beginning  of  the 
blastoccel  or  segmentation  cavity,  which  right  from  its  commence- 
ment is  eccentric  in  position  lying  nearer  the  animal  pole.  This 
increases  in  size  arid  is  quite  well  marked  when  there  are  from  32-64 
cells  present,  and  so  the  embryo  as  a  whole  is  now  a  blastula  whose 
wall  is  much  thinner  at  the  animal  pole  than  elsewhere.  Even 
by  the  time  the  sixty-four-celled  stage  has  been  reached,  we  find  a 
departure  from  the  procedure  found  in  Amphioxus,  for  some  of  the 
cells  have  divided  in  a  plane  tangential  to  the  surface  in  such  a  manner 
that  one  of  the  daughter  cells  comes  to  lie  on  the  inside.  Thus  it 
is  that  all  divisions  are  not  marked  on  the  outside  and  the  wall  of 
the  blastula,  instead  of  consisting  of  a  single  layer  of  cells,  comes  to 
consist  of  several  layers.  The  fully  formed  blastula  is  spherical 
and  slightly  larger  than  the  ovum  and  the  symmetry  indicated  in 
the  egg  has  become  more  obvious.  The  anterior  wall  of  the  segmen- 
tation cavity  is  a  little  thicker  than  the  hinder  while  the  pigmented 
cells  extend  a  little  further  down  on  the  posterior  side.  Also  the 
small  cells  at  the  animal  pole  have  become  differentiated  into  an 
outer  compact  superficial  or  epidermal  layer  and  an  underlying 
more  irregular  and  looser  "nervous  layer."  The  external  area  of 
pigmentation  has  also  spread  out  so  that  it  covers  more  of  the 
outside  of  the  blastula  than  it  did  of  the  ovum. 

The  process  of  forming  a  double  layered  embryo  or  gastrula 
is  very  different  from  that  in  Amphioxus.  The  first  indication  of 
the  change  is  the  appearance  of  a  slight  irregular  horizontal  groove 
on  the  posterior  side  of  the  blastula.  This  soon  assumes  a  crescentic 
shape  and  becomes  more  strongly  marked  as  the  cells  on  its  upper  or 
convex  border  are  deeply  pigmented  while  those  on  the  lower  edge 
are  white.  A  section  through  the  embryo  at  this  stage  reveals  the 
meaning  of  the  crescent.  It  is  due  to  a  double  process  ;  in  the 
first  place  the  cells  of  the  animal  pole  are  actually  growing  downwards 
at  the  point  to  enclose  the  vegetative  cells  and  these  in  their  turn 
are  arching  up  into  the  animal  hemisphere.  The  edge  of  the  crescent 
extends  sideways  further  and  further  over  the  vegetative  cells  until 
finally  its  edges  meet  and  it  forms  a  slightly  oval  area.  This  is  the 
blastopore,  and  owing  to  its  position  with  regard  to  the  orientation 
of  the  embryo  as  a  whole,  we  are  able  to  see  now  that  the  original 
point  at  which  it  started  was  the  dorsal  lip,  and  so  the  ventral  lip 
is  the  last  to  be  formed.  The  blastopore  when  first  completed  lies 
on  the  equator  at  the  posterior  end,  but  as  development  proceeds 
it  becomes  smaller  and  smaller,  and  the  centre  of  gravity  of  the  embryo 


388 


AN   INTRODUCTION  TO  ZOOLOGY 


shifts  slightly,  bringing  about  a  rotation,  so  that  the  blastopore 
comes  to  lie  well  above  the  equator.  It  is  visible  for  some  time  and 
is  filled  with  a  small  group  of  the  white  vegetative  cells,  which  is 
termed  the  yolk  plug.  While  these  external  changes  have  been 
taking  place  no  less  important  ones  have  occurred  internally.  The 


FIG.  132. — Rana,  sections. — From  Marshall. 

The  four  small  figures  represent  the  segmenting  egg,  seen  from  its  lower  pole,  and  illustrate 
the  formation  and  shifting  of  the  blastopore  (BP.).  The  four  large  figures  are  sections  of  the  egg 
taken  vertically  through  its  horizontal  axis  Y.Z.,  and  represent  four  stages  in  the  formation  of  the 
mesenteron.  Notice  the  rotation  of  the  egg  through  more  than  go". 

D.L.,  dorsal  lip  of  blastopore  ;  I.C.,  intermediate  cells  ;  MN.,  mesenteron  ;  SC.,  segmentation 
cavity  or  blastoccel ;  V.L.,  ventral  lip  of  blastopore  ;  WX.,  axis,  of  embryo,  which  is  at  first  vertical 
and  subsequently  antero-posterior ;  YZ.,  axis  of  embryo,  which  is  at  first  horizontal  and  subse- 
quently vertical  or  nearly  so. 

vegetative  cells  push  their  way  up  as  a  moderately  thick  layer 
underneath  the  dorsal  lip  of  the  blastopore  which  becomes  turned 
in  and  moves  slightly  with  them.  They  move  round  the  dorsal 
side  of  the  segmentation  cavity,  leaving  a  small  irregular  cleft 
between  themselves  and  the  ectoderm  cells  growing  over  them, 
and  finally  they  reach  the  yolk  cells  of  the  opposite  or  anterior  end. 


EMBRYOLOGY  389 

This  process  is  completed  about  the  time  that  the.  outline  of  the 
blastopore  is  completed,  and  thus  we  have  produced  a  two-layered 
embryo.  The  outside  pigmented  layer  is  the  ectoderm  and  the 
inside  cells  constitute  the  entoderm  while  between  them  is  the  cleft 
representing  the  blastoccel  which  gradually  becomes  obliterated  as 
in  Amphioxus.  On  the  dorsal  side  the  entoderm  cells  are  in  a  layer 
two  or  three  cells  deep,  while  on  the  ventral  side  they  are  much 
larger,  more  irregular  and  many  cells  deep.  A  group  of  these 
larger  cells  forming  the  yolk  plug  practically  fills  the  blastopore. 
These  ventral  cells  are  heavily  laden  with  yolk  and  constitute  a  food 
store  upon  which  the  embryo  relies  for  some  time  to  come.  The 
space  within  the  entoderm  cells  is,  of  course,  the  archenteron,  and  it 
may  communicate  with  the  exterior  by  a  small  cleft  under  the  dorsal 
lip  of  the  blastopore.  Thus  we  see  that  while  the  details  of  the 
process  differ  considerably  in  Amphioxus  and  Rana,  the  result  of 
gastrulation  in  both  cases,  with  certain  modifications  in  the  frog  that 
will  be  considered  immediately,  is  to  produce  an  embryo  with  an 
ectoderm,  an  entoderm,  an  archenteron  and  a  blastopore  :  a  typical 
diploblastic  condition  and  the  differences  are  in  the  main  due  to  the 
amount  of  yolk  present  in  the  frog's  egg 

While  we  have  not  yet  taken  notice  of  the  mesoderm,  as 
a  matter  of  fact  it  has  been  formed  for  some  considerable  time,  and 
the  above  description  of  the  gastrula  is  not  strictly  speaking  accurate, 
for  when  the  stage  just  dealt  with  is  reached  a  good  deal  of  meso- 
derm is  already  present.  The  simple  diploblastic  condition  in  Rana 
is  more  theoretical  or  potential  than  actual,  and  the  precocious 
appearance  of  the  mesoderm  prevents  its  realisation  in  anything  like 
a  complete  form. 

If  we  examine  closely  a  section  through  the  embryo  when  gastru- 
lation is  commencing  we  shall  find  at  the  dorsal  lip  of  the  blastopore, 
where  the  ectoderm  and  entoderm  cells  are  in  contact,  another 
small  group  of  cells  that  are  destined  to  form  the  mesoderm.  They 
are  right  in  the  bend  of  the  lip  between  the  ecto-  and  entoderm, 
with  both  of  which  layers  they  are  continuous.  As  the  blastopore 
lip  extends  laterally,  these  cells  spread  with  it,  so  that  when  the 
margins  join  up  the  primitive  mesoderm  cells  also  unite  and  thus 
come  to  constitute  a  mesoderm  ring  within  the  lip  of  the  blastopore. 
The  dorsal  lip  commences  fairly  high  up,  and  as  it  extends,  it  first 
moves  down  ventrally  and  daring  this  period  it  leaves  behind  it  a 
band  of  mesoderm,  so  that  the  ring  is  much  deeper  on  the  dorsal 
side  than  on  the  ventral.  Furthermore,  the  dorsal  extension  of  the 
entoderm  carries  the  mesoderm  with  it  so  that  it  soon  becomes 
fairly  extensive.  At  first  more  or  less  closely  connected  with  the 
entoderm,  the  mesoderm  cells  are  gradually  delimited  fairly  sharply 


390 


AN   INTRODUCTION   TO  ZOOLOGY 


from  it,  giving  rise  to  a  more  definite  layer  that  first  appears  in  the 
dorso-lateral  regions.  The  presence  of  the  yolk-laden  cells  in  the 
ventral  half  of  the  embryo  delays  matters  somewhat,  but  cells  are 
given  off  by  the  dorsal  mesoderm  which  pass  further  and  further 
ventrally  and  form  a  more  or  less  continuous  layer  of  mesoderm 
that  separates  the  yolk-laden  entoderm  from  the  ectoderm.  In  the 
actual  mid-dorsal  line,  particularly  near  and  in  front  of  the  blast  opore, 
the  mesodermal  cells  are  slightly  differently  related  to  the  other 
layers  and  the  formation  of  the  notochord  leads  to  certain  modifi- 
cations. 

Along  the  axial  line  the  rudiments  of  the  notochord,  the 
mesoderm,  the  dorsal  entoderm,  and  to  a  certain  extent  also  the 

ectoderm,  are  not 
clearly  separable  and 
form  an  axial  mass, 
while  laterally  to  that 
the  mesoderm  is  sepa- 
rated from  both  ento- 
and  ectoderm.  As  the 
blastopore  closes  the 
slit  between  ecto-  and 
mesoderm  on  each  side 
passes  inwards  towards 
the  middle  line,  but 
before  reaching  it  turns 
downwards  and  stops. 
This  leaves  a  thickened 
layer  of  ectoderm  (the 
neural  plate,  vide  in- 
fra), superficially  and 
beneath  it  a  sort  of 

wedge-shaped  vertical  mass  of  cells,  the  notochord  rudiment  now 
cut  off  from  the  lateral  mesoderm.  The  cleft  separating  ento- 
and  mesoderm  also  passes  inwards,  but  stops  short  of  the  middle 
line  below  and  slightly  lateral  to  the  other  slit,  thus  leaving  the 
entoderm  cells  continuous  right  in  the  middle  line  with  the 
notochordal  cells  and  just  lateral  to  this  continuous  with  the 
lower  portion  of  the  lateral  mesoderm  mass,  by  this  time  fairly 
thick.  Along  the  regions  where  the  dorsal  entoderm  is  con- 
tinuous with  the  mesoderm  in  this  way,  shallow  but  nevertheless 
distinct  grooves  appear  which  are  taken  to  represent  enterocoelic 
invaginations  homologous  with  those  in  Amphioxus.  At  a  later 
stage  the  appearance  of  a  slit  cuts  the  notochord  off  from  the  ento- 
derm leaving  only  a  single  layer  of  its  cells  to  roof  the  enteron,  and  a 


FIG.  133. — Ranafusca,  transverse  section  through 
young  embryo. — -After  O.  Hertwig. 

A.,  archenteron  ;  E.,  ectoderm  ;  E.G.,  traces  of  enterocosls  ; 
En.,  entoderm  ;  M.,  mesoderm  ;  N.,  notochord  ;  N.P.,  neural 
plate  ;  Y.,  yolk  cells. 


EMBRYOLOGY  391 

similar  cleft  cuts  it  off  from  the  ectoderm.     In  this  way  the  noto- 
chord  is  formed. 

We  must  now  go  back  again  to  the  stage  when  the  blasto- 
pore  is  just  completed,  to  consider  the  formation  of  another  important 
structure.  Here  we  find  that  the  "  nervous  "  layer  of  the  ectoderm 
has  thickened  to  form  a  moderately  wide  medullary  plate  which 
extends  forwards  irom  the  dorsal  lip  of  the  blastopore  to  the  front 
end.  As  the  tyastopore  closes  and  the  yolk  plug  is  withdrawn,  the 
lateral  margins  of  this  plate  are  elevated  slightly  to  form  the  lateral 
neural  folds  which  run  from  the  sides  of  the  blastopore  to  the  anterior 
end,  becoming  more  marked  as  they  do  so.  At  the  front  they 
turn  inwards  and  unite  in  the  middle  line,  so  forming  a  transverse 
neural  fold  marking  the  front  limitation  of  the  medullary  plate.  The 
area  contained  within  the  folds,  termed  the  neural  plate,  is  somewhat 
thinner  than  the  margins  and  a  groove  appears  along  its  middle 
line,  the  neural  groove.  When  this  has  been  laid  down  a  cleft  appears 
beneath  the  plate  and  cuts  it  off  completely  from  the  underlying 
notochord  as  noted  above. 

Although  the  embryo  is  now  in  a  triploblastic  condition 
there  is  as  yet  no  ccelom,  and  it  will  be  as  well  to  follow  the  develop- 
mental processes  a  little  further  so  as  to  reach  approximately  the 
same  stage  in  which  we  left  Amphioxus. 

The  formation  of  the  central  nervous  system  is  indicated,  as  we 
have  seen,  by  the  formation  of  the  medullary  plate,  neural  folds  and 
neural  groove.  As  development  proceeds  the  groove  becomes 
deeper  and  narrower  and  the  neural  folds  pass  towards  the  middle 
line.  Finally  they  bend  over  and  meet,  but  not  simultaneously  along 
their  whole  extent.  They  first  come  into  contact  at  a  place  that  will 
form  the  myelencephalon,  so  that  from  here  onwards  the  differentia- 
tion into  brain  and  spinal  cord  regions  can  be  recognised.  From 
this  first  point  the  fusion  extends  in  both  directions,  but  more 
rapidly  towards  the  posterior  end.  As  the  union  of  the  folds  passes 
slowly  forwards  the  transverse  folds  also  meet,  their  fusion  extending 
backwards  until  it  reaches  the  fusion  of  the  lateral  folds  at  a  point 
between  the  future  fore  and  mid  brain.  This  is,  therefore,  the  last 
region  to  fuse  and  so  can  be  considered  as  the  neuropore,  but  it  has 
only  a  transitory  existence  and  soon  closes.  From  the  manner  in 
which  the  lateral  neural  folds  come  together,  the  actual  union  takes 
place  along  the  edges  of  the  neural  plate,  so  that  only  this  structure 
takes  part  in  the  formation  of  the  neural  canal.  The  neural  folds 
themselves  are  left  outside  the  canal,  between  it  and  the  covering 
ectoderm.  Later  they  become  broken  up  into  groups  of  cells  lying 
along  the  dorso-lateral  region  of  the  central  nervous  system,  thus  con- 
stituting the  neural  crests  which  are  concerned  with  the  development 


392 


AN   INTRODUCTION  TO  ZOOLOGY 


of  the  nerves.  As  the  blastopore  closes  it  becomes  slit -like  and 
then  fuses  in  the  middle  region,  leaving  an  opening  at  each  end. 
The  neural  plate  extends  right  back  to  the  dorsal  lip  of  the  blastopore 
and  as  it  bends  over  and  fuses  to  form  the  neural  tube,  it  gradually 
closes  over  the  posterior  end  of  the  upper  aperture,  but  internally 
leaves  at  this  point  an  open  communication,  the  neurenteric  canal, 
leading  from  the  neural  canal  to  the  enteron  as  in  Amphioxus. 

Where  the  lips  of  the  blastopore  with  their  variou%approximated 


FIG.  134. — Rana,  young  embryos. — From  Kellicott,  after  F.  Ziegler. 

A,  blastopore  nearly  closed  ;  neural  folds  just  indicated.  B,  blastopore  becoming  divided  into 
neurenteric  and  proctodaeal  portions  ;  neural  folds  becoming  elevated.  C,  neurenteric  canal 
forming  ;  neural  folds  closing  together.  D,  neural  folds  in  contact  throughout. 

b.,  blastopore,  containing  yolk  plug  ;  bi,  rudiment  of  neurenteric  canal  (dorsal  part  of  blasto- 
poreX ;  bz,  rudiment  of  proctod;val  pit  (ventral  part  of  blastopore)  ;  ba,  branchial  arches ;  g., 
neural  groove  ;  nf,  neural  folds ;  np,  neural  plate  ;  p.,  proctodaeal  pit ;  x.,  neural  folds  roofing 
the  bkistopore  and  establishing  the  neurenteric  canal. 


layers  fuse,  the  cells  of  the  ecto-,  ento-  and  mesoderm  are  brought 
together  into  one  undifferentiated  mass  known  as  the  primitive 
streak  down  the  middle  of  which  runs  a  groove,  the  primitive  groove 
marking  the  point  of  junction.  The  lower  opening  fuses  internally, 
and  so  leaves  an  external  pit  lined  with  ectoderm.  This  is  the 
proctodceum,  and  internally  an  evagination  of  the  entoderm,  the 
future  rectum,  grows  out  towards  it. 

Practically  while  the  neural  folds  are  closing  the  brain  region 


EMBRYOLOGY  393 

becomes  differentiated  into  three  enlargements,  the  fore,  mid  and 
hind  brain  vesicles  clearly  visible  in  longitudinal  section  at  a  slightly 
later  stage.  Very  shortly  after,  a  striking  flexure  occurs  in  the  mid 
brain  and  as  a  result  the  fore  brain  is  bent  down  at  right  angles  to 
the  rest  of  the  central  nervous  system.  The  notochord  extends 
from  the  posterior  end  forward  to  the  end  of  the  mid  brain  where  it 
stops.  Right  in  front  of  the  fore  brain  a  median  tongue  of  cells, 
the  hypophyseal  ingrowth,  passes  inwards  from  the  ectoderm  and  at 
a  later  stage  a  terminal  group  of  cells  leaves  this  and  passes  inwards 
to  take  up  its  position  in  front  of  the  end  of  the  notochord,  under  the 
floor  of  the  hinder  part  of  the  fore  brain.  This  group  of  cells  is  the 
rudiment  of  the  hypophysis  ;  indications  of  the  olfactory  organs 
and  ears  as  ectodermal  thickenings  by  this  time. 

When  the  neural  tube  has  been  completed  and  the  flexure  of  the 
brain  has  taken  place,  the  embryo  has  elongated  noticeably  in  the 
antero-posterior  direction.  Certain  changes  have  also  occurred  in 
the  enteron  during  this  period  of  growth.  With  the  separation  of 
the  notochord  and  mesoderm  the  whole  of  the  dorsal  region  and  the 
anterior  end  of  the  ventral  portion  is  lined  by  a  single  layer  of  cells. 
A  large  number  of  yolk-laden  cells  still  remain,  forming  a  yolk  mass 
in  the  posterior  half  of  the  ventral  region  of  the  enteric  cavity.  Their 
presence  enables  three  regions  of  the  enteron  to  be  clearly  differen- 
tiated. In  front  of  them  the  enteric  cavity  expands  to  form  the 
fore  gut,  a  wide  deep  cavity  lined  by  a  single  layered  epithelium. 
This  region  will  give  rise  first  to  the  pharynx,  and  later  to  the 
oesophagus  and  stomach  and  at  its  antero-ventral  end  in  the  middle 
line  is  a  short  oral  evagination  that  will  later  be  met  by  an  ectodermal 
invagination,  the  stomodceum,  which  is  to  form  the  mouth  and 
buccal  cavity.  The  hinder  end  of  the  enteron  is  characterised  by 
being  in  communication  with  the  cavity  of  the  central  nervous 
system,  via  the  neurenteric  canal,  and  also  by  an  evagination  that  will 
later  produce  the  rectum.  This  region,  also  lined  by  a  single  layer  of 
cells,  is  designated  the  hind  gut.  Between  them  the  dorsal  wall 
is  single  layered,  but  the  ventral  wall  many  layered,  owing  to  the 
presence  of  the  yolk  cells  which  also  reduce  the  enteron  to  a  narrow 
dorsal  cleft.  To  this  region  the  name  of  mid  gut  is  given,  and  it 
will  later  give  rise  to  the  intestine  of  the  adult. 

Finally,  we  have  to  consider  certain  changes  in  the  mesoderm. 
It  has  already  been  noted  that  the  mesoderm  extends  laterally  from 
the  sides  of  the  notochord  and  neural  tube  as  a  solid  sheet  of  cells, 
separating  the  ecto-  and  entoderm,  save  that  at  the  primitive  streak 
region  it  is  confluent  with  the  other  layers.  With  the  development 
and  flexure  of  the  brain  the  mesoderm  cells  extend  up  into  this  head 
region.  Here,  however,  they  do  not  form  a  definite  compact  sheet, 


394  AN   INTRODUCTION   TO  ZOOLOGY 

but  are  represented  by  a  number  of  scattered  somewhat  stellate 

I 


M 


FB 


FIG.  135. — -Rana  temporaries.  Diagrammatic  sections  of  sagittal  sections  of 
young  embryos:  I.,  before  closure  of  blastopore ;  II.,  after  closure  of 
blastopore. 

A.,  anus  ;  B.,  blastopore  ;  E.,  ectoderm  ;  En.,  entoderm  ;  F.B.,  fore  brain ;  F.G.,  fore  gut  ; 
H.,  heart  rudiment  ;  H.B.,  hind  brain  ;  H.G.,  hind  gut :  L.,  beginning  of  liver  diverticulum  ; 
M.,  mesoderm  ;  M.B.,  mid  brain  ;  M.G.,  mid  gut ;  N.,  notochord  ;  N.E.,  neurenteric  canal ; 
O.,  oral  evagination  ;  P.,  pituitary  ingrowth  ;  Pi.,  pineal  outgrowth  ;  Pr.,  proctodosum  ;  R., 
rectum  ;  S.C.,  spinal  cord  ;  Y.,  yolk  cells. 

cells  filling  in  the  interstices  between  the  brain  and  ectoderm  and 
front  end  of  the  enteron.     Mesoderm  in  this  condition  is  frequently 


EMBRYOLOGY 


395 


referred  to  as  mesenchyme.  During  the  period  dealt  with  above, 
the  mesoderm  in  the  middle  region  of  the  embryo  along  the  side  of 
the  notochord  becomes  more  solid  and  thicker,  and  constitutes  the 
segmental  plate  or  myotomal  region.  It  is  fairly  sharply  marked 
off  from  the  thinner  lateral  portion  which  is  the  lateral  plate.  A 
cleft  soon  appears  in  this,  thus  forming  the  beginning  of  the  dcelom, 
or  more  strictly  the  splanchnoccel,  and  this  divides  the  mesoderm 


M.R 


NX:. 


So 


FIG.  136. — Rana  sylvatica.  Part  of  a  section  through  the  anterior 
part  of  the  body  of  a  young  embryo ;  ectoderm  and  entoderm 
represented  in  outline  only. — After  Field. 

C.,  ccelom  ;  E.,  ectoderm  ;  E.G.,  enteric  cavity  ;  En.,  entoderm  ;  M.,  mesoderm  ;  M.P., 
medullary  plate  ;  N.,  notochord  ;  N.C.,  neural  crest  rudiment ;  So.,  somatic  mesoderm  ;  Sp., 
splanchnic  mesoderm  ;  S.P.,  segmental  plate. 

into  somatic  and  visceral  layers.  A  ccelom  originating  in  this  way  as 
a  split  in  a  mesodermal  sheet  is  often  referred  to  as  a  schizocoel  as 
opposed  to  an  enteroccel  as  in  Amphioxus.  At  first  this  cleft  is  con- 
fined to  the  upper  part  of  the  lateral  plate,  and  it  does  not  extend 
ventrally  until  considerably  later,  although  it  passes  up  into  the 
segmental  plate  giving  rise  to  rudimentary  myocoels.  This  stage 
is  reached  as  the  neural  folds  begin  to  approximate,  and  as  they  do 
so  the  longitudinal  bands  of  mesoderm  begin  to  get  cut  up  into  pairs  of 


396  AN    INTRODUCTION  TO   ZOOLOGY 

segments  or  somites.  At  the  time  of  closure  of  the  neural  tube, 
three  or  four  pairs  of  somites  have  been  formed  ;  the  first  of  them 
lies  at  the  end  of  the  fore  gut,  and  thence  the  process  of  somite 
formation  passes  posteriorly.  It  is  noteworthy  that  such  segmenta- 
tion does  not  extend  forward  to  the  head  region  or  laterally  to  the 
lateral  plates.  Here,  then,  we  have  the  embryo  in  approximately 
the  same  stage  of  development  as  was  reached  in  Amphioxus 


Gallus,  the  Fowl. 

In  the  fowl's  egg  we  have  an  example  of  the  extreme 
telocithal  condition,  for  there  is  a  very  large  amount  of  yolk  all 
massed  at  one  pole.  This  is  a  matter  of  considerable  interest  from 
the  comparative  point  of  view,  since  it  leads  to  certain  modifications 
in  the  process  of  development  resulting,  among  other  things,  in  the 
production  of  certain  membranes.  The  mammal,  whose  egg  is  small 
and  almost  yolkless,  has  been  derived  from  an  ancestral  form  with 
a  heavily  yolked  egg,  and  consequently  we  find  certain  peculiarities 
in  its  development  paralleled  in  the  fowl's  egg,  and  the  membranes 
are  put  to  quite  other  uses,  as  we  shall  see.  Fertilisation  is  internal 
and  effected  at  the  top  of  the  oviduct,  and  segmentation  commences 
straight  away,  so  that  by  the  time  it  is  laid  the  process  is  well  under 
way. 

At  oyulation  the  egg  consists  only  of  the  so-called  yolk  which 
passes  to  the  oviduct  (in  the  fowl  only  the  left  ovary  and  oviduct  are 
functional),  where  it  has  added  to  it  the  tertiary  egg  membranes. 
The  ovum  leaving  the  ovary  is  surrounded  by  a  thin  vitelline  mem- 
brane, and  in  spite  of  its  large  size  is  only  a  single  cell  whose  nucleus 
and  active  cytoplasm  is  confined  to  a  small  whitish  area  about 
3  mm.  in  diameter  on  one  side  of  it.  The  germinal  disc,  as  this 
spot  is  called,  is  situated  upon  and  continuous  with  the  yolk,  and  the 
latter  is  composed  of  two  different  varieties  of  material,  one  white 
and  the  other  yellow.  A  layer  of  the  white  yolk  underlies  the 
germinal  disc  and  a  neck  of  it  extends  downwards  into  the  centre 
of  the  yolk,  where  it  swells  out  to  form  a  flask-shaped  mass,  the 
latebra.  Around  this  the  yolk  is  arranged  in  concentric  layers  of 
alternate  thick  layers  of  yellow  and  thin  layers  of  white.  The 
cytoplasm  around  the  disc  where  it  joins  the  white  yolk  is  slightly 
darker  in  colour  and  forms  a  ring  known  as  the  periblast.  When  the 
egg  is  laid  we  find  that  there  has  been  added  to  the  ovum  a  thin 
layer  of  very  dense  albumen,  the  chalaziferous  layer,  which  is 
continued  out  at  each  side  into  a  tough  twisted  cord,  the  chalaza, 
whose  function  it  is  to  keep  the  germinal  disc  uppermost,  however 
much  the  egg  may  be  turned  about.  Outside  this  is  a  thick  layer  of 


EMBRYOLOGY 


397 


less  tough,  but  still  fairly  dense,  albumen,  and  this  in  turn  is  sur- 
rounded by  a  thickish  layer  of  more  fluid  albumen.  These  three 
albuminous  layers  constitute  the  so-called  white  of  the  egg.  The 
albumen  is  confined  in  a  tough  bag,  the  shell  membrane  composed  of 
a  double  layer  of  fibrous  matter,  and  the  two  layers  separate  at  the 
blunt  end  of  the  egg  to  include  between  them  a  small  air  chamber. 
Finally,  overlying  the  shell  membrane  is  the  calcareous  shell  itself, 


ae 


ad 


n 


FIG.  137. — Hen's  egg. — -From  Kellicott. 

A,  entire  "egg,"  modified  from  Marshall.  B,  vertical  section  through  the  vitellus  or  ovum 
proper,  showing  the  concentric  layers  of  white  and  yellow  yolk,  a.,  air  chamber;  ac,  chala- 
ziferous  layer  of  albumen  ;  ad,  dense  layer  of  albumen  ;  «/,  fluid  layer  of  albumen  ;  b.,  blasto- 
derm ;  c,,  chalaza  ;  /.,  latebra  ;  nl,  neck  of  latebra  ;  P.,  nucleus  of  Pander ;  pv,  perivitelline 
space  ;  smi,  inner  layer  of  shell  membrane  ;  smo,  outer  layer  of  shell  membrane  •  v.,  vitellus 
or  "  yolk  "  ;  vm,  vitelline  membrane  ;  wyt  layers  of  white  yolk  ;  yy,  layers  of  yellow  yolk. 

which  when  dry  is  porous  and  allows  of  the  passage  of  gases  and 
water  vapour.  All  these  structures  are  of  the  nature  of  tertiary  egg 
membranes. 

Maturation  in  the  fowl's  egg  does  not  take  place  until  just  after 
it  has  entered  the  oviduct  and  been  fertilised.  With  the  entrance  of 
the  sperm  the  first  polar  body  is  extruded  and  immediately  after 
that  the  second  is  given  off.  Polyspermy  appears  to  ba  the  rule, 
but  although  a  number  of  spermatozoa  enter  the  egg,  only  one  takes 


3g8  AN  INTRODUCTION  TO  ZOOLOGY 

part  in  the  act  of  nuclear  fusion,  the  others  after  a  while  degenerating. 
The  presence  of  such  an  enormous  amount  of  yolk  renders  it  impos- 
sible for  the  tiny  mass  of  cytoplasm  to  control  the  whole  ovum,  and 
as  a  result  we  find  that  the  cleavages  are  confined  to  one  pole  of  the 
egg.  Such  a  type  of  segmentation  is  termed  meroblastic  or  incom- 
plete, in  opposition  to  the  holoblastic  or  complete  type. 

The  first  cleavage  is  indicated  about  three  hours  after 
ovulation  by  the  formation  of  a  furrow  in  the  middle  of  the  germinal 
disc  which  extends  over  about  half  its  diameter.  Not  merely 
does  this  groove  fail  to  stretch  across  the  disc,  but  it  also  does  not 
reach  the  bottom  of  the  cytoplasm.  The  second  cleavage  occurs 
in  a  plane  at  right  angles  to  the  first,  so  marking  off  four  areas,  the 
blastomeres.  Typically  the  third  cleavage  is  represented  by  a  double 
furrow  parallel  with  the  first,  so  that  the  centre  of  the  disc  is  divided 
into  two  rows,  each  containing  four  cells.  Hereafter  the  cleavages 
become  irregular,  although  it  is  sometimes  possible  to  make  out 
a  fourth  fairly  regular  cleavage  in  the  form  of  a  pair  of  furrows  parallel 
with  the  second,  so  cutting  the  four  cells  on  each  side  into  central 
and  peripheral  daughter  cells.  While  the  third  and  fourth  cleavages 
are  making  their  appearance  a  horizontal  cleavage  takes  place  in 
the  region  of  the  central  cells  cutting  off  from  under  them  a  central 
unsegmented  mass  of  cytoplasm.  So  that  there  is  now  a  group  of 
central  cells  completely  circumscribed  and  surrounding  it  the 
marginal  cells  joining  the  periblast.  The  whole  area  of  cells  so  formed 
is  termed  the  blastodisc  or  blastoderm.  The  horizontal  cleavage 
just  noted  widens  until  it  forms  a  slit  which  is  the  segmentation 
cavity  or  blastocoel,  or  as  it  is  often  termed  in  this  form  the  sub- 
germinal  cavity.  Above  this  lie  the  central  cells  and  below  it  the 
unsegmented  cytoplasm  now  designated  the  central  periblast  to 
distinguish  it  from  the  original  or  marginal  periblast. 

Subsequent  cleavages  take  place  somewhat  rapidly,  until  a  large 
number  of  cells  are  produced,  most  of  which  are  central  cells,  while 
the  marginal  cells  are  reduced  to  a  small  band  around  the  periphery. 
Other  cleavages  have  also  occurred  in  the  horizontal  plane,  so  that 
the  roof  of  the  blastoccel  is  several  cells  thick.  The  nuclei  of  the 
marginal  cells  divide  without  accompanying  cytoplasmic  divisions 
and  the  daughter  nuclei  so  produced  wander  out  into  the  peripheral 
and  finally  into  the  central  periblast.  So  that  the  periblast  becomes 
transformed  into  a  syncytium,  that  is,  a  continuous  mass  of  proto- 
plasm containing  a  number  of  nuclei  without  the  corresponding 
cell  divisions.  Hundreds  of  tiny  cells  are  formed  by  this  time  and 
the  blastoderm  begins  to  increase  in  size,  partly  from  the  growth 
and  extension  of  its  own  cells,  and  partly  by  the  addition  of  cells 
cut  off  from  the  periblast.  The  inner  part  of  the  periblast,  which 


EMBRYOLOGY  399 

gradually  becomes  cellular,  is  of  considerable  importance  later,  and 
is  known  as  the  formative  ring  or  germ  wall.  During  the  process 
just  described  the  central  cells  have  become  thinner  and  somewhat 
transparent  so  forming  an  internal  area  known  as  the  area  pellucida, 
while  the  marginal  cells,  composed  in  part  of  cells  derived  from  its 
inner  margin,  is  denser  and  less  transparent  and  constitutes  the 
area  opaca.  This,  then,  may  be  taken  as  the  completion  of  the 
blastula  stage  although  its  formation  and  final  appearance  is  quite 
unlike  those  of  either  Amphioxus  or  Rana  owing  to  the  enormous 
amount  of  yolk  that  is  present  in  the  ovum  at  the  beginning. 

Gastrulation  or  the  differentiation  of  the  two  primary 
germ  layers  now  takes  place,  but  the  method  by  which  it  is  accom- 
plished has  not  been  satisfactorily  determined  in  the  case  of  the  fowl, 
so  that  the  account  given  here  is  based  on  the  process  as  it  occurs 
in  the  Pigeon.  The  area  pellucida  extends  slowly  and  its  increase 
is  accompanied  by  a  rearrangement  of  its  cells,  which  finally  become 
single  layered  towards  the  posterior  end  at  which  point  they  extend 
backwards  forming  a  break  in  the  germ  ring.  At  this  point  a  few 
of  the  cells  on  the  edge  turn  in  underneath  and  constitute  the  fore- 
runners of  the  entoderm.  This  process  increases  in  rate  and  extent 
and  the  place  at  which  it  takes  place  becomes  a  small,  slightly 
raised  crescentic  ridge.  Since  this  contains  both  ectoderm  and 
entoderm  cells,  it  can  be  regarded  as  the  dorsal  lip  of  the  blastopore. 
These  entoderm  cells  spread  forwards  and  outwards  under  the 
ectoderm  of  the  blastoderm  and  above  the  segmentation  cavity  in 
irregular  groups  and  form  a  continuous  sheet  only  at  the  hinder  end. 
As  a  rule  the  egg  is  laid  in  this  condition,  i.e.  with  gastrulation  not 
properly  completed,  and  laying  takes  place  twenty-one  or  twenty- 
two  hours  after  ovulation.  The  blastoderm  has  increased  slightly 
by  this  time  and  may  measure  as  much  as  5  mm.  in  diameter.  It 
remains  in  this  stage  after  it  has  been  laid  unless  the  temperature 
is  raised  either  by  its  being  placed  under  a  sitting  hen  or  in  an 
incubator.  These  conditions  induce  incubation  or  a  continuation 
of  development  outside  the  body  of  the  parent.  During  the  first  few 
hours  of  incubation  the  formation  of  the  entoderm  is  completed  and 
the  diploblastic  condition  established.  In  the  same  period  also, 
we  find  that  the  ectoderm  cells  over  the  central  region  have  become 
decidedly  columnar,  whereas  towards  the  periphery  they  remain 
flat  and  so  a  thicker  median  area,  the  embryonal  shield,  is  produced. 

The  formation  of  the  third  layer  is  heralded  by  the  appear- 
ance of  an  opaque  thicker  band  in  the  middle  line  of  the  hinder  half 
of  the  blastoderm  about  nine  hours  after  the  beginning  of  incubation. 
This  is  the  primitive  streak  and  sections  show  that  it  is  constituted 
at  first  by  an  aggregation  of  ectodermal  cells  in  that  region,  With 


4oo 


AN   INTRODUCTION  TO  ZOOLOGY 


the  appearance  of  the  streak  the  blastoderm  begins  to  increase  fairly 
rapidly,  mainly  by  the  extension  of  the  area  opaca  equally  in  all 
directions.  The  streak  itself  also  grows  quickly  hi  a  posterior 


o 
-P 


FIG.  138. — Chick  blastoderms. — From  Kellicott. 

A  after  Duyal  (modified)  ;  B-D,  after  Lillie.  A,  unincubated  blastoderm,  with  primitive 
streak  just  forming.  B,  primitive  streak  formed,  head  process  not  yet  indicated.  C,  head 
process  formed  ;  head-fold  just  commencing.  D,  just  before  the  establishment  of  the  first  meso- 
dermal  somites,  ac,  amnio-cardiac  vesicle  ;  cr,  crescent-shaped  thickening  at  the  posterior 
side  of  the  blastoderm,  in  the  region  of  entoderm  and  mesoderm  formation  ;  g.,  primitive  groove; 
hf,  head  fold  ;  hp,  head  process  ;  ».,  blood  islands  ;  m.,  axial  thickening  of  mesoderm  ;  mp, 
medullary  plate  ;  mr,  margin  of  mesoderm  ;  n.,  Henson's  node  ;  o.,  area  opaca  ;  p.,  area  pellu- 
cida  ;  pa,  proamnion ;  pi,  primitive  plate  ;  pp,  primitive  pit ;  s.,  primitive  streak ;  L,  sinus 
terminalis  (marginal  sinus)  ;  «.,  area  vasculosa  ;  vi,  area  vitellina  interna. 

direction  its  anterior  end  remaining  relatively  stationary.  With  this 
growth  goes  a  lengthening  of  the  area  pellucida,  which  first  becomes 
oval  and  later  pear-shaped.  As  the  primitive  streak  lengthens  a 
superficial  gutter-like  depression,  the  primitive  groove,  appears. 


EMBRYOLOGY  401 

along  the  middle  of  it  while  its*  raised  edges  are  known  as  the 
primitive  folds.  Its  sides  beneath  the  ectoderm  are  composed  of 
actively  proliferating  cells  which  give  off  their  products  into  the 
space  between  the  ecto-  and  entoderm  and  so  lay  down  the  primitive 
mesoderm  cells.  The  front  end  of  the  primitive  streak  thickens 
and  swells  out  slightly  to  form  the  primitive  knot  or  Henson's  knot, 
and  just  behind  it  the  primitive  groove  terminates  in  a  slight  depres- 
sion the  primitive  pit.  The  mesoderm  is  given  off  very  rapidly  from 
the  sides  of  the  streak,  passing  out  sideways,  and  it  also  aggregates 
in  the  middle  line  beneath  the  streak  and  there  comes  in  contact 
with  the  underlying  entoderm.  This  downward  growth  often  makes 
it  appear  in  section  as  if  this  portion  of  the  mesoderm  were  derived 
from  the  entoderm.  We  have  now  reached  a  stage,  then,  in  which 
the  three  primary  germ  layers  have  been  definitely  established  and 
they  are  confined  to  the  blastodermic  area,  being,  as  it  were,  unable 
to  stretch  round  and  encompass  the  yolk  which  occupies  the  position 
of  the  floor  of  the  enteron. 

However,  this  triploblastic  condition  has  not  yet  brought  the 
embryo  up  to  the  same  stage  of  development  as  in  Amphioxus  and 
Rana,  so  that  we  may  trace  it  a  little  further  and,  as  up  to  the  present 
there  has  been  no  indication  of  the  appearance  of  a  definite  embryo, 
the  immediately  succeeding  stages  are  often  referred  to  as  the  period 
of  formation  or  differentiation  of  the  embryo. 

The  mesoderm,  as  we  have  seen,  is  produced  by  multiplication 
of  the  cells  of  the  primitive  streak  ;  indeed  the  streak  itself  is  not 
really  a  definite  structure,  but  merely  the  result  of  an  active  prolifera- 
tion and  consequent  aggregation  of  cells  along  this  line.  This  meso- 
derm spreads  out  laterally  in  the  form  of  two  wing-like  plates  which, 
as  they  pass  outwards,  extend  forward  at  their  tips,  and  also  finally 
these  anterior  lateral  extensions  turn  inwards  and  meet  in  the  middle, 
line.  Their  front  edges,  therefore,  run  first  outwards,  then  forwards, 
and  finally  inwards  to  meet  well  in  front  of  the  primitive  knot,  thus 
there  is  left  an  area  between  their  union  and  the  end  of  the  primitive 
streak  which  is  free  of  mesoderm  and  this  region  is  termed  the  pro- 
amnion.  By  this  time  not  only  has  the  lateral  plate  of  the  mesoderm 
split  into  somatic  and  visceral  layers,  but  in  the  latter  have  appeared 
a  number  of  compact  masses  of  cells  that  give  this  region  of  the 
blastoderm  a  characteristic  mottled  appearance.  It  is  distinguished 
from  the  remaining  parts  of  the  blastoderm  as  the  area  vasculosa, 
since  these  masses,  appearing  as  dark  patches  when  the  structure 
is  viewed  as  a  whole,  are  destined  to  give  rise  to  the  first  blood- 
vessels and  the  blood  itself.  At  a  later  stage  the  "  blood  islands," 
as  they  are  first  termed,  extend  and  anastomose  forming  an  inter- 
lacing series  of  blood-vessels  over  the  whole  of  the  area  vasculosa. 

2  D 


402 


AN    INTRODUCTION   TO  ZOOLOGY 


By  the  time  the  primitive  streak  has  reached  its  definitive  size, 
a  rod  of  cells,  in  surface  view  appearing  very  similar  to  the  primitive 
streak,  has  started  to  grow  forward  from  the  primitive  knot.  This 
continues  to  grow  at  a  slight  angle  to  the  line  of  the  primitive  streak 
until  it  passes  just  beyond  the  end  of  the  embryonal  shield,  and  as 
it  is  accompanied  by  mesoderm  it  pushes  forward  the  hinder  limit 
of  the  pro-amniotic  area.  It  is  often  termed  the  head  process, 
but  is  more  suitably  designated  the  notochordal  process  since  it  repre- 
sents the  beginning  of  the  notochord.  This  process  is  actually  an 
independent  cellular  strand  growing  forward  from  the  knot,  between 
the  ectoderm  and  entoderm  and  most  probably  composed  of  cells 
derived  from  the  latter  layer,  although  of  course  at  the  knot  itself 
it  is  in  contact  with  all  three  layers.  It  should  not  be^confused 


E.S. 


So 


G.w. 


V.S. 


FIG.  139. — Chick.  Transverse  section  of  part  of  blastoderm  in  region  of 
primitive  streak  in  a  chick  with  six  pairs  of  somites  (about  twenty- 
four  hours). — After  Riickert. 

B.I.,  blood  islets;  C.,  coelom  ;  E.,  ectoderm;  En.,  entoderm;  E.S.,  embryonal  shield; 
E.T.,  endothelial  tube  ;  G.W.,  germ  wall ;  M.,  mesoderm  ;  M.P.,  medullary  plate  ;  P.S.,  primitive 
streak  ;  So.,  somatic  mesoderm  ;  Sp.,  splanchnic  mesoderm  ;  V.S.,  vascular  sinus. 

with  the  primitive  streak  in  spite  of  its  somewhat  similar  superficial 
resemblance,  as  it  is  an  entirely  different  structure  and  it  marks 
definitely  the  axis  of  the  future  embryo. 

As  the  notochordal  process  is  growing  forward  an  alteration 
occurs  in  the  ectoderm  of  the  embryonal  shield  lateral  to  it.  It 
thickens  to  form  the  medullary  plate,  and  this  extends  not  merely 
along  the  sides  of  the  notochord  itself,  but  also  lateral  to  the  front 
end  of  the  primitive  streak.  This  plate  is  indicated  in  the  beginning 
simply  by  a  thickening  of  the  shield  and  is  flat  save  for  the  presence 
of  the  dorsal  groove,  a  shallow  median  groove  that  brings  the  floor 
of  the  plate  into  contact  with  the  notochordal  process.  Soon  after 
its  formation  the  extent  of  the  plate  becomes  sharply  defined  by  its 
edges  rising  up  from  the  general  level  of  the  ectoderm  to  form  the 
neural  or  medullary  folds.  When  the  folds  become  well  developed 


EMBRYOLOGY  403 

they  transform  the  dorsal  groove  into  a  much  wider  and  deeper 
medullary  groove.  At  first  the  folds  are  lateral,  but  later  they  bend 
round  in  front  to  meet  in  the  middle  line,  just  behind  the  anterior 
border  of  the  embryonal  shield,  and  thus  mark  the  limit  of  the 
medullary  plate.  Thus  we  have  laid  down  the  rudiment  of  the  central 
nervous  system. 

A  very  important  change  occurs  after  the  formation  of  the 
transverse  neural  fold,  and  that  is  the  growth  in  this  region  becomes 
unequal.      The  fold  itself  rises  up  considerably  above  the  level 
of  the  surrounding  ectoderm,  and  then  its  dorsal  region  grows  much 
more  rapidly  than  its  ventral  part  or  than  the  front  margin  of  the 
embryonal  shield.     It  follows  as  a  mechanical  result  from  this  that  the 
transverse  fold  region  begins  to  bend  over  and  grow  forward  inde- 
pendently of  and  over  the  rest  of  the  blastoderm.     This  overgrowth 
leads  to  the  formation  of  a  rounded  process  overlying  the  anterior 
end  of  the  blastoderm  in  front  of  the  notochordal  process,  a  region 
which,  it  will  be  recalled,  is  the  proamniotic  area  and  contains  no 
mesoderm.    The  folding  produced  in  this  way  is  known  as  the 
head  fold,  and  it  is  the  beginning  of  an  important  transformation. 
Up  to  the  present,  while  there  have  been  certain  alterations  in  the 
thickness  of  its  various  parts,  etc.,  the  blastoderm  has  grown  as  a 
whole.     Now,  however,  we  find  that  a  portion  of  it  is  becoming 
folded  off  from  the  rest  and  very  sharply  marked  off  from  it.     This 
process  is  known  as  delimitation,  and  we  find  later  that  the  portion 
so  folded  off  is  destined  to  form  the  embryo,  while  the  remaining 
part  of  the  blastoderm  takes  no  direct  part  in  its  formation,  and  is,  in 
consequence,  termed  extra-embryonic.    The   growth  of  the  head 
fold  soon  comes  to  involve  both  layers  of  the  hinder  part  of  the  pro- 
amniotic area,  and  so  in  viewing  the  blastoderm  from  the  surface, 
two  rounded  bay  like  formations  facing  in  opposite  directions  can 
be  distinguished.     The  uppermost  of  these  is  superficial,  and  lies 
between  the  shield  ectoderm  on  the  under  side  of  the  head  fold  and 
the  extra  embryonic  ectoderm  of  the  proamniotic  area.     It  is  known 
as  the  head  fold  bay  and  of  course  opens  forwards  on  to  the  surface 
of  the  blastoderm.     The  other  is  a  deeper-lying  structure  and  is 
brought  into  being  by  the  pulling  forward  and  folding  of  the  ento- 
derm  of  this  region  with  the  extension  of  the  head  fold.     This  bay 
is  enclosed  by  entoderm  and  opens  backwards  into  the  sub-germinal 
cavity.     It  is  at  first  quite  short,  but  nevertheless,  an  important 
structure,  since  it  represents  the  fore-gut  of  the  embryo.     The  rela- 
tions of  all  these  structures  can  be  readily  seen  in  a  sagittal  section. 

When  the  head  fold  is  commencing  the  mesoderm,  as  we 
have  seen,  covers  a  fairly  extensive  area,  spreading  out  sideways 
from  the  primitive  streak  and  noto'chordal  process  and  it  sweeps 


4°4 


AN   INTRODUCTION   TO  ZOOLOGY 


forwards  and  inwards  to  leave  the  proamniotic  area  which  is  meso- 
derm free.  As  the  head  fold  grows  forwards  a  narrow  band  of  the 
mesoderm  situated  along  each  side  of  the  streak  and  notochord 
becomes  thicker  and  more  opaque  than  the  remaining  parts.  This 
band  is  distinguished  as  the  vertebral  plate,  paraxial  or  segmental 

zone,  and  the  peripheral 
thinner  sheet  is  termed 
the  lateral  plate  or 
parietal  zone.  It  has 

..-w  •*»•'*«"•  «*-^r  \   —     been  P°inted   out  also 
/     K     :|W    If  )  that  the  lateral  sheet  of 

mesoderm  has  already 
split  to  produce  the 
coelom  which  is  therefore 
a  schizocoel  as  in  the 
frog.  In  the  region  just 
behind  the  head  process 
the  paraxial  zone  thins 
out  and  loses  its  definite- 
ness,  and  certain  of  its 
cells  migrate  forward  to 
form  a  loose  network  of 
cells  in  the  fold  itself, 
thus  constituting  the 
mesenchyme  of  the  head, 
to  which  additions  are 
made  directly  from  the 
ectoderm  of  the  head 
region.  The  great  im- 
portance of  the  differen- 
tiation of  the  mesoderm 
is  that  the  somites  are 
derived  from  the  paraxial 
zone,  while  the  parietal 
zone  is  distributed  to  the 
rest  of  the  embryo. 

About  twenty  hours 
after  the  beginning  of 
incubation  a  pair  of 
transverse  furrows  appear  in  the  paraxial  zone,  a  short  distance  in 
front  of  the  primitive  knot  and  the  mesoderm  immediately  anterior 
to  the  furrow  aggregates  somewhat  and  forms  an  indistinct  mass 
that  can  be  considered  as  the  first  somite,  although  it  is  never  clearly 
marked  off  from  the  rest  of  the  front  end  of  the  zone.  Shortly 


•Fie.  140. — Chick,  twenty-four  hours. — From 
Lillie. 

a.c.v.,  amnio-cardiac  vesicle  ;  a.o.,  inner  margin  of  area 
opaca  ;  ect.,  ectoderm ;  ent.,  entoderm ;  A./.,  head 
fold  ;  i.s.f.l.,  first  intersomitic  furrow  ;  med.pl.,  medullary 
plate  ;  mes.,  mesoderm  ;  n.gr .,  neural  groove  ;  pr.gr., 
primitive  groove  ;  pr'a.,  proamnion. 


EMBRYOLOGY 


405 


afterwards  the  cells  behind  the  furrows  aggregate,  and  a  second  fur- 
row appears  sharply  delimiting  the  second  pair  of  the  somites. 
From  this  time  on  the  process  of  somite  formation  continues  fairly 
regularly,  each  new  pair  being  formed  behind  those  preceding  and  the 
embryonic  region  grows  at  the  expense  of  the  primitive  streak.  It 
finally  comes  to  an  end  on  the  fifth  day  of  incubation,  by  which  time 
fifty-two  somites  are  formed.  Not  only  is  this  number  and  order 
of  formation  constant,  but  the  fate  of  the  individual  somites  is  also 
the  same.  The  first  pair  of  incomplete  somites  lie  just  behind  the 
region  where  the  ear  will  later  develop,  and  the  next  three  pairs  with 
the  former  take  part  in  the  formation  of  the  occipital  region  of  the 
head.  Although  in  lower  forms  like  Scy Ilium  segments  are  laid 
down  in  front  of  the  auditory  pit,  this  is  not  the  case  in  the  chick, 


MC 


CC 


M 


MC 


So 


FIG.  141. — Chick.  Transverse  section  through  last  somite  of  a 
chick  with  twenty-nine  pairs  of  somites  (about  forty-eight 
hoursx). — Adapted  from  Lillie. 

A.,  aorta  ;  C.,  ccelom  ;  C.C.,  canalis  centralis  ;  E.,  ectoderm  ;  En.,  entoderm  ;  M.,  myotome  ; 
M.C.,  myoccel ;  Me.,  mesothelium  ;  N.,  notochord  ;  N.C.,  neural  crest ;  N.T.,  nephrotome ; 
S.C.,  spinal  cord  ;  So.,  somatic  mesoderm  ;  Sp.,  splanchnic  mesoderm. 

and  there  is  no  indication  of  segmentation  in  front  of  the  ear.  The 
mesodermal  somites  therefore  express  very  clearly  the  primitive 
metamerically  segmented  condition  of  the  body,  which  is  hidden 
to  a  large  extent  in  the  adult.  As  the  rate  of  incubation  varies 
with  different  conditions  and  individuals,  it  is  often  the  custom  to 
distinguish  the  stage  of  development  reached  by  the  number  of 
somites  present  rather  than  by  the  number  of  hours  of  incubation. 

A  transverse  section  shows  clearly  the  relation  between  the  two 
regions  of  mesoderm.  The  outer  cells  of  the  somite  are  seen  to  be 
arranged  in  a  fairly  definite  layer,  the  mesothelium,  around -a  more 
central  mass.  Between  these  two  divisions  is  a  very  narrow 


406  AN   INTRODUCTION  TO   ZOOLOGY 

curved  slit,  the  myocoel  ;  as  a  matter  of  fact  it  is  more  poten- 
tial than  actual  in  the  chick.  The  somite,  then,  is  a  more  or  less 
clearly  marked  oblong  mass  of  cells  having  a  definite  arrangement. 
It  is  joined  on  to  the  parietal  mesoderm,  between  whose  somatic 
and  splanchnic  layers  there  is  by  this  time  a  fairly  large  coelomic 
space,  by  a  narrow  strip  of  cells  variously  known  as  the  intermediate 
cell  mass,  the  somitic  stalk  or  nephrotome.  The  last  name  is  applied 
to  indicate  that  this  small  portion  is  destined  in  the  main  to  give 
rise  to  the  excretory  system,  but  a  part  of  it  also  contributes  to  the 
formation  of  the  mesenchyme. 

It  is  now  necessary  to  return  again  to  the  central  nervous 
system,  whose  beginning  we  have  already  noted  in  the  medullary 
plate.  Practically  the  whole  of  this  structure  in  front  of  the  primi- 
tive knot  is  destined  to  take  part  in  the  formation  of  the  brain  and 
is  consequently  sometimes  distinguished  as  the  brain  plate.  The 
medullary  folds  gradually  get  higher  and  higher,  and  the  medullary 
groove  in  consequence  deeper  and  also  nai  rower.  Finally  the  folds 
turn  inwards  and  grow  towards  the  middle  line,  where  they  meet 
and  fuse,  thus  converting  the  plate  into  the  neural  tube  and  the 
groove  into  the  neural  canal.  They  meet  first  at  a  point  a  little  way 
back  from  the  front  end  of  the  head  fold  in  a  region  that  will  give 
rise  later  to  the  mid-brain.  Before  the  actual  fusion  has  taken 
place,  however,  it  will  be  noticed  that  the  folds  at  the  anterior  end 
are  much  deeper  and  more  strongly  marked  than  posteriorly,  conse- 
quently the  neural  tube  when  formed  is  markedly  larger  in  front 
than  behind.  This  swelling  forecasts  the  division  between  the 
brain  and  spinal  cord  regions  of  the  central  nervous  system.  The 
two  are  not  sharply  demarked  and  pass  insensibly  into  one  another. 
We  have  already  noted  that  the  first  four  pairs  of  somites  are  included 
in  the  head  of  the  adult,  and  therefore  the  hinder  limit  of  the  fourth 
pair  may  be  taken  as  marking  the  end  of  the  brain  and  beginning 
of  the  spinal  cord. 

From  the  very  first  the  brain  is  much  larger  than  the  cord,  and 
even  in  a  chick  of  33  hours'  incubation  (i.e.  with  twelve  pairs  of  som- 
ites), it  occupies  more  than  half  the  total  length  of  the  neural  area, 
a  precocious  development  apparently  necessitated  by  the  relatively 
large  size  of  the  brain  when  fully  grown.  In  the  embryo  of  24  hours, 
before  the  fusion  of  the  folds,  another  specialisation  has  made  its 
appearance,  and  that  is  that  the  front  end  of  the  folds  is  marked  off 
by  a  distinct  constriction,  while  the  hinder  parts  of  the  folds  simply 
decrease  uniformly  as  they  pass  backwards.  The  first  point  of 
fusion  is  behind  this  constriction,  and  from  it  the  union  extends 
forwards  slowly,  leaving  a  small  aperture,  the  neuropore,  for  some 
while  right  at  its  front  end.  The  lips  of  this  opening  when  they  fuse 


EMBRYOLOGY 


407 


•op.  y&s. 


/tA.  Afes 


V.o.m. 


n.T. 


give  rise  to  the  lamina 
terminalis.  The  fusion  of 
the  neural  folds  proceeds 
more  rapidly  in  the  pos- 
terior direction,  although 
the  extreme  hinder  ends 
remain  apart  for  a  con- 
siderable time,  and  as  it 
does  so  a  second  constric- 
tion forms.  The  first  con- 
striction marks  off  the 
fore-brain  vesicle  or  pro- 
sencephalon,  and  the 
second,  the  mid  -  brain 
vesicle  or  mesencephalon, 
while  the  remaining  part 
of  the  brain  region  is 
termed  the  hind  -  brain 
vesicle  or  rhombencepha- 
lon.  Even  at  this  early 
stage  the  ventro  -  lateral 
walls  of  the  fore-brain  have 
bulged  out  laterally  to 
form  the  rudiments  of  the 
optic  vesicles.  Closer  ex- 
amination shows  that 
underlying  this  primary 
division  of  the  brain  there 
is  an  indication  of  segmen- 
tation, so  that  the  brain 
itself  may  be  regarded  as 
consisting  of  a  .series  of 
segments  or  neuromeres, 
marked  off  from  one 
another  by  faint  constric- 
tions. Three  such  neuro- 
meres are  generally  held 
to  be  present  in  the  fore- 
brain,  two  in  the  mid- 
brain,  and  in  the  hind- 
brain  five  can  fairly  easily 
be  distinguished,  of  which 
the  hindermost  is  the  largest  and  usually  considered  to  represent  two. 
The  total  number  of  neuromeres  in  the  chick  therefore  is  eleven. 


Met,. 


pr.str. 


FIG.  142. — 'Chick,  seven  pairs  somites. — 
"*From  Lillie. 

a.c.s.,  anterior  cerebral  suture  ;  ceph.Mes.,  cephalic 
mesoblast  ;  F.G.,  fore-gut  ;  N'ch.,  notochord  ;  n.T., 
neural  tube;  op.Ves.,  optic  vesicle;  pr'a.,  proamnion  ; 
pr.str.,  primitive  streak  ;  sz.,  57.,  second  and  seventh 
somites  ;  V.o.m.,  omphalo-roesenteric  vein. 


408  AN   INTRODUCTION  TO  ZOOLOGY 

The  tops  of  the  neural  folds  as  they  come  together  are  somewhat 
flattened,  so  that  when  they  join,  a  portion  of  the  nervous  matter  is 
left  under  the  superficial  ectoderm  along  the  sides  of  the  line  of 
junction.  This  does  not  take  part  in  the  formation  of  the  tube 
itself,  but  is  left  as  a  pair  of  longitudinal  bands  on  the  dorso-lateral 
sides  of  the  neural  tube.  These  are  the  neural  crests,  and  will  give 
rise  to  the  ganglia  of  the  cranial  and  spinal  nerves. 

We  have  thus  followed  the  development  of  the  chick  up  to 
a  stage  corresponding  fairly  closely  with  those  at  which  we  left 
Amphioxus  and  Rana,  and  in  spite  of  the  considerable  differences 
in  the  method  of  attaining  it,  owing  to  the  presence  of  a  large  amount 
of  yolk,  the  condition  of  the  embryo  in  all  three  species  shows  a 
remarkable  fundamental  resemblance. 


Lepus. 

The  study  of  the  development  of  the  mammalian  ovum  is 
one  that  presents  a  number  of  difficulties.  In  the  first  place  the 
ovum  itself  is  of  small  size,  being  about  '2  mm.  in  diameter  in  man  and 
•17  mm.  in  the  dog  and  the  rabbit ;  this  makes  it  hard  to  find  and 
manipulate  when  obtained.  Secondly,  save  in  the  case  of  the  rabbit, 
there  is  no  definite  relationship  between  ovulation  and  any  external 
happening,  so  that  it  is  largely  a  matter  of  chance  if  early  stages 
are  obtained.  In  the  rabbit  ovulation  occurs  q  or  10  hours  after 
copulation,  so  that  in  this  species  one  difficulty  is  partly  removed. 
Then,  too,  the  mammalian  egg  has  undoubtedly  been  derived  from  a 
large  heavily  yolked  telolecithal  egg,  somewhat  like  that  of  the  fowl. 
In  spite  of  its  small  size  and  the  fact  that  it  is  almost  devoid  of  yolk, 
it  follows  in  its  development  the  same  general  course  as  a  telolecithal 
egg  and  does  not  return  to  the  primitive  homolecithal  type.  Lastly, 
the  segmenting  ovum,  deprived  of  yolk  as  source  of  food  supply, 
takes  on,  at  quite  an  early  stage,  a  definite  and  very  intimate  connec- 
tion with  the  wall  of  the  uterus,  whence  it  obtains  its  nutriment. 
This  profoundly  modifies  the  early  stages  of  development,  and  so 
we  find  a  number  of  processes  occurring,  particularly  in  the  early 
stages  of  segmentation,  that  either  have  no  counterpart  in  other 
forms  or  else  our  knowledge  of  them  is  insufficient  to  allow  of  definite 
homologies  being  established. 

The  extrusion  of  the  first  polar  body  occurs  while  the 
ovum  is  still  in  the  graafian  follicle.  When  the  latter  has  become 
fully  formed,  it  slowly  enlarges  until  it  is  transformed  into  a  hollow 
vesicle  filled  with  follicular  fluid  and  having  its  walls  lined  by  a  layer 
several  cells  thick,  the  membrana  granulosa.  To  one  side  of  this 
the  ovum  is  attached  by  means  of  a  group  of  cells  surrounding  it. 


EMBRYOLOGY  409 

When  the  follicle  is  fully  ripe  in  most  mammals  it  ruptures  spon- 
taneously at  a  time  that  is  associated  with  a  general  physiological 
condition  known  as  heat  or  oestrus.  In  the  rabbit,  as  noted,  the  ripe 
follicle  does  not  burst  until  after  copulation,  which  occurs  during 
oestrus.  The  ovum  thus  released  is  received  by  the  oviducal  funnel, 
and  at  the  top  of  the  oviduct  it  is  fertilised  by  the  entrance  of  a  single 
sperm.  This  is  followed  by  the  formation  and  extrusion  of  the 
second  polar  body. 

The  egg  when  it  leaves  the  follicle  is  a  minute  spherical  cell 
composed  of  semi-transparent  protoplasm,  and  it  contains  a  vesicular 
nucleus  with  little  chromatin  but  a  well-marked  karyosome.  A  few 
yolk  spheres  may  be  present,  but  never  many.  It  is  surrounded 
by  an  egg  membrane,  perhaps  a  secondary  egg  membrane,  which 
is  termed  the  zona  pellucida.  This  is  frequently  striated  for  part 
or  the  whole  of  its  thickness,  suggesting  that  it  is  perforated  by 
minute  canals,  and  so  this  -region  is  sometimes  called  the  zona 
radiata. 

The  first  cleavage  is  complete  cutting  the  ovum  into  two 
equal  or  approximately  equal  cells.  The  second  cleavage  is  also 
complete  and  generally  simultaneous  in  the  first  two  cells,  and  the 
four  resultant  daughter  cells  are  usually  arranged  in  a  characteristic 
cross.  After  this  the  segmentation  is  more  or  less  irregular,  and  as 
a  result  of  it  there  is  produced  a  small  solid  mass  of  cells,  the  morula. 
At  quite  an  early  stage  a  certain  amount  of  organisation  can  be 
distinguished  in  this  morula,  for  it  will  be  seen  that  the  outer  cells 
form  a  definite  peripheral  layer  around  a  more  irregular  internal 
group.  As  these  changes  occur  while  the  egg  is  still  surrounded  by 
the  zona,  the  superficial  layer  is  termed  the  sub-zonal  layer  and 
the  remainder  the  internal  cell  mass.  As  segmentation  continues  a 
cleft  appears  between  the  two  groups  of  cells,  and  it  enlarges  until 
the  internal  cell  mass  is  separated  from  the  sub-zona  layer  by  a 
fluid- filled  cavity,  save  at  one  point  where  they  remain  in  contact. 
Thus  we  have  a  hollow  sphere  with  a  wall  one  cell  thick,  the  blasto- 
dermic  vesicle,  supporting  within  it  at  its  upper  pole  the  inner  cell 
mass.  Its  cavity  is  taken  to  represent  the  yolk  mass  of  an  egg,  like 
that  of  the  fowl.  That  is  to  say,  the  cavity  is  the  subgerminal 
cavity  extended  to  include  the  whole  of  a  hypothetical  yolk  mass, 
which  is  replaced  by  a  small  amount  of  fluid.  The  vesicle  wall  is 
regarded  as  ectodermal,  but  since  it  takes  no  part  in  the  actual 
formation  of  the  embryo  it  must  be  regarded  as  corresponding 
to  the  extra-embryonal  ectoderm  of  the  chick.  By  the  time  this 
stage  has  been  reached  the  egg  has  traversed  the  Fallopian  tube 
and  lies  in  the  uterine  cavity,  and  since  the  vesicle  wall  is  the  part 
brought  into  contact  with  the  uterus  it  is  obvious  that  it  will  be 


4io 


AN   INTRODUCTION  TO  ZOOLOGY 


the  portion  to  be  modified  in  connection  with  fixation  and  nutritive 
requirements  of  the  embryo.  After  reaching  the  uterus  the  blasto- 
dermic  vesicle  enlarges  fairly  rapidly  and  in  some  species  may 
reach  quite  a  large  size.  As  its  wall  later  takes  part  in  the  formation 
of  the  placenta,  which  is  among  other  things  an  organ  of  nutrition, 
it  comes  to  be  termed  the  trophoblast  or  trophoblastic  ectoderm. 
The  remaining  internal  group  of  cells,  which  is  responsible  for  the 


V.W. 


FIG.  143. — Lepus.  .Segmentation  of  egg  of  rabbit.  I.  and  II.,  first  and 
second  cleavages ;  III.,  section  through  morula ;  IV.,  section  through 
fairly  advanced  blastodermic  vesicle. — After  Assheton. 

A.L.,  albuminous  layer  ;  C.,  cavity  of  blastoderm  ;  I.,  inner  cell  mass  ;  P.B.,  polar  body  ; 
V.W.,  wall  of  blastodermic  vesicle  ;  Z.R.,  zona  radiata. 

production  of  the  entire  embryo  and  the  whole  entoderm,  both 
intra-  and  extra-embryonal,  is  distinguished  from  the  former  as  the 
formative  or  embryonal  ectoderm. 

Two  important  processes  now  go  on  simultaneously,  namely, 
the  formation  of  the  germ  layers  and  the  embryonic  rudiment,  and 
secondly  the  implantation  or  embedding  of  the  vesicle  in  the  uterine 
wall.  For  convenience  these  two  processes  will  be  considered 


EMBRYOLOGY 


411 


Xf. 


FIG.  144. — Rabbit,  blastocyst. — From 
Quain. 

ect.,  trophoblastic  ectoderm  ;    ent.,  inner  cell 
mass  ;  g.p.,  zona  pellucida. 


separately,  and  indeed,  since  the  implantation  is  intimately  connected 
with  certain  covering  embryonic  membranes  that  are  produced  in 
the  course  of  development,  its  consideration  will  be  held  over  until 
we  have  followed  the  growth 
of  similar  membranes  in  the 
chick.  It  should  be  borne  in 
mind,  however,  that  this  divi- 
sion is  purely  artificial  and 
the  changes  go  on  side  by  side. 
During  the  subse- 
quent growth  the  internal  cell 
mass  flattens  and  spreads  out 
until  it  forms  a  thin  disc  under- 
lying the  trophoblast  at  one 
pole ;  the  mass,  however, 
soon  becomes  differentiated 
intp  two  layers.  The  outer 
layer  is  the  ectoderm,  and  its 
cells  multiply  rapidly  and  be- 
come cubical,  and  in  this  way 
give  rise  to  the  ectoderm  of 
the  definitive  embryonal  shield. 

The  lower  layer  cells  form  the  entoderm  ;  they  flatten  out  and  grow 
rapidly  and  in  typical  cases  spread  completely  round  the  inside  of  the 
blastocyst,  which  is  thus  made  bilaminar.  This,  then,  concludes  the 
process  of  gastrulation  or  formation  of  the  two  primary  germ  layers. 
At  first,  in  all  cases,  the  inner  cell  mass  is  covered  by  a  layer  of  tropho- 
blastic, often  referred  to  as  Rauber's  layer.  Thus  over  this  area  the 
blastocyst  is  trilaminar.  The  blastocyst  as  a  whole  continues  to 
grow  during  this  period,  and  is  kept  turgid  by  the  infiltration  of  a 
fluid  produced  by  a  series  of  glands  in  the  uterine  wall. 

The  fate  of  Rauber's  layer  is  not  the  same  in  different  groups,  and 
the,  transformations  in  its  neighbourhood  may  be  complex.  In  the 
bats,  many  rodents  and  perhaps  man,  this  layer  persists  and  becomes 
separated 'from  the  embryonal  shield  by  a  cavity  designated  the 
primitive  ammotic  cavity,  since  it  is  destined  later  to  give  rise  to  the 
true  amniotic  cavity.  In  other  species,  notably  the  rabbit,  it  appears 
as  if  the  rapid  growth  of  the  embryonal  shield,  which  fuses  with  the 
trophoblast  around  its  edges,  stretches  the  layer  until  it  breaks 
down  into  a  number  of  loose  cells  lying  under  the  zona  and  between 
it  and  the  shield.  These  cells  may,  perhaps,  merge  in  the  ectoderm 
of  the  shield,  but  at  any  rate  they  disappear  as  separate  structures, 
and  so  the  blastocyst  becomes  bilaminar  throughout.  It  will  be 
clear,  however,  that  while  the  greater  part  of  its  periphery  is 


412  AN  INTRODUCTION  TO  ZOOLOGY 

composed  of  the  retained  trophoblastic  ectoderm  plus  the  entoderm, 
a  circular  area  is  formed  of  embryonal  ecto  and  entoderm.  In  other 
words,  then,  in  this  form  the  embryonal  ectoderm  becomes  exposed 
on  the  surface  of  the  blast ocyst.  Embryos  of  this  type  also  develop 
an  amnion,  but  later  and  by  means  of  the  overgrowth  of  folds  much 
in  the  same  way  as  in  the  chick,  as  we  shall  see  below. 

The  embryonal  shield  now  grows  and  becomes  first  oval  and  then 
pear-shaped.  While  it  is  still  oval  a  primitive  streak  and  primitive 
knot  make  their  appearance  almost  simultaneously  as  an  opaque 
line  and  spot  occupying  the  posterior  two-thirds  of  the  shield.  A 
primitive  groove  also  appears,  and  about  the  same  time  a  less  obvious 
dark  line  running  forwards  from  the  knot  indicates  the  presence  of 
a  notochordal  process.  All  these  structures  recall  the  similar  ones 
in  the  chick,  which  they  resemble  in  origin  and  structure.  The 
primitive  streak  is  the  result  of  the  active  proliferation  of  the  ecto- 
dermal  cells  along  a  median  line,  and  the  cells  budded  off  from  it 
become  arranged  to  form  a  layer  between  ecto-  and  entoderm.  As 
in  the  chick,  then,  the  mesoderm  is  ectodermal  in  origin.  The  primi- 
tive knot  in  the  mammal  is  slightly  different  from  that  in  the  chick. 
In  the  first  place  the  entoderm  is  not  firmly  united  to  it  on  the  under 
side,  and,  secondly,  it  may  have  a  perforation  which  leads  to  a  small 
cavity  in  the  notochordal  process  known  as  the  notochordal  canal. 
This  structure,  while  not  appearing  in  the  chick,  is  characteristic  of 
certain  Reptiles.  In  the  rabbit,  however,  this  canal  is  only  repre- 
sented by  a  groove  on  the  lower  surface  of  the  shield. 

It  is  not  necessary  to  go  into  the  details  of  succeeding  stages  of 
development,  since  they  resemble  those  of  the  chick  fairly  closely. 
A  medullary  plate  is  formed  whose  edges  bend  up  as  medullary  folds 
and  finally  fuse  in  the  middle  line  to  form  a  neural  tube  whose 
primary  brain  vesicles  are  already  indicated  before  the  union  of  the 
folds.  The  mesoderm  sheet  spreads  out  and  soon  passes  out  into 
the  extra-embryonal  regions.  Paraxial  bands  develop  in  it,  and 
segment  into  typical  mesoblastic  somites.  The  coelom  appears  as 
a  split  separating  the  mesoderm  into  somatic  and  splanchnic  layers, 
and  is  divisible  into  a  myoccel  and  splanchnoccel. 

With  this  we  may  conclude  the  account  of  the  early  develop- 
ment of  the  fertilised  ovum  in  four  distinct  types,  Amphioxus, 
Rana,  Callus  and  a  Mammal,  Lepus.  In  spite  of  the  great  differences 
due  to  the  particular  conditions  under  which  each  individually 
develops,  there  is  a  still  greater  fundamental  similarity  which  is  all 
the  more  striking  since  it  is  exhibited  under  such  diverse  conditions. 


CHAPTER  XVI 
LATER  DEVELOPMENT  OF  CHICK  AND  RABBIT 

Chick, 
Vascular  System. 

As  we  have  noted  already,  even  before  the  somites  have  been 
formed,  an  area  vasculosa  is  marked  out  in  the  area  opaca,  and  it 
appears  in  surface  view  as  a  mottled  region.  This  spreads  fairly 
rapidly,  and  by  38  hours  completely  covers  the  embryonic  area, 
having  around  its  edge  a  continuous  dark  line.  These  patches, 
which,  it  should  be  noted,  are  extra  embryonal  and  outside  the  area 
pellucida,  are  actually  closely  packed  irregular  groups  of  cells  lying 
between  the  meso  and  entoderm,  and  receive  the  name  of  "  blood 
islands."  They  soon  connect  up  and  form  an  anastomising  network. 
In  the  stage  when  three  or  four  somites  are  laid  down,  vacuoles 
appear  in  these  groups,  and  the  outer  cells  arrange  themselves  in  a 
flattened  layer  to  form  the  endothelium  of  the  future  blood-vessels, 
while  the  remaining  cells  round  off  and  develop  haemoglobin,  thus 
forming  the  erythrocytes  or  first  blood  cells.  So  that  around  the 
periphery  of  the  area  where  a  similar  process  goes  on  a  continuous 
limiting  vessel,  the  sinus  terminalis,  is  formed,  and  all  over  the 
remaining  parts  a  network  of  capillaries  in  which  larger  trunks  soon 
appear.  These  include  the  anterior  vitelline  veins  running  from 
the  anterior  margin  of  the  area  vasculosa  backwards,  and  the 
lateral  vitelline  veins  coming  in  from  the  side.  The  vessels  thus 
formed  invade,  at  quite  an  early  stage,  the  area  pellucida,  which 
never  possesses  distinct  blood  islands.  As  the  lateral  mesoderm 
splits  it  will  be  found  that  the  vessels  are  confined  to  the  splanchnic 
layer,  while  the  somatic  layer  is  non- vascular. 

l*he  first  embryonal  vessels  to  appear  are  the  dorsal  aortae,  a  pair 
of  tubular  structures  lying  in  the  mesoderm  latero-ventrally  to  the 
neural  tube.  They  pass  forward  to  the  head  region,  and  posteriorly 
they  diverge  at  about  the  level  of  the  last  somite  in  an  embryo  of 
12  somites,  to  run  out  into  the  vascular  area  as  the  vitelline  arteries. 

As  the  head  fold  is  forming,  the  coelom  just  behind  it  enlarges 
markedly  to  form  a  pair  of  sacs,  the  amnio-eardiac  vesicles.  These 

413 


414  AN   INTRODUCTION  TO  ZOOLOGY 

grow  inwards  towards  the  middle  line  between  the  ecto-  and  entoderm 
layers,  thus  making  the  gut  wall  at  this  point  very  deep.  The 
second  vessels  to  appear  are  a  pair  of  ventral  aortse,  beneath  and 
ventral  to  the  fore  gut,  and  these  run  backwards  and  pass  out  behind 
the  opening  leading  into  the  fore  gut,  i.e.  the  anterior  intestinal 
portal,  into  the  vascular  network  as  the  vitelline  veins.  These  two 
thin  tubes  run  along  the  inner  side  of  the  vertical  portion  of  meso- 
derm  of  the  mesial  walls  of  the  amnio-cardiac  vesicles,  whose  walls 
are  thickened  in  this  region  to  form  the  myocardium,  which  is 
destined  to  give  rise  to  the  main  mass  of  the  muscles  of  the  heart. 
Very  soon  after,  the  vesicles  fuse  in  the  middle  line  to  form  a  median 
space,  the  future  pericardium,  and  the  two  endothelial  tubes  also 
fuse  over  part  of  their  length  to  form  a  single  vessel,  the  endo- 
cardium, which  provides  the  lining  of  the  various  chambers  of  the 
heart.  The  fusion  of  the  vesicles  forces  the  anterior  intestinal  portal 
backwards.  The  heart  rudiment  then  comes  to  be  a  median  tube 
continuous  with  two  posterior  limbs  diverging  as  the  vitelline  veins, 
and  passing  forwards  into  the  two  ventral  aortae,  which  communicate 
around  the  front  end  of  the  fore  gut  with  the  dorsal  aortae  by  con- 
necting trunks  termed  the  first  or  mandibular  arches. 

The  heart  tube  itself  increases  in  length,  but  as  its  anterior  and 
posterior  ends  are  relatively  fixed,  the  enlargement  results  in  a 
bulging  out  to  the  right  side,  and  so  allows  two  divisions  to  be 
recognised,  an  anterior  ventricular  portion  and  a  posterior  atrial 
portion.  To  the  hinder  end  a  further  region,  the  sinus  venosus,  is 
added  by  a  fusion  of  a  part  of  the  vitelline  veins.  Further  growth 
leads  to  a  still  more  pronounced  bending,  and  the  ventricular  region, 
the  anterior  end  of  which  is  by  this  time  differentiated  to  form  a 
bulbus  arteriosus,  gradually  moves  down  ventral  to  the  atrial  region, 
ultimately  coming  to  lie  behind  it.  Next  the  atrium  is  divided  into 
two  by  the  growth  of  the  inter-atrial  septum,  and  a  union  and  en- 
largement of  the  proximal  ends  of  the  vitelline  veins  forms  the  sinus 
venosus.  Finally,  the  bulbar  portion  becomes  absorbed  into  the 
right  side  of  the  ventricle,  so  that  with  the  division  of  the  ventricle 
into  two,  as  a  result  of  the  development  of  the  interventricular  septum, 
the  bulbus  arteriosus  comes  to  be  incorporated  with  the  right 
ventricle  only. 

As  noted  previously,  at  the  thirtieth  hour  the  dorsal  and 
ventral  aortae  are  united  by  but  a  single  arch  on  each  side,  but  during 
the  second  day,  firstly,  the  second  or  hyoid  arch  is  formed  and  then 
a  third  arch.  By  the  end  of  the  third  day  a  fourth  arch  has  been 
completed,  and  the  fifth  and  sixth  arches  are  established  during  the 
fourth  and  fifth  days.  Of  these  arches  the  fourth  and  sixth  are  most 
strongly  marked,  while,  the  fifth  is  only  transitory  and  incomplete. 


LATER   DEVELOPMENT   OF  CHICK  AND   RABBIT    415 


KB. 


The  adult  condition  is  reached  as  a  result  of  the  disappearance  of  the 
whole  fifth  arch,  the  proximal  parts  of  the  first  and  second  arches, 

and  certain  of  the  connecting 
vessels.  The  presence  of  the 
six  arterial  arches  passing 
round  the  pharynx  in  relation 
to  rudimentary  gill  clefts  is 
characteristic  of  all  vertebrates 
from  Amphibia  up,  and  recalls 
the  condition  that  obtains 
permanently  in  the  adult  fish. 
In  all  these  higher  groups, 
too,  the  same  arches  persist 
throughout  life,  and  practically 
the  same  portions  are  lost  or 
cease  to  function. 

The  first  embryonal  veins  to 
appear,  during  the  second  day, 


or.pl 


•A  te. 


~"'fr.  str. 


FIG.    145. — Chick,  twelve  pairs  somites  (33  hours). — From  Lillie. 

H.B.,  hind-brain ;  op.Ves.,  optic  vesicle  ;  M.B.,  mid-brain  ;  F£.,  fore-gut ;  Ht .,  heart ; 
V .o.m.,  omphalo-mesenteric  vein  ;  sa.,  512.,  second  and  twelfth  somites  ;  Ao.,  aorta.  ;  pr.str., 
primitive  streak;  or.pl.,  oral  plate;  A.C.S.,  anterior  cerebral  suture;  v.Ao.,  ventral  aorta; 
ai.p.,  anterior  intestinal  portal. 

are  the  anterior  cardinal  veins,  a  pair  of  vessels  running  back  along 
the  ventro-lateral  walls  of  the  brain,  each  receiving  an  external  jugular 
vein  from  the  floor  of  the  pharynx.  Proximally  they  enlarge  and 


4i6  AN   INTRODUCTION   TO  ZOOLOGY 

turn  inwards  to  form  the  two  ducti  Cuvieri,  which  enter  the  sinus 
venosus,  one  on  each  side.  The  posterior  cardinal  vein  arises  as  an 
outgrowth  from  the  ductus  Cuvieri  and  passes  backwards  laterally 
to  the  somites  and  above  the  intermediate  cell  mass  to  the  tail. 
At  a  later  stage  the  two  lateral  vitelline  veins  unite  to  form  a  single 
vessel,  the  ductus  venosus  or  main  splanchnic  vein,  opening  into  the 
posterior  end  of  the  sinus  venosus.  The  two  anterior  vitelline 
veins  fuse  near  the  sinus  terminalis  and  the  right  vein  then  dis- 
appears, leaving  the  left  to  open  into  the  posterior  end  of  the 
heart. 

Delimitation  of  the  Embryo, 

The  manner  in  which  the  head  end  of  the  embryo  grows  off  the 
blastoderm  as  a  head  fold  has  already  been  discussed,  and  it  was 
noted  that  it  produced  a  head  fold  bay,  which  may  appropriately 
be  termed  the  anterior  limiting  sulcus,  since  it  separates  the  anterior 
end  of  the  embryo  from  the  blastoderm.  This  general  embryonic 
growth,  early  marked  at  the  front  end,  goes  on  somewhat  more 
slowly  over  the  whole  embryo  in  length,  breadth  and  thickness. 
After  the  eighteenth  pair  of  somites  have  been  formed,  the  primitive 
streak  as  such  disappears,  and  is  represented  only  as  an  area  of  active 
cell  proliferation,  the  tail  bud.  By  the  time  twenty-six  somite  pairs 
are  established,  i.e.  about  46  hours,  this  point  grows  off  the  blasto- 
derm in  a  similar  manner  to  the  head,  thus  bringing  into  being  a 
tail  fold  and  a  posterior  limiting  sulcus.  This  growth  involves  also 
the  entoderm,  giving  rise  to  a  hind  gut.  The  intervening  part  of  the 
embryo,  too,  has  been  getting  thicker,  and  so  rising  above  the 
blastoderm,  and  a  little  later,  as  this  upstanding  part  widens,  we 
have  produced  the  lateral  limiting  sulci,  connecting  up  the  anterior 
and  posterior  sulci.  In  this  manner,  then,  the  actual  embryo  becomes 
sharply  marked  off  from  the  surrounding  blastoderm  by  an  encircling 
groove. 

Thus,  from  being  just  a  thickening  in  the  middle  region  of 
the  blastoderm,  and  passing  out  insensibly  into  the  extra-embryonal 
region,  the  embryo  becomes  a  sharply  delimited  structure.  Even 
before  this  marking  off  is  complete  general  changes  have  been  taking 
place  in  the  conformation  of  the  embryo  as  a  whole  that  need  to  be 
considered  before  passing  on.  Up  to  the  stage  where  twleve  pairs 
of  somites  are  present  (about  33  hours)  the  axis  of  the  embryo  is 
fairly  straight,  but  when  15  somites  have  appeared  (about  36  hours) 
it  will  be  noted  that  the  head  end  is  turning  definitely  to  the  right ; 
the  bend,  or  cranial  flexure,  is  brought  about  by  the  rapid  growth 
of  the  roof  of  the  mid-brain.  The  turning  is  not  due  to  the  left  side 
growing  faster  than  the  right,  but  to  the  combined  twisting  and 


LATER   DEVELOPMENT   OF  CHICK   AND   RABBIT    417 

bending  of  the  front  end  of  the  embryo  in  such  a  manner  that  not 

cf    ms 


mt 


CrSJ. 


MyelencJ. 
au.P. 


3./0. 


frstr. 


B 


FIG.  146. — A.  Chick  embryo  -with  twenty  pairs  of  somites  (45  hours),  dorsal 
view.  B.  Chick  embryo  with  twenty-seven  pairs  of  somites  (48  hours), 
viewed  from  above. — From  Kellicott,  after  Lillie. 

A.  A . o.m. ,  vitelline  artery  ;  au. P. .auditory  pit ;  Cr .Fl.,  cranial  flexure  ;  D.C.,  ductus  Cuvieri ; 
Dienc.,  diencephalon ;    mesenc.,  mesencephalon ;     Metenc.,  metencephalon ;    Myelenc.,  i  and  2, 
anterior  and  posterior  divisions  of  the  myelencephalon  ;    Op.Ves.,  optic  vesicle  ;    Ph.,  pharynx  ; 
pr.str.,  primitive  streak  ;   S2,  55,  etc.,  second,  fifth,  etc.,  somites  ;  Telenc.,  telencephalon  ;   Vel.tr., 
velum  transversum  ;   Ven.,  ventricle. 

B.  a.,  auricle  ;   am.,  posterior  margin  of  amnionic  folds  ;  c.,  carotid  loop  ;    cf.,  cranial  flexure  ; 
d.,  diencephalon  ;  d.C.,  ductus  Cuvieri ;  gi,  g2,  gs,  first,  second,  and  third  gill  clefts  ;  i.,  isthmus  ; 
/.,  lens  ;  ma.,  mandibular  arch  ;    ms.,  mesencephalon;  mt.,  metencephalon  ;  o.,  otocyst  (aud:tory 
sac)  ;  just  to  the  right  of  the  otocyst  is  a  thickening  representing  the  ganglion  of  the  VII.  and  VIlJ. 
cranial  nerves  ;  r .,  retinal  layer  ;  S2,  sio,  sao,  second,  tenth,  and  twentieth  somites  ;  t.,  tail-bud  ; 
v.,  ventricle;    v.a.,  vitelline  artery;    v.v.,  vitelline  vein;    i,  2,  3,  first,  second,  and  third  aortic 
arches  ;   V '.,  ganglion  of  V.  cranial  nerve. 

only  does  the  anterior  end  turn  to  the  right,  but  the  right  side  of  the 

2  E 


4i8  AN   INTRODUCTION   TO  ZOOLOGY 

head  turns  uppermost  and  the  left  side  downwards.  It  is  probably 
a  mechanical  result  of  the  cranial  flexure,  for  the  head  is  unable  to 
bend  straight  downwards,  owing  to  the  presence  of  the  yolk.  At  the 
stage  of  20  somites  (about  43  hours)  this  is  very  clearly  marked. 
Five  hours  later  (27  somites)  another  bend  has  made  its  appearance, 
this  is  the  cervical  flexure,  and  is  produced  by  the  increased  growth 
of  the  roof  of  the  hind-brain,  causing  a  curving  of  the  whole  hind- 
brain  region.  The  result  of  these  two  flexures  and  the  enlargement 
of  the  fore-brain  vesicle  is  that  the  front  end  of  the  head  becomes 
directed  backwards  and  finally  inwards  also.  The  maximum 
amount  of  bending  is  reached  in  the  stage  of  35  pairs  of  somites 
(about  72  hours).  The  appearance  of  the  actively  growing  tail-bud 
leads  to  a  similar  but  much  less  extensive  bending  of  the  small 
posterior  end  of  the  embryo.  This  also  turns  to  the  right,  and  is 
well  marked  at  72  hours.  Indeed,  the  whole  embryo  tends  to  turn 
in  the  same  direction,  save  in  the  region  of  the  vitelline  veins,  so 
that  it  comes  to  lie  nearly  on  its  left  side,  and  the  heart  also  becomes 
displaced  to  the  right. 

Foetal  Membranes. 

The  foetal  membranes  are  four  in  number,  viz.  :  the  amnion, 
the  chorion,  the  yolk-sac  and  the  allantois.  The  first  three  of  these 
are  produced  by  the  extra-embryonal  blastoderm,  while  the  last  is 
an  actual  outgrowth  from  the  body  of  the  embryo  itself. 

The  first  two  are  developed  together  as  the  result  of  one  process, 
and  we  may  commence  with  them.  The  actual  details  of  their 
formation  are  somewhat  complicated,  but  it  will  suffice  if  only  the 
general  outlines  are  considered.  Their  formation  is  initiated  in  a 
chick  about  30  hours  old  by  the  appearance  of  a  transverse  ridge 
across  the  pro-amniotic  area  in  front  of  the  head.  This  increases 
in  height,  and  a  section  across  it  shows  that  originally  it  involves  the 
ectoderm  and  the  entoderm  ;  at  a  later  stage,  however,  the  mesoderm 
and  ccelom  invade  this  area,  and  so  the  fold  ultimately  involves  only 
the  ectoderm  and  the  somatic  mesoderm,  while  the  entoderm,  with  its 
accompanying  splanchnic  mesoderm,  sinks  back  again  to  a  lower 
level.  The  head  fold  of  the  amnion,  as  this  ridge  is  termed,  assumes 
a  crescentic  shape,  and  its  ends  pass  back  as  the  lateral  amniotic 
folds.  The  fold  gets  higher  and  grows  back  fairly  rapidly,  com- 
pletely enclosing  the  head,  and  at  48  hours  (26-28  somites)  has 
reached  back  almost  to  the  level  of  the  vitelline  veins.  At  this  time 
a  tail  fold  of  the  amnion  appears,  similar  to  the  anterior  one,  save 
that  it  consists  of  ectoderm  and  somatic  mesoderm  from  the  very 
commencement,  and  never  involves  the  entoderm.  Its  lateral  folds 
join  up  with  the  pre-existing  ones,  so  that  the  whole  embryo  is 


LATER   DEVELOPMENT   OF  CHICK   AND   RABBIT    419 

covered  save  for  a  long  oval  opening  towards  the  hinder  end.  This 
gradually  narrows  down,  and  is  completely  closed  up  by  the  end  of 
the  third  day  of  incubation.  As  the  folds  meet  over  the  embryo  the 
junction  between  them  breaks  down  save  over  a  limited  area,  where 
it  persists,  and  their  constituent  layers  fuse.  Thus  it  comes  about 
that  the  embryo  becomes  covered  superficially  by  two  very  thin 
membranes.  The  innermost  of  these  is  the  amnion,  and  it  is  com- 
posed of  a  layer  of  ectoderm  continuous  with  the  embryonal  ectoderm 
on  the  inside  and  a  layer  of  somatic  mesoderm  on  the  outside.  The 
embryo  itself  then  comes  to  lie  in  the  floor  of  a  hollow  cavity,  the 


LA 


EC 


Sp 


FIG.  147. — Chick  embryo.  Transverse  section  of  embryo  with  twenty-eight 
pairs  of  somites  (about  48  hours)  in  region  where  grit  remains  open.- — 
Adapted  from  Duval. 

D.A.,  dorsal  aorta  ;  E.,  embryonic  coelom  ;  EC.,  ectoderm  ;  En.,  entoderm  ;  Ex.,  exocoel; 
G.,  enteric  groove;  L.A.,  lateral  amniotic  folds;  N.,  notochord  ;  S.M.,  somatic  mesoderm; 
Sp.C.,  spinal  cord  ;  Sp.M.,  splanchnic  mesoderm  ;  V.,  vitelline  veins. 

amniotic  cavity,  which  becomes  filled  with  a  clear  liquid,  the  amniotic 
fluid.  Outside  this,  and  separated  from  it  by  an  extension  of  the 
extra-embryonic  coelom,  is  the  second  layer,  the  chorion  or  false 
amnion.  This  is  composed  of  the  somatic  mesoderm,  internally  and 
externally,  by  ectoderm  continuous  with  the  general  extra-embryonic 
ectoderm.  It  has  been  noted  that  along  the  line  marking  the  final 
point  of  closure  of  the  amniotic  folds  the  junction  between  them  does 
not  break  down.  The  folds  remain  firmly  united  in  this  region, 
which  is  termed  the  sero-amniotic  connection,  and  this  affects  the 
future  arrangements  of  the  membranes  to  a  considerable  extent. 


420  AN   INTRODUCTION  TO  ZOOLOGY 

In  a  previous  chapter  the  manner  in  which  the  blastoderm 
spreads  out  over  the  yolk  has  been  described,  and  this  expansion 
has  been  going  on  at  the  same  time  as  the  above  phenomenon.  The 
result  is  that  the  ring  of  peripheral  periblast,  with  its  accompanying 
sinus  terminalis,  has  passed  a  good  way  round  the  yolk  by  the  time 
the  formation  of  the  amnion  is  completed.  The  actual  growing 
edge  is  formed  by  the  union  of  ecto-  and  entoderm,  but  it  is  closely 
followed  by  the  mesoderm,  which  splits  as  it  progresses  into 
splanchnic  and  somatic  layers  separated  by  extra- embryonal 
ccelom.  This  process  is  not  completed  until  the  twelfth  day,  and 
then,  as  will  be  realised,  the  yolk  will  be  enclosed  in  a  bag  composed 
on  the  inside  of  entoderm  and  on  the  outside  of  splanchnic  meso- 
derm. The  yolk  sac,  as  this  structure  is  called,  is  separated  from  the 
outside  layer  of  the  blastoderm,  composed  of  ectoderm  and  somatic 
mesoderm,  by  the  extra-embryonic  ccelom.  Long  before  the  com- 
pletion of  this  process  this  layer  over  the  yolk,  the  splanchnic 
mesoderm  and  entoderm,  or  splanchnopleure,  is  spoken  of  as  the  yolk 
sac,  and  from  its  walls  a  series  of  unfoldings,  the  yolk  sac  septa, 
arise.  At  first  its  connection  with  the  embryonic  gut  is  a  wide  one, 
but  it  gradually  becomes  restricted  with  the  formation  of  the 
limiting  sulci,  the  fore  and  hind  guts,  and  the  fusion  of  the  amnio- 
cardiac  vesicles.  Independently  of  these,  however,  the  splanchno- 
pleure continues  to  constrict,  until  finally  it  is  reduced  to  a  narrow 
tube,  the  yolk  stalk,  putting  the  cavity  of  the  sac  in  communication 
with  the  lumen  of  the  gut  in  the  region  of  the  vitelline  veins.  Some- 
what later  the  limiting  sulci,  i.e.  the  somatopleure,  composed  of 
ectoderm  and  somatic  mesoderm,  also  constricts,  until  it  leaves  only 
a  narrow  opening,  the  umbilicus,  around  the  yolk  stalk. 

The  allantois  is  entirely  different  from  the  other  membranes 
we  have  discussed,  inasmuch  as  it  is  of  embryonal  origin.  We  have 
seen  that  the  tail  fold  brings  with  it  the  formation  of  a  short  hind 
gut,  which  later  becomes  lengthened,  not  only  by  direct  growth,  but 
also  by  the  processes  resulting  in  the  formation  of  the  yolk  stalk  and 
umbilicus.  At  an  early  stage  a  ventral  depression  appears  in  the 
floor  of  the  hind  gut,  and  this  grows  outwards  as  a  tubular  structure 
from  the  tail  fold  bay  into  the  extra-embryonal  ccelom.  It  has  just 
commenced  to  do  this  by  the  end  of  the  third  day.  It  will  be 
noticed  that  from  the  beginning  it  is  composed  of  entoderm  in- 
ternally, ectoderm  externally,  and  mesoderm  between  them  :  the 
latter  being  continuous  with  the  somatic  mesoderm  of  the  embryo. 
After  a  short  while  the  outgrowth  develops  a  characteristic  arrange- 
ment of  blood-vessels.  Later,  the  proximal  part  of  this  outgrowth, 
which  remains  narrow,  is  termed  the  allantoic  stalk,  while  the  distal 
portion  reaches  the  chorion  and  spreads  out  to  form  an  enormous 


LATER   DEVELOPMENT   OF   CHICK   AND   RABBIT    421 

flattened  sac,  which  by  the  twelfth  day  has  spread  almost  entirely 
around  the  blastoderm.  It  fuses  with  the  chorion  to  form  a  com- 
bined structure,  the  allanto-chorion.  This  is  a  very  important 
organ  in  the  physiological  life  of  the  actively  growing  embryo.  The 
circulatory  system  of  this  organ  is  composed  of  two  large  allantoic 
arteries  and  one  large  allantoic  vein  connected  by  a  complicated 
plexus  of  anastomosing  vessels.  The  allanto-chorion  receives  the 
nitrogenous  excretion  of  the  embryo,  is  the  organ  of  respiration,  and 
also  greatly  assists  in  the  absorption  of  the  albuminous  white  of 
the  egg. 

Gut  and  Related  Structures. 

In  the  formation  of  the  gut  three  regions  can  be  distinguished  : 
the  first  is  the  fore  gut,  formed,  as  we  have  seen,  by  the  pulling  in  of 
the  entoderm  during  the  formation  of  the  head  fold  ;  the  second  in 
order  of  formation  is  the  hind  gut,  resulting  from  the  formation  of 
the  tail  fold ;  and  the  third  is  the  mid  gut,  which  joins  the  two 
preceding  portions,  and  is  completed  by  the  closing  in  of  the 
splanchnopleure  all  along,  save  at  the  yolk  sac  opening.  I  he  first 
two  are  the  most  important. 

As  the  head  fold  forms,  the  ectoderm  on  its  underside  bends  up, 
touches  and  fuses  with  the  entoderm  of  the  front  end  of  the  fore  gut 
to  form  the  oral  plate.  This  is  at  first  superficial,  but,  with  the 
flexure  of  the  front  end  of  the  embryo,  it  becomes  overhung  by  a 
ridge,  and  shortly  after  the  mesoderm  at  its  sides  thickens  to  form 
two  ridges,  the  mandibular  arches,  and  so  the  plate  comes  to  lie  at 
the  bottom  of  a  shallow  depression,  the  oral  fossa  or  stomodoeum. 
At  the  beginning  of  the  third  day  the  oral  plate  ruptures,  putting  the 
lumen  of  the  fore  gut  in  communication  with  the  stomodceum. 
Even  before  this  time  the  dorsal  stomodceal  wall  gives  off  an  out- 
growth that  grows  in  under  the  floor  of  the  fore-brain,  and  later 
its  terminal  portion  becomes  detached  and  forms  the  glandular 
portion  of  the  hypophysis  cerebri.  The  formation  of  the  jaws 
greatly  enlarges  the  stomodoeum,  which  gives  rise  to  the  buccal 
cavity  of  the  adult,  and  hence  the  latter  is  lined  by  ectoderm. 

From  the  fore  gut  arise  the  pharynx  with  the  branchial  pouches, 
lungs,  oesophagus,  stomach,  duodenum,  liver  and  pancreas.  The 
front  end  of  the  gut  is  the  pharyngeal  region,  and  along  four  trans- 
verse lines  on  each  side  it  bulges  to  form  a  series  of  grooves,  the 
visceral  or  branchial  pouches.  The  first  of  these  to  appear  is  the 
hyomandibular  pouch,  which  is  noticeable  at  38  hours  (14-16 
somites)  as  a  groove  on  each  side  just  in  front  of  the  level  of  the 
auditory  pits.  Its  deeper  portion  reaches  and  fuses  with  the  ecto- 
derm to  form  a  branchial  membrane.  The  second  pouch  appears  at 


422  AN   INTRODUCTION  TO  ZOOLOGY 

43  hours  (19-20  somites),  and  the  third  and  fourth  at  45  hours 
(23  somites)  and  3  days  (35  somites)  respectively,  and  in  each  case  a 
branchial  membrane  is  formed.  Externally  the  ectoderm  opposite 
each  pouch  becomes  grooved  inwards,  constituting  the  ectodermal 
moiety  of  the  pouch  and  the  membranes  of  all  save  the  fourth  break- 
down for  a  while,  thus  giving  rise  to  three  transitory  visceral  clefts. 
Between  the  pouches  the  mesenchyme  thickens  to  form  the  visceral 
arches,  of  which  five  can  be  recognised.  Of  these,  the  first  or  mandi- 
bular  arch  lies  in  front  of  the  hyomandibular  pouch,  and  the  second 
arch,  between  this  cleft  and  the  next,  is  the  hyoid  arch.  In  these 
arches  develop  a  vascular  system,  composed  of  afferent  and  efferent 
vessels,  and  the  rudiments  of  skeletal  supports,  so  that  we  have 
produced  a  pharyngeal  complex  characteristic  of  a  water-dwelling 
Vertebrate.  The  mandibular  arches  extend  downwards,  uniting 
in  the  middle  line,  and  their  skeletal  elements  give  rise  to  the  lower 
jaws. 

The  visceral  pouches  have  but  a  short  existence,  and  finally 
disappear,  save  for  certain  remnants.  The  dorsal  region  of  the 
hyomandibular  cleft  takes  part  in  the  formation  of  the  Eustachian 
tube.  The  dorsal  entoderm  of  the  third  cleft,  and  to  a  lesser  extent 
of  the  fourth  cleft,  gives  rise  to  proliferations  that  constitute  the 
rudiments  of  the  thymus  gland.  On  the  second  day  a  small  cell 
thickening  appears  on  the  floor  of  the  fore-gut  between  the  bases 
of  the  second  gill  pouches.  Later  it  bulges  downwards  and  then 
becomes  cut  off  as  a  closed  vesicle.  By  the  seventh  day  it  has 
divided  into  two,  and  finally  these  give  rise  to  the  thyroid  gland  of 
the  adult. 

In  the  region  just  behind  the  fourth  visceral  pouch  the  gut 
narrows  down  to  form  the  oesophagus,  and  on  its  floor  in  the  transition 
region  a  shallow  groove  makes  its  appearance  on  the  second  day. 
This  structure  is  well  marked  by  the  end  of  the  third  day,  and  its 
hinder  end  has  grown  downwards  and.  broadened  considerably. 
The  groove  is  known  as  the  laryngeo-tracheal  groove,  since  it  gives 
rise  to  both  the  larynx  and  the  trachea,  while  the  wider  posterior 
portion  bifurcates  and  develops  into  the  paired  lungs. 

The  oesophagus  is  only  a  short  tube,  and  behind  it  the  gut  en- 
larges slightly  in  the  third  day,  indicating  the  future  position  of  the 
stomach,  and  then  narrows  down  again  to  form  the  duodenal  region. 
The  liver  appears  at  the  close  of  the  second  day  as  two  outgrowths 
just  in  front  of  the  anterior  intestinal  portal  on  the  ventral  wall  of 
the  gut,  i.e.  in  close  proximity  to  the  ductus  venosus  and  the  ductus 
Cuvieri.  The  anterior  one,  slightly  the  earlier  to  appear,  grows 
round  these  vessels  upwards  and  to  the  left,  while  the  posterior  one 
grows  upwards  and  to  the  right.  In  the  third  and  fourth  days  they 


LATER  DEVELOPMENT   OF  CHICK   AND   RABBIT    423 

anastomose  and  branch  freely,  giving  rise  to  a  network  of  liver 
tissue  around  the  venous  trunks.  With  the  further  growth  of  this 
glandular  structure  its  branches  penetrate  the  ductus  venosus, 
pushing  its  wall  in  front  of  them,  and  finally  they  cut  it  up  into  a 
canalisation  of  small  vessels  superficially  resembling  capillaries,  but 
termed  sinusoids,  to  indicate  their  different  origin.  These  are 
retained  as  the  venous  vessels  of  the  adult  liver,  and  so  the 
ductus  pours  its  biood  into  them  instead  of  directly  into  the  sinus 
venosus,  and  forms  the  foundation  of  the  definitive  hepatic  portal 
system. 

The  pancreas  arises  a  little  later  than  the  liver,  and  by  three 
diverticula.  The  dorsal  outgrowth  appears  on  the  dorsal  gut  wall 
about  opposite  the  posterior  liver  diverticulum  during  the  third  day. 
The  two  ventral  rudiments  appear  behind  the  latter,  and  soon  after 
the  dorsal  one.  In  a  short  time  the  three  separate  pancreatic  growths 
fuse  to  form  a  common  mass. 

Later  History  of  the  Mesoderm, 

The  formation  of  the  mesoderm  and  its  differentiation  into 
somites,  intermediate  cell  mass  and  lateral  plate  have  already  been 
treated,  but  certain  points  in  connection  with  their  later  history  call 
for  brief  notice.  By  the  fourth  day  42  somites-have  been  laid  down, 
and  after  that  10  more  are  formed,  but  disappear  later.  The  somites 
are  distributed  in  the  following  manner :  1-4  enter  into  the  skull ; 
5-16  between  the  skull  and  fore  limb  ;  17-19  fore  limb  ;  20-25 
between  fore  and  hind  limb  ;  26-32  hind  limb  ;  33-35  cloaca!  region  ; 
and  36-42  caudal  region.  The  transitory  somites  are  situated 
behind  the  last  of  these,  and  their  disappearance  suggests  that  an 
ancestral  form  possessed  a  longer  tail. 

At  an  early  stage  in  each  somite,  as  we  have  seen,  an  epithelial 
and  a  more  solid  portion  can  be  recognised,  but  later  we  find  that 
three  parts  become  differentiated,  viz.  the  dermatome,  the  myotome, 
and  the  sclerotome.  The  original  dorsal  epithelial  part  becomes 
more  distinct,  and  its  lateral  region  becomes  fairly  sharply  delimited 
from  the  remainder  as  the  cutis  plate  or  dermatome.  The  mesial 
dorsal  portion  also  becomes  marked  out  as  the  muscle  plate  or 
myotome.  This  becomes  thin  and  turns  in  under  the  cutis  plate. 
It  grows  outwards,  and  finally  reaches  and  fuses  with  the  free  lateral 
edge  of  the  cutis  plate,  so  that  the  two  form  a  double-layered  plate, 
sometimes  referred  to  as  the  dermo-myotome.  With  the  ingrowng 
of  the  lateral  limiting  sulcus  this  plate  becomes  disposed  practically 
vertically.  Its  outer  layer,  the  cutis  plate,  is  gradually  transformed 
into  the  dermis  of  the  adult,  and  as  it  does  so  it  becomes  more  and 
more  closely  attached  to  the  ectoderm,  which  provides  the  epidermis. 


424  AN   INTRODUCTION  TO  ZOOLOGY 

The  cells  of  the  muscle  plate  elongate  to  form  characteristic  "  myo 

EZ-m   G?Y 
cewf/.  &          MM  \  QpMJffff      &M. 

-n-k rr-r-. ^-> •?-> — jrsf  -j-r    ^  i ^-^-'-7— 77    '7? — *?~tt  '  \ 

^/tW\ 

erf) 


FIG.  148. — 'Chick  embryo  with  thirty-five  pairs  of  somites  (72  hours). — 

From  Lillie. 

a.a.,  1,2,3,4,  first,  second,  third,  and  fourth  aortic  arches  ;  Ar.,  artery  ;  A.V.,  vitelline  artery  ; 
cerv.  Fl.,  cervical  flexure  ;  cr.Fl.,  cranial  flexure  ;  D.C.,  duct  of  Cuvier  ;  D.V.,  ductus  venosus  ; 
Ep.,  epiphysis  ;  Gn.V.,  ganglion  of  trigeminus  ;  Isth.,  isthmus  ;  Jug.ex.,  external  jugular  vein  ; 
Md.,  mandibular  arch  ;  M.M.,  maxillo-mandibular  branch  of  the  trigeminus  ;  olf.P.,  olfactory  pit  ; 
Ophth..  ophthalmic  branch  of  the  trigeminus  ;  Ot.,  otocyst  ;  V.,  vein  ;  W.B.,  wing  bud  ;  V.c.p., 
posterior  cardinal  vein  ;  V.umb.,  umbilical  vein;  V.V.,  vitelline  vein  ;  V.V.p.,  posterior  vitelline 
vein. 

blasts,"  that  give  rise  later  to  the  muscles.     They  are  responsible 
for  the  whole  of  the  voluntary  musculature  of  the  adult,  while  the 


LATER   DEVELOPMENT   OF   CHICK  AND   RABBIT    425 

involuntary  muscles  are  derived  from  the  splanchnic  mesoderm. 
To  return  now  to  the  more  ventral  mass  of  the  somite,  we  find  that 
its  cells  proliferate  actively  and  take  up  a  position  near  the  noto- 
chord,  and  it  is  then  distinguished  as  the  sclerotome.  This  spreads 
around  the  notochord,  the  neural  canal  and  the  dorsal  aorta,  and  its 
cells  secrete  an  intercellular  substance.  Some  time  afterwards  the 
sclerotomes  acquire  a  secondary  segmentation,  and  each  becomes 
divided  transversely  in  the  middle  of  the  somite.  The  posterior 
half  of  each  one  unites  with  the  anterior  half  of  the  next  succeeding, 
and  the  block?  of  tissue  formed  in  this  way  of  course  come  to  alternate 
with  the  original  somites.  The  sclerotome  tissue  gives  rise  to  the 
skeletal  elements  of  the  axial  skeleton  save  for  a  large  part  of  the 
skull. 

The  intermediate  cell  mass  is  also  termed  the  nephrotome,  in 
order  to  indicate  its  close  relation  with  the  excretory  system,  and 
in  it  are  developed  the  constituents  of  the  kidneys.  In  all  higher 
Vertebrata  three  successive  excretory  organs  make  their  appearance, 
and  are  known  as  the  pro-,  meso-  and  meta-  nephros  respectively. 

The  first  of  these,  the  pronephros,  is  never  functional  in  the  chick 
at  any  time,  and  is  represented  only  by  degenerate  remnants.  Its 
vestigial  tubules  may  appear  in  the  fifth  to  the  sixteenth  somites, 
but  are  only  marked  in  the  last  six  or  so,  and  there  is  a  certain 
amount  of  variation  in  their  number  and  arrangement.  The  first 
show  in  this  region  in  the  middle  of  the  second  day  in  the  form  of 
small  solid  cellular  buds  on  the  postero-dorsal  surface  of  the  nephro- 
tome. These  buds  elongate,  and  their  distal  extremities  bend  over 
and  unite  with  the  corresponding  portion  of  tubules  behind,  so  that 
they  come  to  form  a  continuous  rod  of  cells,  which  represents  the 
Wolffian  duct.  Although  we  term  these  structures  the  tubules,  it  is 
somewhat  of  a  misnomer,  for  they  are  never  hollow,  and  in  the  same 
way  too  the  duct  is  solid  when  it  is  laid  down,  but  a  hollow  appears 
towards  the  front  end  during  the  second  day.  Its  posterior  end 
grows  back  above  the  nephrotomes  quite  independently  without 
contributions  from  the  posterior  members  of  the  series,  until  at 
2 \  days  it  reaches  and  fuses  with  the  cloaca.  The  lumen  also  spreads 
slowly  down  it  and  reaches  the  end  about  the  third  day.  This 
Wolffian  duct  then  is  composed  of  two  parts  ;  anteriorly  it  is  a 
fusion  of  portions  of  separate  tubules,  and  posteriorly  it  is  a  con- 
tinuous structure.  The  only  indication  of  a  cavity  in  the  tubules 
themselves  is  to  be  found  at  their  lower  end,  where  a  few  of  them 
contain  short  hollows  continuous  with  the  ccelom,  and  this  hollow 
is  taken  to  represent  the  vestigial  nephrotome.  Furthermore,  no 
indications  of  a  Malpighian  body  are  to  be  found  in  any  of  them. 
Just  after  the  appearance  of  the  tubule  bud,  the  cells  of  the 


426 


AN   INTRODUCTION  TO  ZOOLOGY 


nephrotome  adjoining  the  somite  become  converted  into  a  scattered 
mesenchyme,  and  so  connection  with  the  somite  is  lost.  By  the 
fourth  day  the  pronephfic  tubules  have  disappeared. 

The  second  kidney  or  mesonephros  develops  in  the 
segments  from  the  thirteenth  or  fourteenth  back  to  the  thirtieth, 
and  some  of  them  therefore  overlap  the  pronephros.  However,  it 
is  only  those  in  segments  20-30  that  develop  typically,  and  these 
do  not  begin  to  appear  until  the  fourth  day.  In  this  region  the 
nephrotomes  separate  from  the  somitesand  form  a  continuous  band 


A.C. 


S.M. 


Ex 


SpM 


FIG.  149. — Chick  embryo.   Transverse  section  of  embryo  with  thirty-five  pairs 
of  somites  (about  72  hours). 

A.,  amnion  ;  A.C.,  amniotic  cavity  ;  C.,  chorion  ;  D.,  dermatome  ;  D.A.,  dorsal  aorta  ;  E., 
embryonic  ccelom  ;  EC.,  ectoderm  ;  En.,  Entoderm  ;  Ex.,  exocoel ;  M.,  mesonephric  tubule 
My.,  myotome  ;  N.,  notochord  ;  N.C.,  neural  crest :  beginning  of  spinal  ganglion  ;  P.,  post-cardinal 
vein  ;  S.,  sclerotome  ;  S.A.,  sero-amniotic  junction  ;  S.C.,  sub-cardinal  vein  ;  S.M.,  somatic 
mesoderm  ;  Sp.C.,  spinal  cord  ;  Sp.M.,  splanchnic  mesoderm  ;  V.,  vitelline  artery  ;  W.,  Wolffian 
duct. 


of  tissue  lying  between  the  somite,  the  lateral  mesoderm,  the 
dorsa  aorta  and  the  Wolffian  duct.  Dorsal  to  this  region  at  quite 
an  early  stage  a  venous  trunk,  the  posterior  cardinal  vein,  is  laid 
down.  Each  tubule  first  appears  on  the  ventral  side  of  the  nephro- 
tome as  a  group  of  cells  which  soon  acquire  a  hollow,  and  so  is 
vesicular  from  the  commencement.  This  vesicle  becomes  elongated 
and  bent,  and  one  end  acquires  an  opening  into  the  Wolffian  duct, 
while  the  other  enlarges  to  form  the  rudiment  of  a  Bowman's  capsule. 
Later,  secondary  capsules  to  the  number  of  four  or  five  arise  in  each 
somite,  and  altogether  they  form  quite  a  noticeable  swelling  on  the 


LATER  DEVELOPMENT  OF   CHICK  AND   RABBIT    427 

dorsal  ccelomic  wall.    This  is  the  mesonephros  or  Wolffian  body, 
which  is  the  first  functional  kidney  of  the  embryo. 

The  permanent  kidney  of  the  adult,  the  third  to  develop, 
is  the  metanephros,  and  its  mode  of  origin  is  slightly  different  from 
that  of  the  two  preceding.  Towards  the  end  of  the  fourth  day  a 
sac-like  outgrowth  starts  from  the  Wolffian  duct  near  its  point  of 
entry  into  the  cloaca.  This  grows  forward  as  a  tubular  structure 
on  the  inner  side  of  the  posterior  cardinal  vein  and  above  the 
mesonephros.  The  tube  itself  is  the  ureter,  and  it  gives  rise  to  a 
series  of  complexly  branched  outgrowths,  the  future  collecting 
tubules.  The  actual  secretory  tubules  are  derived  from  the  nephro- 
tome  tissue  of  somites  33-35,  which  accompanies  the  ureter  as  it 
grows  forward.  The  metanephros  has  therefore  a  twofold  origin. 

Nervous  System. 

The  early  development  of  the  nervous  system  has  already  been 
treated  and  its  development  outlined  up  to  the  stage  when,  after  the 
closure  of  the  neural  folds,  the  three  primary  divisions  of  the  brain 
had  been  marked  out  and  eleven  neuromeres  indicated.  Certain 
points  in  the  subsequent  differentiation  of  the  brain  call  for  notice. 

At  quite  an  early  stage  an  outgrowth  appears  on  each  ventro- 
lateral  aspect  of  the  anterior  end  of  the  prosencephalon,  and  it  is 
termed  the  primary  optic  vesicle,  to  mark  the  fact  that  it  is  the  fore- 
runner of  the  eye.  Across  the  floor  of  the  brain,  between  the  two 
vesicles,  is  a  depression,  the  optic  recess.  The  region  in  the  middle 
line  passing  forwards  from  this  to  the  final  point  of  closure  of  the 
neuropore  is  the  lamina  terminalis,  and  although  it  is  morpho- 
logically the  anterior  end  of  the  brain,  as  a  result  of  the  flexures  it 
becomes  turned  in  a  posterior  and,  finally,  even  a  postero-dorsal 
direction.  While  at  first  the  vesicles  are  in  wide-open  communica- 
tion with  the  cavity  of  the  prosencephalon,  their  proximal  ends 
become  closed  in  dorsally  until  they  are  reduced  to  two  narrow  tubes, 
the  optic  stalks,  with  small  apertures,  one  on  each  side  of  the  optic 
recess  ;  the  future  development  of  these  will  be  considered  separately 
later.  The  first  neuromere  swells  out  markedly,  and  at  40  hours 
has  produced  on  the  dorsal  side  a  slight  indentation  between  it 
and  the  next  neuromere.  This  is  the  velum  transversum,  and  a  line 
from  it  to  the  optic  recess  marks  the  posterior  limit  of  the  telence- 
phalon,  which  is  therefore  constituted  by  the  first  neuromere.  Near 
the  end  of  the  second  day  a  pair  of  dorso-lateral  outgrowths  push  out 
from  the  telencephalon.  These  are  the  beginnings  of  the  cerebral 
hemispheres,  and  they  expand  rapidly  dorsally,  anteriorly  and 
posteriorly.  They  are  hollow,  and  their  cavities,  the  lateral  or  first 
and  second  ventricles,  are  in  open  communication  with  the  cavity  of 


428  AN  INTRODUCTION  TO  ZOOLOGY 

the  prosencephalon,  which  is  therefore  designated  the  third  ventricle, 
by  wide  apertures,  the  foramina  of  Munro.  At  first  the  wall  of  the 
ventricle  thickens  slowly  and  fairly  evenly,  but  soon  the  latero- 
ventral  region  increases  enormously  to  form  the  basal  ganglia  or 
corpora  striata,  which  almost  obliterate  the  lateral  ventricles. 

The  next  two  neuromeres  merge  to  give  rise  to  the  thalamence- 
phalon,  whose  dorsal  limits  are  the  velum  transversum  in  front  and  a 
broad  depression  behind.  On  the  ventral  side  the  front  end  is 
marked  by  the  optic  recess  and  posteriorly  by  an  elevation  in  the 
floor  of  the  brain,  the  tuberculum  posterius.  Its  cavity  contributes 
to  the  formation  of  the  third  ventricle.  Just  behind  the  optic  recess 
its  floor  thickens  to  form  the  optic  chiasma,  and  then  behind  this 
again  sends  down  a  median  diverticulum,  the  infundibulum.  During 
later  development  its  walls  thicken  greatly  to  form  the  optic  thalami, 
whose  enlargement  reduces  the  ventricle  to  a  narrow-  vertical  cleft. 
The  roof  remains  thin,  and  in  the  region  of  the  velum  transversum, 
together  with  the  adjacent  roof  of  the  telencephalon,  it  becomes 
modified  to  form  the  choroid  plexus  of  the  third  and  later  also  of  the 
lateral  ventricles.  During  the  third  day  a  small  tubular  median 
outgrowth  arises  from  the  hinder  end  of  the  thalamencephalon  in 
the  mid-dorsal  line.  This  is  the  epiphysis  cerebri,  which  in  the  chick, 
as  in  the  .rabbit,  only  develops  into  a  glandular  structure.  The 
hinder  limit  of  this  part  of  the  brain  is  definitely  laid  down  somewhat 
later  by  the  appearance  of  a  transverse  thickening,  the  posterior 
commissure. 

The  mid-brain  or  mesencephalon  comprises  two  neuromeres,  i.e.  the 
fourth  and  fifth,  and  it  comes  to  occupy  the  most  anterior  position  in 
the  embryo  topographically,  indeed,  it  is  largely  owing  to  the  great 
growth  of  its  roof  that  the  cranial  flexure  is  brought  about.  Its 
anterior  limits  have  already  been  noted,  and  posteriorly  it  is  marked 
off  from  the  hind-brain  dorsally  by  a  constriction,  the  isthmus, 
while  ventrally  its  limits  are  ill-defined.  At  the  end  of  the  third  day 
little  specialisation  has  taken  place  in  it,  and  it  is  not  until  later 
that  outgrowths  from  its  thickened  dorsal  region  form  the  optic  lobes, 
and  its  ventro-lateral  walls  thicken  to  form  the  crura  cerebri.  These 
various  thickenings  of  the  wall  lead  to  the  reduction  of  the  originally 
large  cavity  to  a  narrow  lumen,  the  iter. 

The  whole  of  the  cavities  of  the  neuromeres  constituting  the  hind- 
brain  remain  in  wide  open  communication,  and  form  one  indivisible 
fourth  ventricle.  Its  walls,  on  the  other  hand,  differentiate  into 
the  metencephalon  and  the  myelencephalon.  The  metencephalon 
includes  only  one  neuromere,  and  its  dorsal  limit  can  be  distinguished 
on  the  third  day  by  the  fact  that  it  forms  the  hinder  almost  vertical 
wall  of  the  isthmus,  and  is  much  thicker  than  the  corresponding 


LATER  DEVELOPMENT   OF  CHICK  AND   RABBIT    429 
part  of  the  succeeding  neuromeres.     Its  walls  thicken  slowly  and 


ao 


A 


vt 


ao 


vt 


FIG.  150.- — Sagittal  sections  through  the  head  of  the  chick.     In  A 
the  heart  is  shown  in  optical  section. — From  Kellicott. 

A,  of  an  embryo  with  twenty-two  or  -three  pairs  of  somites  (about  forty-four  hours)  ;  B,  of  an 
embryo  with  thirty-nine  pairs  of  somites  (end  of  the  fourth  day),  a.,  auricle  ;  am.,  amnion  ;  ao., 
dorsal  aorta  ;  ba.,  bulbus  arteriosus  ;  c/.,  cranial  flexure  ;  D.,  diencephalon  ;  dv.,  ductus  venosus; 
<?.,  epiphysis  ;  h.,  hypophysis  ;  »'.,  infuudibulum  ;  ip.,  anterior  intestinal  portal ;  is.,  isthmus  ; 
/.,  rudiment  of  lung  ;  li.,  liver  ;  m.,  mandibular  arch  ;  Ms.,  mesencephalon  ;  Mt.,  metencephalon 
My.,  myelencephalon  ;  n.,  notochord  (degenerating)  ;  o.,  oral  membrane  (oral  plate)  ;  oe.,  oeso- 
phagus ;  p.,  pharynx  ;  P.,  parencephalon  ;  r.,  optic  recess  ;  S.,  Seessel's  pocket  (preoral  gut)  ; 
st.,  stomach  ;  sv.,  sinus  venosus  ;  Sy.,  synencephalon  ;  T.,  telencephalon  ;  t h.,  rudiment  of  thyroid 
body  ;  tp.,  tuberculum  posterius  ;  v.,  ventricle  ;  vt.,  velum  transversum. 

steadily,  and  finally  its  roof  becomes  transformed  into  the  cerebellum 
and  its  floor  into  the  pons  Varolii. 


430  AN   INTRODUCTION  TO  ZOOLOGY 

The  posterior  division  of  the  brain- is  the  myelencephalon.  It 
includes  the  seventh  to  the  eleventh  neuromeres,  and  its  roof  remains 
very  thin,  non-nervous,  and  is  finally  transformed  into  the  choroid 
plexus  of  the  fourth  ventricle.  Its  walls  and  floor  thicken  enormously 
later  on  to  constitute  the  medulla  oblongata. 

Behind  the  hind-brain  the  neural  canal  continues  backwards 
and  shows  practically  no  signs  of  segmentation.  Even  from  the 
first  its  sides  are  thicker  than  its  roof  and  floor,  so  that  the  lumen  is 
elongated  in  the  dorso-ventral  direction.  During  the  first  three  days 
of  incubation  the  walls  increase  slightly  in  thickness,  but  the  chief 
development  is  to  be  found  in  the  cells  composing  them,  which  have 
differentiated  into  two  varieties  :  epithelial  cells  lining  the  tube  and 
stretching  to  its  outer  limits,  and  more  rounded  germinal  cells 
occupying  the  interstices  between  them.  From  the  former  come  the 
epithelial  cells  forming  the  lining  of  the  central  canal,  and  known  as 
the  ependyma.  The  germinal  cells,  on  the  other  hand,  provide  the 
actual  nerve  cells  constituting  the  grey  matter  of  the  spinal  cord, 
and  these  neuroblasts,  as  they  are  termed,  develop  into  typical 
ganglion  cells. 

Sense  Organs, 

The  three  main  organs  of  special  senses,  taking  them  in  the  order 
in  which  they  appear,  are  the  eye,  the  ear  and  the  olfactory  organs. 

Eye.  :.  , 

Early  in  the  second  day,  before  the  neural  folds  have  met,  the 
lower  side  walls  of  the  fore-brain  region  show  distinct  outbulgings. 
These  are  more  marked  upon  the  closure  of  the  folds,  and,  as  has 
been  noted  above,  the  aperture  between  them  and  the  fore-brain 
cavity  becomes  considerably  constricted,  forming  at  last  a  tubular 
optic  stalk.  The  distal  portion  is  dilated  to  form  the  primary  optic 
vesicle,  and  its  outer  wall  almost  touches  the  ectoderm,  which  com- 
mences to  thicken  in  this  region  at  quite  an  early  stage,  giving  rise 
to  the  lens  rudiment.  The  outer  and  ventral  wall  of  the  vesicle 
also  thickens  and  sinks  inwards  towards  the  inner  wall.  Simul- 
taneously with  this  invagination  process  the  rudiment  of  the  lens 
also  sinks  in  to  form  a  thick-walled  depression  that  at  first  almost 
fills  the  inside  of  the  cavity  on  the  outside  of  the  optic  vesicle.  The 
invagination  continues  until  the  thicker  originally  outer  wall  comes 
to  lie  close  against  the  thinner  original  mesial  wall,  and  so  the  cavity 
of  the  vesicle  is  obliterated.  In  this  way  a  double-walled  optic  cup 
or  secondary  optic  vesicle  is  formed,  whose  cavity  represents  the 
posterior  or  vitreous  chamber  of  the  eye.  At  the  same  time  the 
lens  invagination  gets  deepei,  becoming  transformed  into  a  relatively 


LATER  DEVELOPMENT  OF  CHICK  AND   RABBIT    431 

large  pit.  The  edges  of  this  come  together,  and  finally  it  becomes 
cut  off  from  the  superficial  ectoderm  as  a  thick-walled  spherical 
vesicle,  that  almost  fills  the  opening  of  the  optic  cup  by  about  the 
end  of  the  third  day.  The  inside  thickened  layer  of  the  optic  cup 
is  the  beginning  of  the  sensitive  layer  of  the  retina,  while  the  outside 
part  is  the  pigment  layer,  and  pigment  is  formed  in.  it  during  the 
fourth  day. 

It  has  been  noted  that  not  only  is  the  original  outer  wall  of  the 
primary  vesicle  involved  in  the  invagination,  but  also  the  ventral 
wall  back  to  the  optic  stalk.  Thus  it  comes  about  that  when  the 
optic  cup  is  first  formed  it  is  incomplete  on  the  ventral  side.  The 
adjacent  regions  expand  and  fill  in  this  gap  to  a  certain  extent,  but 


Lens  p-Cfi 


Lj, 


Lf/B. 


FIG.  151. — -Transverse  section  through  the  eyes  and  heart  of  an  embryo  of 
about  thirty-five  pairs  of  somites. — From  Lillie. 

ch.Fis.,  choroid  fissure  ;  D.C.,  duct  of  Cuvier  ;  Lg.,  lung  ;  pl.gr.,  pleural  groove  ;  V.c.,  posterior 
cardinal  vein  ;  Y.S.,  yolk-sac ;  Ao.,  aorta  ;  Chor.,  chorion  ;  Atr.,  atrium  ;  Dienc.,  diencephalon; 
p.C.,  parietal  cavity  ;  p.  Ch.,  posterior  chamber  of  the  eye  ;  am.,  amnion;  B.A.,  bulbus  arteriosus. 


for  quite  a  while  a  narrow  cleft,  termed  the  choroid  fissure,  is  left 
running  from  the  rim  of  the  cup  back  to  the  optic  stalk.  The 
presence  of  this  fissure  is  of  considerable  importance  to  the  future 
course  of  development  of  the  eye.  In  the  first  place  it  leaves  the 
whole  of  the  inside  layer  of  the  retina  in  communication  with  the 
stalk,  and  so  when  the  nerve  cells  of  the  inside  of  this  layer  send  out 
their  axons  these  can  grow  back  along  the  stalk  as  the  optic  tract 
into  the  brain.  This  leads  to  the  disappearance  of  the  cavity  of  the 
stalk,  and  its  transformation  into  the  definitive  optic  nerve.  Secondly, 
the  fissure  allows  that  part  of  the  adjacent  mesoderm  from  which 
the  vitreous  humour  will  be  derived  to  spread  into  the  optic  cup, 
and  it  also  allows  for  the  ingrowth  of  the  retinal  arteries  and 
veins. 


432  AN   INTRODUCTION   TO  ZOOLOGY 

At  a  later  stage  the  lens  withdraws  from  the  ectoderm,  leaving  a 
space  representing  the  anterior  or  aqueous  chamber.  The  super- 
ficial ectoderm  becomes  thinner  and  transparent,  forming  the 
conjunctiva.  Thus  it  will  be  seen  that  the  parts  of  the  eye  essential 
to  vision,  i.e.  the  conjunctiva,  the  lens,  the  retina  and  the  optic 
nerve,  are  derived  directly  from  the  ectoderm  as  in  the  case  of  the 
first  two,  or  indirectly  from  it,  via  the  brain  in  the  case  of  the  second 
two.  All  the  remaining  or  accessory  parts  are  derived  from  the 
mesoderm. 

Ear. 

At  thirty  hours,  or  soon  after,  a  thickened  patch  of  ectoderm 
appears  on  each  side  of  the  hind-brain,  just  in  front  of  the  first  pair 
of  somites  and  nearly  above  the  hyoid  arch.  These  are  the  auditory 
plates,  the  rudiments  of  the  membranous  labyrinths.  These  plates 
increase  in  size  and  sink  in  the  middle  to  form  the  auditory  pits, 
whose  edges  approximate  until  by  the  third  day  we  find  a  large 
auditory  sac,  formed  and  connected  with  the  surface  by  a  narrow 
tubular  canal.  This  canal  is  the  endolymphatic  duct,  homologous 
with  the  similarly  named  structure  in  the  dogfish.  At  the  end  of  the 
third  day  it  loses  its  connection  with  the  exterior,  and  so  the  auditory 
sac  comes  to  lie  freely  in  the  head  mesenchyme.  Some  days  later 
the  distal  end  of  the  ductus  enlarges  to  form  an  endolymphatic  sac, 
that  finally  extends  along  the  mesoderm  above  the  dorsal-lateral 
surface  of  the  myelencephalon.  The  auditory  sac  elongates  dorso- 
ventrally,  and  soon  an  internal  ridge  appears,  marking  its  division 
into  a  dorsal  portion,  the  utriculus,  and  a  ventral  region,  the  sacculus. 
At  first  the  end  of  the  duct  opens  into  the  upper  corner  of  the' 
utriculus,  but  this  part  subsequently  expands  dorsally  on  the  outer 
side  of  the  ductus,  which  thus  comes  to  open  into  the  inner  side  of  the 
utriculus. 

About  the  fifth  day  three  narrow  grooves  appear  in  the  superior 
chamber  in  the  relative  positions  of  the  semicircular  canals.  They 
grow  out  as  thin  hollow  expansions,  their  inner  margins  fuse  save 
at  two  ends,  thus  converting  them  into  tubular  canals  opening  at 
each  end  into  the  utriculus.  Gradually  they  move  outwards,  carry- 
ing along  the  line  of  fusion  of  their  edges  a  thin  sheet  of  the  sac 
wall  with  them,  but  with  the  breaking  down  of  this  sheet  and  the 
enlargement  of  one  end  of  each  to  form  an  ampulla  they  become 
transformed  into  typical  semicircular  canals.  The  sacculus  gives 
rise  to  an  outgrowth  that  later  develops  into  the  cochlea.  At  first 
the  walls  of  the  auditory  sac  are  moderately  thick,  but  with  its 
enlargement  they  become  much  thinner  save  in  certain  areas,  which 
mark  the  positions  of  the  maculae,  cristee  and  papillae  of  the  adult  ear. 


LATER  DEVELOPMENT   OF   CHICK   AND   RABBIT     433 

The  essential  part  of  the  ear  then,  like  the  eye,  is  derived  from  the 
ectoderm. 

From  the  mesenchyme  surrounding  the  auditory  sac  are  developed, 
first,  a  membranous  covering  for  the  labyrinth,  with  which  it  becomes 
closely  associated  ;  secondly,  a  loose  tissue  that  becomes  converted 
into  perilymph,  and  outside  this  a  denser  layer,  in  which  the  cartilage 
and  afterwards  the  bone  is  laid  down. 

The  hyomandibnlar  pouch  arises  in  two  portions  :  a  large  ventral 
part  similar  to  the  remaining  gill  pouches,  and  a  smaller  dorsal 
portion  which  is  perforated  only  for  a  short  time.  The  ventral 
portion  is  transitory,  while  the  dorsal  piece  is  persistent  throughout 
life  as  a  part  of  the  tympanic  cavity.  The  remaining  portion  of  the 
cavity  and  the  Eustachian  tube  are  differentiated  from  the  dorsal 
wall  of  the  pharynx  in  this  region.  The  external  auditory  meatus 
arises  as  an  ectodermal  ingrowth  from  the  surface  of  the  head  in  a 
region  that  was  originally  between  the  dorsal  and  ventral  portions  of 
the  hyomandibular  pouch. 

Olfactory  Organ. 

The  olfactory  organs  take  their  origin  in  much  the  same  way 
as  the  ear,  as  a  pair  of  thickened  patches  of  ectoderm,  the  olfactory 
plates,  lying  on  the  side  of  the  head  in  the  fore-brain  region  in  front 
of  the  eye.  They  appear  towards  the  close  of  the  second  day.  The 
plate  invaginates  and  forms  a  fairly  deep  olfactory  pit  with  a  wide 
opening,  which,  owing  to  a  more  rapid  growth  of  the  dorsal  region  of 
the  head,  comes  to  lie  on  the  ant ero- ventral  side  of  the  head.  The 
openings  of  the  two  pits  are  separated  in  the  middle  line  by  a  broad 
band  of  tissue,  the  fronto-nasal  process,  which  thus  forms  their 
inner  margins  as  well  as  the  anterior  boundary  of  the  primitive 
mouth.  The  lateral  wall  of  the  aperture  is  elevated  during  the 
fourth  day  to  form  the  external  nasal  process.  In  the  fifth  day  this 
external  process  becomes  linked  with  fronto-nasal  process  by  a  bridge 
of  tissue  that  separates  the  olfactory  opening  into  parts,  one  above  it 
and  one  below.  The  bridge  itself  is  a  rudiment  of  part  of  the  upper 
jaw,  and  consequently  as  it  enlarges  the  two  apertures  become 
more  and  more  widely  separated  ;  the  upper  one  moving  dorsally 
to  form  the  external  nans,  and  the  lower  one,  lying  in  the  stomodceal 
area,  passes  into  the  buccal  cavity  to  become  the  internal  naris. 

Lepus. 

In  the  previous  chapter,  when  considering  the  development  of  the 
mammal,  only  the  changes  in  the  embryo  itself  were  taken  into 
account,  and  it  was  indicated  that  while  these  were  in  progress  others 

2  F 


434  AN   INTRODUCTION   TO  ZOOLOGY 

more  external  and  extra-embryonic  were  going  on.  These  lead  to 
the  formation  of  the  characteristic  membranes  enclosing  the  embryo 
and  its  attachment  to  the  uterine  wall,  and  we  must  now  consider 
them.  The  membranes  are  on  the  whole  closely  similar  to  those  in 
the  chick  in  origin,  but  have  a  quite  different  fate.  The  amnion 
loses  most  of  its  value  as  a  protection,  the  yolk  sac  is  practically 
a  vestigial  remnant,  the  allantois  loses  most  of  its  respiratory  and 
excretory  functions,  being  mainly  concerned  with  bringing  the 
embryo  into  relation  with  its  food  supply,  and  the  chorion  either 
wholly  or  in  part  becomes  the  chief  organ  of  nutrition  and  excretion. 
There  is  considerable  diversity  in  the  parts  played  by  the  various 
structures  in  different  groups  of  mammals. 

By  the  time  the  ovum  reaches  it  the  mucous  membrane  lining 
the  wall  of  the  uterus  has  become  enlarged,  highly  vascularised,  and 
thrown  into  a  series  of  folds,  and  the  trophoblastic  wall  of  the  vesicle 
is  soon  brought  into  contact  with  it.  In  the  rabbit  and  certain 
other  forms  it  becomes  attached  to  the  wall  of  the  main  cavity  of  the 
uterus,  a  condition  known  as  central  implantation.  In  man  and  other 
species,  however,  the  vesicle  bores  its  way  through  the  superficial 
layers  of  the  mucous  membrane,  which  closes  over  behind  it,  and  so 
it  becomes  buried,  a  condition  known  as  interstitial  implantation. 
In  the  rabbit  a  horse-shoe-shaped  area  of  the  trophoblast,  behind  and 
lateral  to  the  embryonic  rudiment,  becomes  modified  to  form  a  layer 
of  enlarged  actively  growing  cells,  termed  the  trophoderm,  and  this  is 
responsible  for  its  attachment  to  the  mucosa.  In  man  this  change 
takes  place  over  the  whole  surface  of  the  vesicle. 

Amnion  and  Chorion. 

The  processes  resulting  in  the  formation  of  the  Amnion  and  the 
Chorion  in  the  rabbit  closely  resemble  those  in  the  chick.  The  wing- 
like  extension  of  the  mesoderm  from  the  axial  line  leads  to  the 
formation  of  quite  a  large  proamniotic  area.  The  first  of  the  folds 
to  appear  is  the  tail  fold,  and  while  this  is  in  progress  the  head  bends 
down  and  pushes  its  way  into  the  proamniotic  area,  whose  anterior 
margin  soon  rises  up  into  a  fold.  Lastly,  the  lateral  folds  appear 
joining  the  preceding  ones,  and  they  all  grow  upwards  to  fuse  over 
the  top  of  the  embryo.  The  fusion  is  more  complete  than  in  the 
chick,  and  so  only  a  small  sero-amniotic  knot  is  left.  The  separation 
of  the  walls  of  the  folds  then  leads  to  the  formation  of  (a)  the  amnion 
immediately  above  the  embryo,  and  (b)  the  chorion  in  close  contact 
with  the  trophoblast.  Within  the  former  is  the  amniotic  cavity, 
and  between  it  and  the  chorion  is  the  extension  of  the  extra- 
embryonic  ccelom.  The  relations  of  ectoderm  and  mesoderm  in 
these  two  folds  is  just  as  in  the  chick.  The  mesoderm  later  invades 


LATER   DEVELOPMENT   OF   CHICK  AND   RABBIT    435 

the  proamniotic  area,  and  the  folds  pass  in  under  the  embryo  until 

h     ta 


pa 


vb 


FIG.  152. — Diagrams  of  the  formation  of  the  embryonic  membranes 
and  appendages  in  the  rabbit. — From  Kellicott,  after  Van 
Beneden  and  Julin  (partly  after  Marshall). 

Sagittal  sections.  A,  at  the  end  of  the  ninth  day  ;  B,  early  the  tenth  day  ;  C,  at  the  end  of 
the  tenth  day.  Ectoderm  black  ;  endoderm  dotted  ;  mesoderm  grey,  al.,  allantois  ;  as.,  allan- 
toic  stalk  ;  b.,  tail  bud  ;  c.;  heart  ;  d^  trophoderm  ;  e.,  entoderm  ;  ex.,  exoccelom  ;  /.,  fore-gut  ; 
h.,  hind-gut  ;  m.,  mesoderm  ;  n.,  central  nervous  system  ;  p.,  pericardia!  cavity  ;  pa.,  proamnion  ; 
s.,  marginal  sinus  (sinus  terminalis)  ;  t.,  trophoblast  ;  ta.,  tail-fold  of  amnion  ;  ».,  trophodermal 
villi  ;  vb.,  trophoblastic  villi ;  y.,  cavity  of  yolk  sac  ;  ys.,  yolk  stalk. 

they  completely  surround  it,  save  at  the  places  where  the  allantoic 
and  yolk  sac  stalks  leave  it.     In  the  rabbit  the  amniotic  cavity  never 


436  AN   INTRODUCTION   TO   ZOOLOGY 

becomes  very  large,  but  the  extra-embryonic  ccelom  or  exocoel  dilates 
until  it  practically  fills  the  cavity  of  the  blastodermic  vesicle. 

In  man  the  formation  of  the  amnion  is  not  accompanied  by  the 
production  of  folds,  but,  as  pointed  out  in  the  last  chapter,  the 
primitive  amniotic  cavity  appears  as  a  space  between  Rauber's 
layer  and  the  embryonal  shield.  The  manner  of  its  formation  is 
such  that  no  proamnion  appears.  The  final  relations  of  the  folds 
are  the  same  in  the  two  cases,  however,  in  spite  of  their  different 
origin.  The  human  amniotic  cavity  enlarges  very  rapidly,  and 
finally  fills  the  whole  of  the  inside  of  the  vesicle  save  for  the  part 
occupied  by  the  yolk  sac  and  allantoic  stalks,  which  therefore  come 
to  be,  as  it  were,  bound  together  to  form  a  combined  structure,  the 
umbilical  cord.  The  dilatation  of  the  amnion  is  so  great  that  it 
completely  obliterates  the  exoccel,  and  the  external  mesoderm  layer 
of  the  amnion  comes  to  lie  close  to  the  inner  mesoderm  of  the  chorion 
and  the  two  fuse. 

In  the  rabbit  a  vascular  area  is  established  in  a  normal  way  in 
the  splanchnopleure,  and  it  is  bounded  by  the  sinus  terminalis. 
The  rapid  extension  of  the  exocoel,  which  soon  reaches  the  sinus 
terminalis,  limits  the  extension  of  the  vascularisation  to  the  upper 
side  of  the  yolk  sac.  Beyond  the  vascular  area  the  mesoderm  never 
extends,  so  that  the  remaining  hemisphere  of  the  vesicle,  termed  the 
omphalopleure,  is  simply  bilaminar.  Thus  the  chorion  is  limited 
to  the  upper  regions. 

Yolk  sac, 

At  an  early  stage  the  yolk  sac  in  the  rabbit  occupies  the  main 
part  of  the  blastodermic  vesicle,  and  its  splanchnopleuric  portion  is 
separated  from  the  chorion  by  the  exoccel.  In  this  species  the 
entoderm  of  the  sac  develops  but  slowly,  so  that  for  a  considerable 
time  it  is  incomplete  on  the  ventral  side.  Finally,  it  is  completed, 
and  this  last  portion  lies  in  contact  with  the  chorionic  ectoderm  and 
the  mesoderm  but  slowly  pushes  its  way  in  between  them.  Thus 
it  is  that  in  this  form  the  yolk  sac  is  only  splanchnopleuric  in  its 
upper  portion,  while  below  its  entoderm  is  in  contact  with  the 
blastodermic  ectoderm.  The  extra-embryonic  ccelom  expands 
markedly  in  the  rabbit,  and  so  compresses  the  cavity  of  the  yolk 
sac  until  this  structure  finally  takes  on  a  sort  of  umbrella  shape. 
The  long  narrow  yolk  sac  stalk  represents  the  handle,  and  the 
flattened  expanded  sac  itself  the  cover.  The  splanchnopleure 
becomes  richly  vascularised  and  has  a  well-developed  sinus  terminalis, 
and  its  main  vessels  are  termed  the  vitelline,  or,  perhaps  more 
frequently,  the  omphalo-mesenteric  arteries  and  veins  respectively. 
The  veins  penetrate  the  liver  and  enter  the  hinder  portion  of  the 


LATER  DEVELOPMENT  OF  CHICK  AND  RABBIT  437 

sinus  venosus.     In  the  adult  their  embryonic  portions  form  the 
hepatic  portal  veins. 

In  man  the  yolk  sac  is  very  different :  it  is  completed  at  a  fairly 
early  stage,  and  is  quite  small.  It  never  reaches  the  far  side  of  the 
blastodermic  vesicle,  and  so  appears  as  a  small  bladder  suspended 
in  the  exoccel  by  the  yolk  sac  stalk.  It  then  diminishes  in  size,  and 


V. 


AS 


N 


FIG.  153. — Generalised  diagram  of  the  foetal  membranes  of  a  mammal, 
adapted  from  Hertwig. 

A.,  amnion  ;  A.C.,  amniotic  cavity  ;  Al.,  allantois  ;  Al.C.,  allantoic  cavity  ;  Al.S.,  allantoic 
stalk;  B., brain;  C.,chorion  ;  E.,enteron  ;  E. E.,  embryo  nic  ectode  rm  ;  En. ,  embryonic  entoderm  ; 
M.,  embryonic  mesoderm  ;  N.,  notochord ;  N.C.,  neural  canal ;  T.,  trophoblast ;  V.,  villus ; 
Y.,  yolk  sac  ;  Y.S.,  yolk  sac  stalk. 

as  the  expansion  of  the  amniotic  cavity  brings  about  the  formation 
of  the  umbilical  cord,  the  yolk  sac  becomes  first  of  all  reduced  to  a 
small  blind  ccecum  and  finally  disappears  altogether. 

Allantois. 

The  allantois  is  also  subject  to  considerable  variation  among 
various  groups  of  the  Mammalia,  but  on  the  whole  its  relations  are 


AN   INTRODUCTION  TO  ZOOLOGY 

similar  to  those  in  the  chick.  Despite  the  differences  in  its  fate  in 
various  forms  it  is  always  of  importance,  because  its  blood-vessels 
form  the  vascular  supply  of  the  placenta  and  are  very  similar 
throughout  the  mammalian  series.  The  allantois  appears  as  an 
outgrowth  of  the  hind  gut  and  expands  freely  in  the  exoccel,  which  it 
traverses  and  reaches  the  chorion  in  the  trophodermic  region. 
It  unites  with  this  layer  to  form  the  allanto-chorion,  and  so  almost 
from  its  beginning  forms  a  link  between  the  embryo  and  the  portion 
of  the  blastoderm  most  intimately  related  to  the  uterine  mucosa. 
The  vessels  consist  of  a  pair  of  umbilical  arteries  and  a  pair  of 
umbilical  veins,  and  through  them  the  chorionic  region,  previously 
without  vessels,  becomes  highly  vascularised.  This  region  is 
destined  to  form  the  placenta,  which  therefore  receives  its  embryonic 
blood  supply  via  the  allantonic  vessels,  and  it  is  important  to  re- 
member that  this  is  true,  however  small  or  large  the  actual  allantois 
may  be. 

Placenta. 

The  term  placenta  is  applied  to  the  organ  that  in  the  Mammalia 
forms  the  connection  between  the  lining  wall  of  the  maternal  uterus 
and  the  membranes  surrounding  the  growing  embryo.  From  the 
entirely  different  mode  of  development  in  the  chick  it  is  obvious 
that  this  structure  is  not  represented  in  it.  Structurally  the 
placenta  is  a  complex,  consisting  of  a  very  close  apposition  between 
the  foetal  membranes  and  the  uterine  tissues,  or  more  frequently 
and  typically  it  is  an  actual  and  complicated  interpenetration  of  the 
two  that  results  in  bringing  their  blood  streams  in  close  proximity 
to  one  another.  Functionally  it  serves  in  the  early  stages  to  anchor 
the  embryo  to  the  uterine  wall,  and  later  it  is  the  centre  wherein  the 
dissolved  salts  and  nutrient  material  of  the  parental  blood  can 
transfuse  into  the  foetal  blood,  thus  providing  the  materials  necessary 
for  growth.  Also  it  provides  a  means  whereby  the  excretory  matters, 
both  nitrogenous  substances  and  carbon  dioxide,  can  diffuse  from 
the  fcetal  to  the  maternal  blood,  and  so  it  also  serves  as  an  organ 
of  excretion. 

In  the  early  stages  of  development  the  vascular  walls  of  the  yolk 
sac  may  play  a  part  in  these  functions,  and  so  we  speak  of  a  yolk  sac 
or  omphalopleural  placenta,  but  sooner  or  later  in  all  the  higher 
mammals,  i.e.  the  Eutheria,  an  allantoic  placenta  is  developed,  and 
this  is  one  of  the  distinguishing  characteristics  of  the  order. 

We  have  already  considered  briefly  different  forms  of  behaviour 
in  the  early  implantation  of  the  developing  ovum  ;  it  may  be 
central,  or  interstitial,  or  in  certain  cases  it  is  eccentric,  that  is  to  say, 
lies  in  a  fold  of  the  mucosa  to  one  side.  It  is  obvious  that  this  will 


LATER  DEVELOPMENT   OF   CHICK   AND   RABBIT     439 

affect  the  relation  of  the  membranes  to  the  uterine  wall,  and  so  the 
type  of  placenta  formed,  but  it  gives  no  indication  of  the  enormous 
variations  that  are  to  be  met  with  in  the  details  of  the  origin,  forma- 
tion and  final  constitution  of  this  highly  important  structure. 
These  are  matters  of  considerable  complexity,  and  outside  the  scope 
of  this  work,  so  that  we  can  only  consider  the  outlines  of  them  in  the 
case  of  the  rabbit. 

The  uterine  mucosa  in  the  non-pregnant  rabbit  is  arranged  in  a 
series  of  longitudinal  folds,  of  which  two,  situated  one  on  each  side 


Ex 


Ex 


YE 


FIG.  154. — Diagram  of  section  of  fully  formed  blastodermic  vesicle  of 
Lepus,  adapted  from  Hertwig. 

A.,  Amnion  ;  Al.,  allantois  ;  Al.S.,  allantoic  stalk  ;  E.,  embryo  ;  Ex.,  exoccel ;  S.T.,  sinus 
terminalis  ;  T.,  trophoblast ;  V.,  villi ;  V.L.,  vasuclar  layer  of  yolk  sac  ;  Y.,  yolk  sac  ;  Y.E.,  yolk 
sac  entoderm  ;  Y.S.,  yolk  sac  stalk. 

of  the  line  of  attachment  of  the  broad  ligament,  are  larger  and  more 
important  than  the  remainder  :  they  are  termed  the  placental  ridges. 
They  are  separated  by  a  groove,  and  generally  when  the  blastocyst 
adheres  to  the  uterine  wall  it  is  in  such  a  position  that  the  embryo 
is  near  and  parallel  to  this  groove.  As  we  have  seen  previously,  a 
horse-shoe-shaped  band  of  trophoderm  is  established  at  an  early 
date,  and  this  lies  in  the  region  touching  the  ridges.  The  cells  of  this 
delimited  area  multiply  rapidly  and  fuse,  so  that  they  form  a 
syncytium,  which  is  extremely  active,  and  in  which  cell  walls  cannot 
be  made  out.  The  uterus,  and  particularly  the  placental  ridges, 


440 


AN   INTRODUCTION  TO  ZOOLOGY 


enlarge  greatly  in  early  pregnancy,  and  the  ridge  capillaries  dilate 
considerably.  The  uterine  epithelium  in  touch  with  the  trophoderm 
disintegrates  and  is  absorbed,  and  the  syncytium  actively  grows 
down  into  the  underlying  mucosa  in  the  form  of  a  series  of  thin  plates 
arranged  in  the  form  of  an  irregular  honeycomb.  These  grow  in, 
absorbing  the  tissue  of  the  mucosa  as  they  do  so,  and  come  to  sur- 
round a  number  of  the  maternal  capillaries.  Finally,  even  the 


Pi. 


f.yk. 


FIG.  155. — 'Diagrammatic  transverse  section  of  a  pregnant  uterus, 
illustrating  the  later  phase  of  placentation. — From  Bourne. 

all.,  allantois  ;  am.,  amnion ;  bv.,  uterine  blood-vessels;  em.,  embryo;  gl.,  uterine  glands 
lu'2,  secondary  lumen  of  uterus  ;  ms.m.,  mesometrium  ;  mus.,  muscular  wall  of  uterus  ;  PL, 
allantoidean  placenta  ;  p.yk.,  proximal  wall  of  yolk  sac  ;  umb.,  stalk  of  yolk  sac  ;  Ut.,  uterus. 

walls  of  the  capillaries  break  down,  and  so  the  parental  blood  comes 
to  circulate  in  a  series  of  sinus-like  spaces  in  the  trophoderm. 

While  this  has  been  going  on  the  allantois  has  grown  out  and 
fused  with  the  chorion,  bringing  with  it  its  mesoderm  in  the  form  of 
a  richly  vascularised  mesenchyme.  This  mesenchyme  grows  down- 
wards between  the  trophoblastic  lamellae,  and  so  forms  a  series  of  long 
papillae,  known  as  villi.  Thus  we  have  a  series  of  tongues  of  mesen- 
chyme containing  embryonal  blood-vessels  and  surrounded  by  the 
trophoblastic  tissue,  in  whose  sinuses  circulates  the  maternal  blood. 


LATER   DEVELOPMENT  OF   CHICK   AND   RABBIT    441 

This  is  the  essential  arrangement  of  the  adult  placenta,  but  it  is 
somewhat  masked  by  the  facts  that  in  the  first  place  the  embryonic 
capillaries  enlarge  considerably,  and  in  the  second  the  trophoblastic 
tissue  and  allantoic  mesenchyme  thin  out  until  they  become  almost 
negligible.  So  it  is  that  the  embryonic  capillaries  are  practically 
bathed  in  maternal  blood,  and  an  interchange  of  substances  between 
the  two  blood  streams  is  simply  a  question  of  diffusion.  It  is 
important  to  notice,  however,  that  in  spite  of  their  proximity  the 
two  blood  streams  never  actually  intermingle,  owing  to  the  fact 
that  the  embryonal  capillaries  always  retain  intact  their  endothelial 
walls. 

We  see  then  that  the  placenta  when  fully  formed  is  a  very  intimate 
union  between  foetal  membranes  and  uterine  mucosa.  It  has  the 
form  of  a  flattened,  disc-shaped  (hence  the  term  placenta = a  flat 
cake),  spongy,  vascular  thickening  composed  of  two  lobes,  one  related 
to  each  original  placental  ridge.  The  embryo  is  attached  to  this  by 
means  of  the  allantoic  stalk  carrying  the  blood-vessels,  and  around 
which  the  yolk  sac  stalk  becomes  slightly  twisted  to  form  a  common 
umbilical  cord.  From  its  shape  this  type  of  placenta  is  termed 
discoidal,  and  it  is  somewhat  similar  but  not  bilobed  in  man. 
Owing  to  the  fact  that  the  fusion  is  so  complete,  a  superficial  part  of 
the  uterine  mucosa,  mainly  blood,  comes  away  from  the  deeper 
layers  with  the  allantoic  portion  at  birth,  and  so  the  placenta  is 
spoken  of  as  deciduate.  At  parturition  the  amnion  and  chorion 
rupture  and  the  embryo  is  born  attached  to  the  placenta  and 
remains  of  the  membranes  by  the  umbilical  stalk,  which  is  gnawed 
through  by  the  parent.  The  biting  squeezes  together  the  blood- 
vessels and  the  small  remnant  soon  shrivels  up,  leaving  only  an 
almost  indistinguishable  umbilical  scar  or  navel,  which  marks  the 
point  where  the  animal  when  an  embryo  was  attached  to  the 
placenta. 

We  have  seen  then  that  as  the  mammal  has  descended  from 
an  ancestor  laying  a  large  heavily  yolked  telolecithal  egg,  it  is  provided 
with  a  series  of  membranes  on  the  whole  very  similar  to  those  of  the 
chick  and  homologous  with  them.  Their  original  functions  in 
relation  to  the  absorption  of  the  yolk  and  protection  of  the  embryo 
disappear,  and  they  take  on  entirely  different  ones,  still  to  a  certain 
extent  protective,  but  in  the  main  concerned  with  excretion  and 
nutrition. 


CHAPTER  XVII 
EVOLUTION,   VARIATION   AND    HEREDITY 

THE  study  of  Biology  is  usually  divided  for  convenience 
into  Botany  and  Zoology,  and  so  far  we  have  been  concerned  in  the 
main  with  the  investigation  of  the  facts  of  these  subjects,  particu- 
larly as  they  relate  to  animals.  The  scientist  is  naturally  concerned 
with  facts,  for  these  of  themselves  are  of  great  importance  and  often, 
as  in  the  study  of  medicine,  of  practical  utility  in  conducting  the 
affairs  of  life.  But  while  in  the  early  stages  it  is  necessary  to  devote 
considerable  time  to  acquiring  a  knowledge  of  these  facts,  and  the 
methods  by  which  they  can  be  verified  and  fresh  ones  discovered, 
sooner  or  later  one  inevitably  tries  to  put  them  in  some  sort  of  order, 
to  relate  them  to  one  another  and  generally  to  try  and  find  the 
causes  that  are  working  to  produce  them.  This  leads  us  into  a  field 
of  work  where  theories  and  facts  are  intermixed,  often  to  such  an 
extent  that  it  is  impossible  to  draw  a  distinction  between  them. 
When  we  pass  on  to  examine  certain  of  these  theoretical  points  the 
separation  of  Botany  and  Zoology,  that  is,  the  treatment  of  plants 
and  animals  as  beings  that  can  be  placed  in  isolated  compartments, 
is  no  longer  convenient  or  possible.  We  may  indeed  draw  our  illus- 
trations mainly  from  one  or  other  kingdom,  but  they  are  only 
examples  and  as  a  rule  serve  to  illustrate  principles  that  apply 
directly  or  with  only  slight  modification  to  both.  Sometimes  we 
are  able  to  see  as  the  result  of  a  number  of  observations  that  certain 
definite  conditions,  which  we  term  the  cause  or  causes,  are  inevitably 
followed  by  the  appearance  of  definite  phenomena  which  we  call 
the  effect.  When  we  can  express  this  relationship  in  such  a  general 
way  that  it  includes  a  number  of  different  phenomena,  we  say  that 
we  have  formulated  a  natural  "  law."  It  will  be  seen,  however,  that 
the  word  "  law"  has  a  different  meaning  from  that  intended  when 
used  in  ordinary  conversation.  There  is  no  implication  that  there 
is  a  code  of  regulations  drawn  up,  the  breach  of  which  is  followed  by 
punishment,  but  simply  that  a  certain  effect  is  always  produced  as 
the  result  of  certain  previous  conditions. 

The  most  fundamental  of  the  theories  to  be  considered  are  those 
that  concern  Evolution,  or  better,  Organic  Evolution.     This  idea  of 

442 


EVOLUTION,    VARIATION   AND   HEREDITY         443 

successive  change  or  evolution  has  come  to  be  one  of  the  most 
widespread  generalisations  of  modern  times  and  is  not  limited  to 
the  sciences  in  which  it  was  first  propounded,  but  has  now  been  applied 
to  practically  every  branch  of  human  thought.  We  can  recognise 
clearly  three  distinct  sorts  of  evolution  :  (i)  Inorganic  evolution, 
that  is,  the  formation  of  the  world  and  the  inorganic  materials  in 
it ;  (2)  Organic  evolution,  that  is,  the  production  of  the  many  and 
varied  forms  of  living  beings  that  fill  the  world  to-day  ;  and  (3)  the 
evolution  of  things  with  which  man  has  dealt,  such  as  the  growth 
of  languages,  religions  and  human  institutions,  the  development  of 
art,  of  buildings,  of  machinery  and  so  on.  It  is  with  the  second  of 
these,  namely,  Organic  Evolution,  that  we  are  practically  entirely 
concerned. 

Two  very  different  things  must  be  clearly  borne  in  mind  from  the 
outset :  one  is  that  the  occurrence  of  organic  evolution  is  now  generally 
accepted  as  a  fact  by  all  who  study  even  a  little  biology  ;  and  the 
other  is  that  there  is  probably  no  explanation  of  it  as  a  process,  of 
the  exact  causes  at  work,  and  the  precise  manner  in  which  they 
operate  has  been  generally  admitted  to  be  satisfactory.  The  two 
things,  however,  that  is,  evolution  itself  and  the  theories  to  explain 
it,  must  be  kept  quite  distinct,  and  the  failure  to  do  so  has  led  to  much 
confusion  of  thought,  particularly  in  semi-popular  writings,  but  also 
in  some  scientific  works.  Thus  it  is  by  no  means  uncommon  to  find 
in  poorly  informed  quarters  a  statement  to  the  effect  that,  in  view 
of  the  criticisms  that  have  been  levelled  at  Darwin's  theory  of 
evolution,  evolution  itself  is  no  longer  believed  in  by  them.  Nothing 
could  be  further  from  the  truth,  for  the  fact  of  evolution  remains 
unchallenged  and  it  is  only  the  explanation  of  the  causes  at  work 
that  is  questioned. 

It  is  desirable,  in  order  to  appreciate  the  more  modern 
views  on  evolution  and  some  of  the  theories  that  have  been  put 
to  explain  it,  to  examine  quite  briefly  the  opinions  that  were  previously 
held  and  so  approach  our  present  conceptions  on  the  subject  from 
the  historical  standpoint. 

From  the  time  of  the  Greeks,  who  had  a  very  good  knowledge  of 
Biology  and  indeed  made  some  attempts  to  explain  the  origin  of 
the  diversity  of  living  things,  up  to  the  beginning  of  the  eighteenth 
century,  Biological  Science  was  in  a  chaotic  state  and  in  the  first 
part  of  that  period,  instead  of  advancing,  fell  back.  Throughout 
the  Middle  Ages  little  thought  was  given  to  the  subject,  and  it  was  a 
matter  of  general  belief  that  there  had  been  a  special  act  of  creation 
and  all  the  animals  and  plants  we  now  see  were  created  exactly  as 
they  are  at  present.  Each  particular  type  was  considered  fixed 
and  unalterable  and  spoken  of  as  a  species.  When  fossil  remains  of 


444  AN   INTRODUCTION   TO  ZOOLOGY 

animals  long  since  extinct  were  found,  they  naturally  presented  some 
difficulty  and  were  regarded  by  some  as  misfits,  animals  of  which 
models  had  been  made  and  then  rejected,  others  saw  in  these  fossils 
traps  laid  by  the  evil  one  for  the  undoing  of  the  faithful. 

In  the  revival  of  natural  sciences  in  the  eighteenth  century, 
Biology  became  once  more  a  subject  of  study  in  which  many 
valuable  observations  were  made,  and  it  soon  became  evident  that 
enquiring  minds  demanded  some  satisfactory  explanation  of  the 
enormous  variety  of  living  beings  inhabiting  the  earth. 

To  enable  biologists  to  deal  with  the  mass  of  observations  that 
had  gradually  accumulated  and  was  fast  being  added  to,  it  early 
became  necessary  for  them  to  be  reduced  to  some  sort  of  order  which 
in  the  first  place  meant  a  satisfactory  system  of  classification.  The 
English  naturalist,  John  Ray  (1627-1705),  was  the  first  to  really 
attempt  a  classification  of  living  things  on  the  basis  of  anatomical 
resemblance.  He  framed  a  definition  of  a  species  to  be  used  as  a 
unit,  and  upon  this,  subsequent  classifications  were  based.  Another 
noteworthy  contribution  that  he  made  was  the  separation  of 
flowering  plants  into  Monocotyledons  and  Dicotyledons,  and  he  also 
called  attention  to  the  important  fact  that  it  was  impossible  to  rely 
entirely  upon  one  organ  in  a  group  of  organisms  in  any  system  of 
classification. 

Karl  Linne  or  Linnaeus  (1741-1789),  a  Swede,  rendered  the 
growing  biological  science  a  great  service  by  devising  a  system  of 
naming  animals  and  plants.  To  each  he  gave  two  names,  the 
first  or  generic  name  designating  the  genus  or  group  of  similar 
types  to  which  it  belonged,  and  the  second  or  specific  name 
designating  the  species  or  collection  of  almost  identical  forms  among 
which  it  could  be  included.  He  himself  held,  "  There  are  as  many 
different  species  as  there  were  different  forms  created  in  the  beginning 
by  the  Supreme  Being."  The  species  was  thus  considered  as  estab- 
lished once  and  for  all  without  the  possibility  of  being  changed. 
He  did  a  great  amount  of  important  work,  naming  and  describing 
a  large  number  of  animals  and  plants  and  arranging  them  in  groups, 
and  many  of  his  names  are  retained  to-day.  In  consequence  of  this 
his  name  had  considerable  authority,  and  his  idea  of  the  fixity  of 
species  was  widely  accepted. 

Cuvier  (1769-1832)  carried  the  Linnaean  system  a  step  further, 
by  grouping  genera  together  in  larger  categories  united  by  a  common 
basis  of  structural  similarity,  and  in  this  way  laid  the  foundations 
of  the  science  of  Comparative  Anatomy.  Further,  he  was  the  first 
biologist  to  study  fossil  forms,  and  more  noteworthy  still,  he  dis- 
covered the  striking  palaeontological  fact  that  the  lower  and  conse- 
quently more  remote  the  layer  from  which  fossils  are  obtained  the 


EVOLUTION,   VARIATION   AND   HEREDITY         445 

j# 

less  the  fossil  species  resembled  modern  ones.  This  fact  he  was 
unable  to  interpret  correctly,  owing  to  his  belief  in  the  immutability 
of  species,  and  he  held  the  disappearance  of  the  fossil  forms  to  be  the 
result  of  sudden  cataclysms,  such  as  earthquakes  or  floods,  which 
annihilated  successive  faunae  or  sets  of  animals  living  on  the  earth. 

Another  French  naturalist,  Buff  on  (1708-1788),  opposed  this  view 
of  sudden  geological  changes,  and  maintained  that  these  fossil  remains 
were  the  result  of  topographical  changes  in  the  configuration  ot 
land  aijd  water  and  to  climatic  changes,  leading  to  the  extinction 
of  certain  forms  of  life. 

Several  thinkers,  notably  Goethe,  1790,  Oken  and  Erasmus 
Darwin,  1794  suggested  the  possibility  of  change  or  transmutation. 
Goethe  held  that  the  various  organs  of  a  plant  were  modifications 
of  the  one  organ  the  leaf.  Darwin  called  attention  to  the  similarity 
between  the  arm  of  man  and  the  wing  of  a  bird,  and  claimed  that  this 
indicated  a  relationship  between  the  two.  Apart  from  these  men  the 
generally  accepted  opinion  was  that  species  were  unalterable. 

The  first  serious  attempt  to  grapple  with  the  problem 
was  made  by  Lamarck  (1744-1829),  in  his  "  Philosophic  Zoologique," 
in  1809,  a  little  more  than  a  century  ago.  In  this  book  we  see 
recognised  for  the  first  time  the  fact  that  animals  ate  not  just  isolated 
beings,  but  bear  some  relationship  one  to  another,  and  also  that  they 
can  be  arranged  in  a  series  from  the  most  primitive  to  the  highest 
form.  His  series  is  not  the  one  we  now  recognise,  but  it  was  a  series, 
and  indicated  an  evolution  which  the  author  not  only  recognised 
but  tried  to  account  for.  At  the  same  time  he  saw  that  vestigial 
structures  and  remarkably  well- developed  organs  needed  some  sort 
of  explanation.  He  held  that  new  forms  were  made  in  the  past 
and  are  still  being  made  by  the  modification  of  pre-existing  species. 
The  modifications  were  due  to  the  surroundings  in  which  the  animal 
lived  and  its  attempts  to  suit  its  life  to  them.  Thus  it  was  suggested 
that  the  giraffe  got  its  long  neck  by  constantly  trying  to  feed  on  leaves 
higher  and  higher  up  the  trees  ;  wading  birds  have  long  legs  because 
they  wished  to  go  into  the  water  after  their  food  but  at  the  same 
time  avoid  wetting  their  feathers.  Conversely  the  mole's  eyes  are 
very  small  because  it  no  longer  uses  them,  the  teeth  of  the  whale 
disappear  as  it  swallows  its  food  without  mastication,  and  so  on. 

These  views  were  based  on  the  well-known  fact  that  during  the 
life  of  an  individual  those  organs  that  are  used  increase  in  size, 
e.g.  the  arm  of  a  blacksmith,  and  those  that  are  not  used  decrease. 
The  actual  alteration  in  the  individual  would  be  small  but  it  would 
be  transmitted  to  the  offspring,  and  when  continued  generation  after 
generation  the  effect  would  be  cumulative.  The  particular  view  is 
termed  "  use  inheritance,"  or,  since  the  character  is  acquired  during 


446  AN   INTRODUCTION   TO  ZOOLOGY 

\ 

the  lifetime  of  the  individual  and  handed  on  to  the  young,  "the 
inheritance  of  acquired  characters,''  a  point  to  which  we  shall  return 
later.  No  satisfactory  evidence  has  been  obtained  to  show  that 
such  inheritance  occurs  at  any  rate  in  the  crude  form  in  which  it 
has  just  beenstated,  and  this  doctrine  and  all  it  implies,  usually  spoken 
of  as  Lamarckism,  is  not  generally  held  to-day.  It  shows  a  clear 
recognition  of  certain  important  underlying  principles  however. 
The  first  is  the  idea  of  modification  or  mutation  of  species,  the  second 
is  the  fact  that  structural  characteristics  are  handed  on  from  parent 
to  offspring,  and  thirdly  that  animals  are  on  the  whole  well  suited 
for  the  life  they  lead. 

Here  we  meet  for  the  first  time  with  a  definite  recognition  of  and 
expression  of  the  idea  of  Organic  Evolution,  which  has  since  been  much 
extended,  and  in  its  modern  form  states  that  the  various  members 
of  the  animal  and  vegetable  kingdoms,  as  we  know  them  to-day,  have 
not  existed  for  all  time,  but  are  the  result  of  a  long  continuous 
series  of  changes.     These  slow  changes  have  been  in  progress  since 
an  early  period  of  the  earth's  history,  and  are  still  going  on  and  will 
continue  until  organisms  cease  to  exist.     They  have  resulted  in  the 
production  of  higher  and  higher  forms  of  life,  or,  to  put  it  in  another 
way,  the  forms  we  see  to-day  have  been  evolved  or  developed  from 
lower  forms,  and  these  from  still  lower  ones,  and  so  on.     There 
has  been  a  gradual  progress  from  the  simple  to  the  more  and  more 
complex  and  specialised.    This  change  in  the  case  of  certain  domestic 
animals  and  plants  is  an  observed  and  observable  phenomenon 
even  in  the  course  of  a  man's  lifetime.     We  now  grow  in  our  gardens 
many  kinds  of  plants  that  were  unknown  to  our  forebears,  and,  we 
all  know,  new  "•  varieties,"  as  they  are  termed,  are  added  year  by 
year.     Not  only  does  this  apply  to  flowers  such  as  roses,  etc.,  but 
to  our  crops  like  wheat,  fruit  and  potatoes,  and  also  to  animals. 
Still  it  has  been  questioned  in  the  past  and  is  sometimes  questioned 
to-day  by  people  with  little  biological  knowledge,  whether  such  a 
conception  of  change  is  generally  applicable  to  animals  and  plants 
in  the  state  of  nature.     The  answer  is  undoubtedly  yes,  but  before 
going  on  to  consider  the  theories  which  attempt  to  explain  it,  we  shall 
do  well  to  stop  for  a  while  and  examine  the  evidences  for  the  occur- 
rence of  organic  evolution.     These  may  be  dealt  with  conveniently 
under  four  heads :  Anatomical,  Embryological,   Geographical  and 
Palseontological  evidence. 

Anatomical  Evidence. 

We  have  had  in  this  course  many  concrete  examples  of 
facts  relating  to  evolution,  so  that  it  is  only  necessary  to  call  attention 
to  the  general  plan  of  the  evidence.  We  find  that  animals,  and 


EVOLUTION,   VARIATION   AND   HEREDITY         447 

plants  too  for  that  matter,  are  not  simply  isolated  forms  unrelated 
to  one  another,  but  can  be  arranged  in  groups,  showing  marked 
similarity  among  themselves.  The  members  of  the  smaller  groups, 
the  genera,  are  very  closely  related  and  often  differ  mainly  in  size, 
proportion  of  parts,  and  colour.  Consider,  for  example,  the  cat  family, 
the  Felidce,  we  have  Felis  leo  the  lion,  F.  tigris  the  tiger,  F.  pardus 
the  leopard,  Ff  lynx  the  lynx,  F.  concolor  the  puma,  F.  domesticus  the 
domestic  cat,  and  other  less  known  species.  Every  one  is  sufficiently 
familiar  with  the  appearance  of  all  these  to  recognise  that  the  differ- 
ences between  them  are  mainly  those  of  size,  colour,  markings,  and 
proportion  of  parts.  They  are,  however,  all  obviously  similar 
animals,  and  indeed  in  a  standard  text-book  of  mammalian  anatomy 
we  read,  "  The  Tiger  (F.  tigris)  is  so  closely  related  to  the  Lion  that  it 
is  chiefly  by  external  characters  that  the  two  species  are  distin- 
guished." This  likeness  is  most  reasonably  explained  by  supposing 
them  all  to  be  the  modified  descendants  of  one  original  distant 
ancestral  "  cat."  Larger  groups,  e.g.  Amphibia,  Reptilia,  Aves, 
and  Mammalia,  although  differing  enormously  in  many  respects,  are 
very  obviously  built  on  the  same  general  plan.  They  have  a  skeleton 
consisting  of :  a  vertebral  column,  i.e.  a  number  of  essentially  similar 
bones  in  a  series  forming  the  main  support  of  the  body,  and  a  canal 
for  the  reception  of  the  spinal  cord  ;  a  skull,  i.e.  a  collection  of  bones 
giving  lodgment  to  the  brain,  the  olfactory  optic  and  auditory 
organs,  and  furnishing  a  pair  of  jaws  ;  pectoral  and  pelvic  girdles, 
giving  support  to  paired  pentadactyl  limbs,  and  so  on  with  the  other 
systems.  All  of  them  exhibit  the  same  basal  plan,  and  where 
differences  occur  we  can  see  very  often  that  the  change  enables  the 
animal  to  be  better  suited  to  its  environment.  The  most  obvious 
explanation  of  this  is  to  suppose  that  they  have  all  descended  from  an 
ancestral  form  that  exhibited  the  general  plan  in  a  simple  unmodified 
condition.  Similar  evidence  confronts  us  in  every  group,  large  or 
small,  in  the  animal  and  vegetable  kingdoms.  Why  is  it  that  our 
own  fore  limb,  the  fore  limbs  of  a  cat,  a  whale,  a  bat,  and  a  bird  are 
all  composed  of  essentially  similar  parts  although  adapted  for  quite 
different  purposes  ?  The  simplest  answer  is  that  they  all  represent 
modifications  of  one  and  the  same  thing. 

We  saw  in  Scyllium  that  the  ear  is  composed  of  a  complex  mem- 
branous labyrinth,  and  lies  near  the  first  gill  cleft,  and  is  concerned 
largely  with  equilibration.  In  the  frog,  which  no  longer  needs  a 
gill  cleft,  since  it  is  an  air-breathing  form,  the  outside  of  the  cleft  is 
covered  by  a  tightly  stretched  membrane,  the  tympanum,  which 
becomes  accessory  to  the  ear.  The  old  ear,  as  in  Scyllium,  is  still 
present  though  more  developed,  but  hearing  becomes  a  more  im- 
portant function,  and  for  increased  efficiency  further  structures,  the 


448  AN   INTRODUCTION  TO  ZOOLOGY 

tympanum,  columella  auris,  etc.,  are  added  to  it.  Finally,  in  the 
rabbit  the  process  of  improvement  is  carried  still  further,  and  we 
have  added  an  external  auditory  meatus,  a  pinna,  a  chain  of  ear  bones. 
Also  to  meet  the  much  higher  function  of  hearing  we  find  a  part  of 
the  labyrinth  specialised  to  form  the  cochlea,  a  very  complex  appa- 
ratus for  receiving  and  analysing  sound  vibrations.  So  that  we  can 
actually  trace  the  building  up  of  the  structures  of  the  higher  animals 
through  a  successive  series  of  stages  in  .lower  forms.  The  reverse 
is  also  true,  for  the  complex  branchial  skeleton  of  Scyllium  can  be 
traced  through  the  stage  in  Rana  up  to  that  in  the  mammal,  where 
it  has  been  reduced  to  the  hyoid  bone,  and  is  utilised  for  a  different 
purpose  from  its  original  one. 

Yet  another  form  of  evidence  is  that  supplied  by  what  is  known 
as  vestigial  structures.  We  all  of  us  possess  muscles  for  moving  our 
ears  and  scalp,  yet  only  a  few  are  able  to  employ  them,  and  even 
then  the  movements  serve  no  useful  purpose.  Why  should  such 
structures  be  present  at  all  ?  They  are  handed  down  to  us  from 
ancestors  to  whom  they  were  useful.  The  appendix  in  man,  the  tiny 
limbs  in  certain  snake-like  lizards,  a  small  spur  representing  a  hind 
limb  in  Boa  constrictor,  the  ligamentum  arteriosum,  and  a  multitude 
of  other  structures  are  to  be  similarly  explained. 

Embryological  Evidence. 

In  the  portions  dealing  with  embryology  another  variety 
of  evidence  has  been  brought  out.  Thus,  for  example,  we  find 
that  the  bird  and  the  mammal  pass  through  a  stage  of  develop- 
ment in  which  the  heart,  the  blood-vessels,  and  the  pharyngeal 
region  are  in  a  condition  resembling  that  found  permanently  in 
the  adult  fish,  so  that  the  whole  type  of  circulation  is  the  same 
and  adapted  to  the  aeration  of  the  blood  in  the  gills.  Attention 
was  first  called  to  facts  of  this  sort  in  "  Vestiges  of  the  Natural 
History  of  Creation,"  a  book  published  in  1844  by  Robert  Chambers, 
which  appeared  anonymously,  since  the  author  feared  it  would 
damage  his  business.  Observation  of  a  large  number  of  instances 
similar  to  this  soon  led  to  the  formulation  of  what  has  been  termed 
the  Recapitulation  Theory,  or  the  Fundamental  Law  of  Biogenesis. 
According  to  this  law,  the  reason  for  the  appearance  in  the  embryo 
of  conditions  or  structures  that  obtain  permanently  in  the  lower 
groups  is  that  each  animal  in  its  development  recapitulates,  or  at 
any  rate  hints  at,  the  past  development  of  the  race.  This  has  been 
expressed  in  the  aphorism,  Ontogeny  (the  history  of  the  individual) 
reproduces  Phylogeny  (the  history  of  the  race). 

It  is  unnecessary  to  labour  the  point,  however,  that  the  record  of 
the  race  history  kept  in  the  developmental  stages  of  any  animal  is 


EVOLUTION,   VARIATION   AND   HEREDITY          449 

modified,  blurred  and  incomplete.  For  while  there  is  a  circulatory 
system  in  the  chick  similar  to  that  in  a  water-dwelling  fish,  yet  the 
embryo  chick  is  not  a  fish,  and  could  not  utilise  this  arrangement  for 
aquatic  respiration.  The  history  is  often  enormously  modified  to 
suit  the  actual  requirements  of  the  young  animaL  and  so  on. 

Striking  applications  of  this  law  are  to  be  met  with  in  certain 
groups  of  the  animal  kingdom.  Thus  in  the  Cirripedia,  a  group 
containing  the  ship  barnacles  and  the  rock  barnacles,  and  in  the 
parasitic  Copepoda,  a  group  parasitic  on  fish,  crabs,  etc.,  the  adult 
animal  gives  practically  no  indication  of  its  relationships.  Yet  in 
both  cases  its  life  history  shows  that  it  is  really  a  much  modified 
crustacean,  i.e.  an  animal  allied  to  shrimps,  etc.  The  sea  squirts 
or  Ascidians,  almost  shapeless  masses  of  leathery  jelly  on  the  rocks, 
were  also  classified  satisfactorily  when  their  life  history  had  been 
elucidated.  Thus  this  law  has  aided  in  placing  various  groups  in 
their  proper  position  in  the  animal  series,  because  their  developmental 
history  has  revealed  their  relationship  to  other  forms. 

When  there  is  doubt  as  to  the  exact  place  that  any  species  holds 
in  a  genus,  use  can  often  be  made  of  the  fact  that  generally  the 
embryos  of  closely  related  species  resemble  one  another  more  nearly 
than  those  of  less  closely  related  species  or  than  the  adults  themselves 
do.  Similarly  we  find  that  embryos  of  closely  related  groups  tend 
to  resemble  one  another  more  closely  than  the  adult  members  of  fhe 
group. 

It  will  be  seen  then  that  all  these  phenomena  point  to  the 
conclusion  that  the  present-day  forms  have  been  derived  from 
less  specialised  pre-existing  forms,  and  that  from  one  such  ancient 
generalised  type  a  number  of  species  have  arisen.  There  is  no  logical 
reason  to  stop  at  the  species,  but  larger  and  larger  groups  with  a  com- 
mon structural  basis  have  come  from  a  more  remote  form,  and  so  on. 

Geographical  Evidence. 

On  studying  the  geographical  distribution  of  living  beings 
we  find  that  closely  allied  animals  and  plants  are  usually  to  be 
found  living  in  neighbouring  districts.  Conversely,  large  tracts  of 
land  usually  contain  a  number  of  allied  species  fairly  widely 
scattered  over  it.  When,  however,  a  sea  or  a  mountain  range  or  a 
desert  or  some  such  "  barrier  "  that  has  been  in  position  for  a  long 
time,  geologically  speaking,  intervenes,  the  animals  and  plants  on 
the  two  sides  of  it  are  in  general  not  closely  related.  This  is  because 
the  closely  allied  species  have  had  a  common  origin  and  have 
spread  and  spread  until  further  expansion  has  been  hindered  by 
barriers.  If  closely  allied  species  were  not  circumscribed  in  this 
way,  and  had  not  originated  in  the  same  place,  there  is  no  reason 

2  G 


450  AN   INTRODUCTION   TO  ZOOLOGY 

why  they  should  not  appear  in  all  parts  of  the  world.  Whereas  we 
find  in  Madagascar,  Australia  and  New  Zealand,  which  have  been 
separated  from  the  nearest  land  for  long  periods  of  time,  that  they 
have  a  number  of  animals  distinctive  of  themselves  and  not  found 
elsewhere.  In  the  case  of  Australasia  we  find,  for  example,  that  the 
lowest  mammals,  Echidna  and  Ornithorhynchus ,  are  limited  to  that 
region  and  found  nowhere  else.  When  we  do  find  species  of  animals 
that  are  widely  spread  there  is  an  explanation  forthcoming.  For 
example,  marine  forms  like  some  fish  live  in  the  sea,  which  does  not 
present  so  many  barriers  as  the  land  ;  or  birds,  butterfles,  etc.,  being 
winged  forms,  can  overcome  obstacles  that  would  prove  barriers 
to  other  less  easily  moving  forms. 

Apparent  contradictions  to  this  rule  are  to  be  met  with  in  certain 
groups,  e.g.  the  wingless  birds  which  appear  in  New  Zealand  (the 
Kiwi),  Australasia  (the  Emu  and  the  Cassowary),  Africa  (Ostrich), 
and  South  America  (the  Rhea  or  South  American  Ostrich)  ;  and 
again,  the  Marsupials  or  pouched  mammals  which  occur  in  Austral- 
asia and  South  America.  The  explanation  of  this  seeming  anomaly' 
is  that  both  groups  originated  in  or  near  the  land  masses  of  the 
Northern  Hemisphere  and  spread  widely,  as  their  fossil  remains  show. 
Later  on  more  highly  developed  forms  appeared  on  the  old  land  area, 
and  the  competition  drove  the  earlier  forms  to  more  and  more  remote 
places.  The  Northern  expansion  was  limited  by  climatic  factors,  and 
the  result  was  that  they  were  driven  into  the  southern  projecting 
points,  where  they  have  remained.  In  the  case  of  the  marsupials 
in  Australia,  the  land  connections  broke  down  before  the  higher 
mammals  reached  it,  so  that  the  marsupials  became  the  dominant 
forms.  Hence  we  see  that  the  apparent  exceptions  in  reality  only 
add  a  further  proof  of  this  gradual  change. 

Palceontological  Evidence. 

If  evolution  has  actually  taken  place,  and  we  could  obtain 
specimens  or  records  of  past  animals  and  plants,  then  of  course 
we  should  be  able  to  clinch  the  argument  by  showing  the  actual 
series  of  modifications.  Such  records  are  fortunately  preserved 
to  a  certain  extent  in  the  actual  remains  or  other  evidences  of 
pre-existing  animals  and  plants  embedded  in  the  rocks  of  the  earth, 
and  to  which  we  give  the  name  of  fossils.  Had  examples  of  all  past 
animals  been  preserved,  it  would  then  be  a  straightforward  though 
endless  task  to  show  how  all  forms  were  related  to  one  another  and 
bridge  the  gaps  at  present  separating  them,  by  intermediate  forms. 

A  little  thought  will  show  that  we  cannot  expect  the  geological 
record  to  be  complete.  Only  a  small  part  of  the  surface  of  the  earth 
is  possible  of  access,  and  only  an  infinitesimal  part  of  this  has  been 


EVOLUTION,   VARIATION   AND   HEREDITY          451 

thoroughly  dug  over  and  examined.  Of  the  millions  of  animals 
that  die  yearly,  but  few  are  so  situated  at  the  time  of  their  death 
that  they  can  be  preserved.  Preservation  usually  takes  place  in  a 
river  or  sea  deposit  or  in  formations  like  a  bog.  It  is  only  the  hard 
parts  that  will  be  preserved,  save  in  exceptional  cases,  and  so  as  a 
rule  only  those  animals  with  hard  parts  are  capable  of  leaving  a 
record  behind  them.  The  geological  record  is  for  these  and  many 
other  reasons  very  incomplete,  but  in  spite  of  this  the  evidence  it 
affords  is  overwhelming.  We  shall  now  examine  a  case  in  some  little 
detail  in  order  to  see  the  nature  of  the  evidence.  The  classical 
example  is  that  of  the  horse,  but  before  considering  the  actual  details 
of  this,  we  must  note  briefly  how  the  records  were  made. 

Geology,  the  study  of  the  earth's  crust,  tells  us  that  in  remote 
times  the  land  surface  of  the  globe  was  composed  of  granites  and 
basalts  and  similar  hard  rocks.  The  action  of  the  rain,  the  wind, 
the  gases  of  the  atmosphere,  etc.,  led  to  the  slow  breaking  up  of  these 
rocks,  and  the  loose  matter  was  carried  away  by  the  rivers  to  be 
accumulated  in  layers  or  deposits.  As  the  conditions  under  which 
these  deposits  were  made  varied,  so  the  character  of  the  layers 
themselves  altered.  We  find  these  sedimentary  rocks,  as  they  are 
termed,  superimposed  one  upon  another  in  a  succession  of  layers  or 
strata.  It  naturally  follows  from  their  mode  of  formation  that  the 
lower  the  stratum  is  in  a  series  the  older  it  is,  and  also  that  the  thick- 
ness and  number  of  layers  will  give  us  some  idea  at  any  rate  of  the 
time  taken  in  their  formation.  A  number  of  such  strata  have  been 
recognised  by  Geologists,  and  they  have  been  divided  into  three 
main  divisions  or  eras  :  the  Palceozoic,  the  Mesozoic,  and  the  Canozoic, 
each  representing  an  enormously  long  period  of  time,  and  each  com- 
posed of  a  number  of  smaller  periods.  It  is  only  the  last  of  these, 
the  Caenozoic  or  Tertiary  period,  that  immediately  concerns  us,  for 
no  distinct  ancestral  horse  form  has  been  found  prior  to  it.  This 
division  itself  is  divided  into  Eocene,  Oligocene,  Miocene,  Pliocene, 
Pleistocene,  and  Recent  periods,  each  of  which  is  in  turn  composed 
of  several  distinct  layers.  It  will  be  seen  then  that  under  certain 
conditions  it  is  possible  for  an  animal  to  be  swept  away  and  deposited 
by  the  sediment  and  so  its  bones  or  hard  parts  preserved. 

The  surface  of  the  earth  is  constantly  undergoing  slow  movements, 
whereby  certain  portions  are  subsiding  and  others  being  elevated. 
Sometimes  this  transformation,  always  a  gradual  one,  takes  place 
relatively  faster  than  at  others.  In  this  way  it  comes  about  that  the 
strata  laid  down  under  water  get  raised  up  to  form  part  of  the  land, 
and  they  bring  with  them  animal  remains  or  fossils.  Sometimes  they 
are  raised  so  high  that  they  form  mountain  ranges,  to  become  subject 
to  the  same  weathering  forces  and  so  help  to  form  further  sediments. 


452 


AN   INTRODUCTION   TO   ZOOLOGY 


FIG.  156.— Restoration  of 
four-toed  horse,  Eohip- 
pus,  Lower  Eocene,  North 
America. — After  Lull. 


To  return  now  to  the  horse,  a  long  series  of  fossil  forms  have 
been  discovered,  going  back  into  the  Eocene  period,  and  these  are 
separated  up  into  groups  of  allied  species,  from  each  of  which  we  can 
take  as  a  representative  a  typical  species.  These  are  by  no  means  all, 
but  they  represent  so  far  as  we  know  the  main  stages  in  the  evolution 
of  the  horse.  It  is  quite  impossible  to  consider  even  superficially 
all  of  these  or  to  examine  all  the  various  points  of  the  skeletons  of 
the  forms  selected.  We  shall  consider  then  some  of  the  characters 
of  a  few  of  the  best  known  and  striking  of  these  stages. 

Eocene. — The  first  animal  that  can  be  undoubtedly  considered 
as  on  the  ancestral  line  of  the  horse  is  Eohippus,  which  is  found  in 

the  lower  Eocene  beds.  It  has  been  found 
both  in  Europe  and  in  North  America,  but 
it  is  not  known  with  certainty  where  it 
originated  or  what  it  originated  from.  It 
is  interesting  to  note  in  this  connection, 
however,  that  the  most  primitive  and 
earliest  known  member  of  this  group  is 
Hyracotherium,  which  was  found  in  the 
London  clay,  and  is  preserved  in  the  British 
Museum.  If  it  originated  in  W.  Europe, 
then  it  must  have  migrated  to  North  America,  which  was  then 
entirely  forest  clad,  across  what  is  now  Bering  Strait,  and  in 
North  America  most  of  the  succeeding  stages  were  passed  through. 
Echippus  was  a  small  animal  intermediate  in  size  between,  say 
a  cat  and  a  fox,  about  n  inches  at  the  shoulder. 
The  fore  limb  had  four  well- developed  toes  with 
hoofs,  and  the  fifth  was  represented  by  a  splint 
bone,  and  the  hind  limb  had  three  well- developed 
digits  with  hoofs,  and  the  fourth  was  represented 
by  a  splint  bone.  The  teeth  indicate  that  it  was  a 
vegetable  feeder,  although  they  are  not  nearly  so 
specialised  as  in  the  present-day  horse.  Its  teeth 
and  limbs  throw  out  hints  of  a  still  more  remote 
much  less  horse-like  ancestor.  You  will  see,  how- 
ever, that  this  animal  does  not  resemble  a  horse  at 
all  closely,  and  indeed,  it  is  highly  probable  that  if 
no  intermediate  forms  were  known  it  would  not 
have  been  regarded  as  having  close  affinity  with 
the  horse.  Between  Eohippus  and  Equus,  however, 
are  a  large  number  of  intermediate  forms  that  grade  into  one  another 
so  completely  that  it  becomes  a  matter  of  difficulty  to  draw  a  line 
between  one  species  and  another. 

Later,  in  the  middle  Eocene,  appeared  Orohippus.      This  form 


FIG.  157. — Hand 
(A)andfoot(B) 
of  Eohippus. 
One-fourth 
natural  size. — 
After  Marsh, 
from  Lull. 


EVOLUTION,   VARIATION   AND   HEREDITY          453 

shows  a  distinct  increase  in  size,  and  was  about  14  inches  at  the 
shoulder,  and  in  size  and  proportions  resembed  a  whippet.  The 
fore  foot  had  lost  all  trace  of  the  first  digit,  and  the  hind  foot  all  trace 
of  both  the  first  and  the  fifth  digit.  In  addition  to  this,  certain 
alterations  are  also  observable  in  the  teeth. 

Oligoeene. — In  the  lower  formations   of  this  period  appeared 


FIG.  158. — -Restoration  of  four-toed 
horse,  Orohippus,  Middle  Eocene, 
Wyoming. — After  Lull. 


FIG.  159. — Restoration  of  three- toed 
horse,  Mesohippus,  Middle  Oligoeene, 
North  America. — -After  Lull. 


r 


Mesohippus,  which  was  a  slightly  larger  animal,  about   18  inches 

high  at  the  shoulder.     This  is  known  as  the  three-toed  horse,  because 

in  the  fore  foot  the  fifth  digit  was  present  only 

as  a  splint  bone  and  therefore  quite  rudimentary, 

while  the  hind  foot  remained  much  as  before. 

Thus  both  fore  and  hind  feet  possesses  three 

toes  with  hoofs. 

Miohippus  was  a  still  later  three-toed  horse, 
from  the  Upper  Oligoeene  beds.  It  represents 
an  advance  in  size  on  Mesohippus,  being  24 
inches  high  at  the  shoulder,  i.e.  about  as  large 
as  a  sheep.  In  its  skeleton  it  is  not  much  more 
advanced  save  that  the  splint  of  the  fifth  digit 
is  in  a  still  more  rudimentary  condition.  Both 
types  had  the  middle  digit  larger  than  the 
lateral  digits,  indicating  that  it  was  the  most 
important  in  locomotion,  and  in  both  also  the 
teeth  show  some  advance  in  complexity. 

Miocene. — This  period  also  furnishes  a  group 
of  horse-like  forms.  Miohippus  continues  over 
into  the  lower  formations,  while  in  the  middle  formations  we 
find  Pamhippus  and  Hypohippus.  These  are  types  which,  while 
showing  certain  advances  in  size,  etc.,  also  indicate  specialisation 


LG.  160.  —  Hand 
(A)  and  foot  (B)  of 
Mesohippus.  One- 
fourth  natural  size. 
— After  Marsh. 


454 


AN   INTRODUCTION  TO  ZOOLOGY 


in  the  directions  ^  of  becoming  forest  dwellers.  On  the  other 
hand,  Merychippus  from  the  middle  and  upper  layers,  while  it 
also  shows  a  slight  increase  in  size,  is  more  important  in  that  it  is 
modified  for  a  life  on  the  plains.  Its  permanent  teeth  are  adapted 
lor  a  grass  diet,  while  its  milk  teeth  still  retain  the  characteristics 


FIG.  161. — Restoration  of  the  Miocene  prairie 
horse,  Merychippus. — 'After  Lull. 


^. 


of  an  animal  feeding  on  shrubs  and  softer 
vegetation.  Further,  while  still  three  toed, 
in  ordinary  locomotion  its  lateral  digits 
did  not  reach  the  ground.  In  the  upper 
Miocene  appears  Protohippus,  which  stood 
36  inches  at  the  shoulder.  There  is  not 
much  structural  advance  shown  save  that 
the  lateral  digits  are  slightly  smaller  and 
further  removed  from  the  ground.  The 
principal  point  of  interest  is  the  teeth, 
whose  enamel  has  become  more  complex, 
and  even  the  milk  teeth  are  of  the  grass- 
eating  type. 

Pliocene. — This  period  is  represented 
by  Pliohippus,  which  first  occurs  in  the  middle  beds. 
48  inches  at  the  shoulder,  a  fairly  large  modern  horse  being  60  inches, 
i.e.  14  hands,  and  was  more  horselike  in  its  general  proportions. 
In  this  animal  the  two  lateral  toes  on  fore  and  hind  feet  become 
more  reduced,  and  in  some  of  the  species  have  almost  entirely 
disappeared,  or  better  become  nearly  as  ludimentary  as  in  our 
modern  horse. 


FIG.  162. — Hand  (A)  and 
foot  (B)  of  the  first  one- 
toe  horse,  Pliohippus 
pernix,  Pliocene,  Ne- 
braska. One-fourth 
natural  size. — After 
Lull. 


It  was 


EVOLUTION,    VARIATION   AND   HEREDITY         455 

Pleistocene. — Finally,  in  the  lower  Pleistocene  appeared  the 
genus  Equus,  the  relatives  of  our  modern  horse,  and  represented  by 
numerous  skeletons  of  E.  Scotti.  It  is,  as  is  known,  one  of  the  best- 
adapted  animals  for  locomotion  by  means  of  running  that  we  know, 
and  is  marked  off  from  all  other  living  genera  by  the  fact  that  it 
possesses  but  a  single  toe  on  fore  and  hind  feet.  The  relics  of  the 
next  two  lateral  bones  are  to  be  found  as  a  pair  of  splint  bones, 
remains  of  metacarpals.  At  present  there  are  three  main  types  of 
the  horse  family  found  wild  in  the  world,  known  as  horses,  asses,  and 
zebras.  They  are  mainly  to  be  distinguished  by  external  characters, 
and  their  skeletons  are  remarkably  alike. 

The  slow  development  of  the  horse  from  the  "  dawn  horse  " 
Eohippus  of  the  early  Eocene  up  to  the  appearance  of  the  modern 
genus  Equus  was  contemporaneous  with  the  disappearance  of  much 
of  the  forest  land  in  North  America  and  its  gradual  replacement  by 
the  plains  and  prairies  we  now  find  there.  From  a  wood  dweller  the 
horse  gradually  become  fitted  for  a  life  on  grassy  plains,  from  an 
animal  with  teeth  for  browsing  on  herbs  and  shrubs  to  one  with 
teeth  for  grazing  on  grass.  The  specialisation  not  only  affected 
the  feet  and  size  of  the  body,  but  the  whole  structure  of  the  horse 
including  the  head.  This  alteration  is  particularly  noticeable  in 
the  Miocene  times,  when  much  of  North  America  became  grass  land 
and  several  types  suited  to  grazing  appeared  and  also  became 
extinct. 

Throughout  the  Pleistocene  times  horses  of  the  genus  Equus 
are  to  be  found  in  North  America,  where  the  genus  was  apparently 
widespread  and  numerous.  Every  now  and  again  from  Oligocene 
times  horses  reinvaded  Europe,  as  the  discovery  of  their  fossil  remains 
testify,  but  they  do  not  appear  to  have  flourished  there  as  they  did 
in  North  America.  Strangely  enough,  however,  during  the  Pleisto- 
cene period  the  invasion  was  successful,  and  the  genus  Equus  became 
very  widespread  over  the  plains  of  Eurasia.  For  some  reason  at 
present  unknown,  horses  died  out  completely  at  a  subsequent  period 
in  America,  which  became  isolated  from  Asia  by  the  breakdown  of 
the  land  connection  and  the  formation  of  Bering  Strait.  All  the 
herds  of  horses,  the  mustangs,  found  all  over  the  prairies  of  North 
America  by  the  early  colonists  are  due  to  horses  re-introduced  in  his- 
toric times.  They  are  the  descendants  of  the  horses  that  escaped 
from  the  old  Spanish  explorers.  The  disappearance  of  the  original 
species  of  Equus  in  North  America  is  all  the  more  remarkable  since 
on  the  re-introduction  of  the  same  genus  they  flourished  so  well. 

This  past  history  of  the  horse  at  which  we  have  only  briefly 
glanced  is  undoubtedly  one  of  the  most  complete,  but  it  is  paralleled 
by  that  of  other  groups,  e.g.  the  elephant,  the  rhinoceros,  the  camel, 


CO 

< 

(E 
hJ 

EPOCHS 

Indicating  approximately 
relative     duration      as 
estimated   by    thickness 
of     deposits 

Range  of  Animal  Groups 

Probable  dominant 
form  of  Animol 
Life. 

1     „ 

J5     „    £       S 

)            U>             f»              F—  '              -A              t- 

1  1  i  j  1  1  1 

_c  iz   <     o;    03    s    E 

& 

o> 
°< 

St 

z  a 
—  Hi 

<  h 

O 

Recent  <*  Pleistocene  4pOOft 

i      . 

A<$e  of  Man. 

Pliocene            1  3,000ft 

1 
i 

Age      of 
Mammals 

Miocene           I4,OOO  Ft 

1 

Oligocene        12,000  Ft. 

i 
i 

Eocene               2O,OOO  ft. 

i 
i 

ME  SO  ZOIC  or 
SECONDARY 

C  reteceous     44,OOO  ft. 

1 
1 
l 

!      ' 
i      ' 

Age      of 
Reptiles 

Jurnasic              8,OOOft. 

1     ; 

Triassic          17;OOOft. 

i 
i 

i 

PALAEOZOIC  or  PRIMARY 

Permion           t2,OOOFt. 

i 
i 

Age.   of 
Amphibia 

Carboniferous  SaOOQFt. 

1 

i 

1       : 
1 
1 

Devonian          22,OOOft. 

1 
1 

Age      of 
fishes 

Silurian           l5,OOOft. 

OrdoOicion      l7;OOOft. 

i 

Age      of 
Higher 
Invertebrates 

Cambrian          26,CXX>ft. 

i 
PRE*-  CAMBRIAN     (Proterozoic  &  Archeon)         Extent  not  Known 

FIG.  163. 


EVOLUTION,   VARIATION   AND   HEREDITY         457 

etc.,  etc.     In  each  case  they  can  be  traced  back  to  more  primitive, 
less  specialised  forms. 

These  cases  are,  however,  specific  instances,  but  when 
we  turn  to  the  general  evidence  from  palaeontology  we  shall  find  that 
it  is  just  as  strong.  Let  us  glance  for  a  few  moments  at  the  sequence 
in  which  the  various  classes  of  the  Vertebrata  appear  ;  but  in  order 
to  do  so  we  must  first  fill  in  the  subdivisions  of  the  other  two  geo- 
logical eras  (vide  diagram).  Throughout  the  whole  of  the  Archaean 
or  Proterozoic  and  major  part  of  the  Cambrian  we  find  no  trace  of 
any  animal  with  a  vertebrate  structure.  They  first  appear  possibly 
in  late  Cambrian,  but  certainly  in  the  Silurian,  and  these  are  the  fish. 
In  the  next  period,  the  Devonian,  we  get  more  fish  and  the  earliest, 
most  primitive  Amphibia,  and  soon  after  in  the  Carboniferous  other 
and  more  highly  developed  Amphibia  and  very  primitive  amphibia- 
like  reptiles.  The  Permian  period  saw  the  origin  of  a  number  of 
new  and  more  developed  Reptilia,  which  however  increased  greatly 
in  variety  and  organisation,  so  that  they  were  the  dominant  forms 
during  the  greater  part  of  the  Mesozoic  Era,  which  is  in  consequence 
termed  the  "  Age  of  Reptiles."  One  line  of  reptilian  development 
led  in  the  upper  Triassic  to  the  primitive  Mammalia.  These  are 
undoubted  mammals,  and  the  group  has  gradually  gone  on,  and  in 
the  Caenozoic  Era  it  replaced  the  Reptilia  as  the  dominant  group. 
The  culminating  and  most  recent  addition  to  the  Mammals  is  man 
himself,  who  first  appears  in  the  Pliocene  period.  With  the  spread 
of  our  knowledge  of  fossil  forms  that  has  taken  place  in  the  last  twenty 
or  so  years,  it  has  been  increasingly  difficult  to  separate  off  the 
Reptilia  from  the  Amphibia  on  the  one  hand  and  the  Mammalian 
on  the  other. 

The  birds  appear  as  a  highly  specialised  offshoot  from  the  reptiles 
in  the  Jurassic,  which  also  furnishes  us  in  its  early  layers  with  that 
remarkable  mixture  of  bird  and  reptile  Archaeopteryx. 

This  short  account  suffices  to  show  the  kind  of  evidence  that 
we  derive  from  Anatomical,  Embryological,  Geographical  and 
Palaeontological  sources.  The  amount  of  evidence  that  has  now 
been  amassed  is  colossal,  and  it  is  being  added  to  year  by  year.  It 
all  points  most  clearly  to  the  fact  that  living  matter  was  at  first 
simple,  but  through  countless  ages  it  has  gradually  become  more  and 
more  specialised  and  produced  an  endless  variety  of  forms.  This 
long-continued  and  continuous  process  of  change  or  Evolution  has 
resulted  in  the  production  of  all  the  many  animal  and  plant  forms  at 
present  inhabiting  the  earth. 

Darwin. — The  most  masterly  contribution  to  the  subject  of 
Evolution  was  not  made  until  fifty  years  after  the  appearance  of 
Lamarck's  book,  when  Charles  Darwin  published  his  "  Origin  of 


458  AN   INTRODUCTION   TO  ZOOLOGY 

Species  "  in  1859.  This  immortal  work  gave  the  first  satisfying 
and  truly  scientific  account  of  the  process  of  organic  evolution,  and 
further  tried  to  account  for  the  manner  in  which  it  is  brought  about 
in  nature.  The  theory  of  Natural  Selection,  as  it  is  called,  was  first 
conceived  by  Darwin  in  1838,  but  it  was  not  until  he  had  given  the 
matter  twenty  years'  further  thought  that  he  published  a  paper 
conjointly  with  Alfred  Russel  Wallace  in  the  Transactions  of  the 
Linnaean  Society,  and  followed  it  a  year  later  by  his  book.  A.  R. 
Wallace  was  a  younger  biologist  working  in  Melanesia,  who  quite 
independently  arrived  at  conclusions  very  similar  to  Darwin's. 
This  joint  publication  was  the  beginning  of  a  life-long  friendship 
between  these  two  great  men. 

Charles  Darwin  was  born,  strangely  enough,  in  1809,  the  year  of 
the  publication  of  "  Philosophic  Zoologique."  In  1825,  he  went  to 
Edinburgh  to  study  medicine  ;  he  stayed  about  two  years,  and  in 
1828  went  to  Cambridge.  He  was  appointed  naturalist  to  the 
Beagle  in  1831,  and  went  on  a  voyage  round  the  world  in  her  that 
lasted  till  1836.  This  voyage  played  a  great  part  in  shaping  his 
ideas,  and  he  has  left  a  charming  account  of  it  in  "  The  Voyage  of  the 
Beagle."  In  addition  to  writing  "  The  Origin  of  Species,"  Darwin 
did  an  enormous  amount  of  biological  work.  ' '  The  Origin  of  Species/' 
or,  to  give  it  its  full  title,  "  The  Origin  of  Species  by  means  of  Natural 
Selection,  or  the  Preservation  of  Favoured  Races  in  the  Struggle 
for  Life,"  is  a  book  that  has  probably  influenced  human  thought  more 
than  any  other,  and  certainly  was  the  foundation  stone  of  modern 
biology.  We  must  consider  briefly  its  main  points. 

The  tripod  upon  which  this  theory  rests  are  the  three  factors 
Variation,  Heredity,  and  the  Struggle  for  Existence,  and  we  must 
glance  briefly  at  the  meaning  of  each  of  these. 

Variation. — Variations  are  extremely  well  known  in  our  domestic 
animals  and  plants,  for  no  matter  how  much  the  offspring  may  re- 
semble the  parents  it  is  never  precisely  the  same.  The  various 
members  of  one  family  are  not  absolutely  alike.  By  recognising 
and  taking  advantage  of  these  variations  man  has  been  able  to 
achieve  much.  Thus,  probably  all  the  various  species  of  cabbage, 
cauliflower,  broccoli  and  Brussels  sprouts,  etc.,  are  derived  from  the 
one  wild  species  Brassica  oleracea.  In  the  same  way  apples,  of  which 
we  now  grow  about  1000  varieties,  come  from  the  wild  crab  apple. 
The  standard  examples  in  the  animal  kindgom  are  the  various  kinds 
of  pigeon  (Columba  livia),  of  dogs,  cattle,  etc.  In  all  these  cases  the 
result  has  been  attained  by  selecting  and  breeding  from  the  forms 
with  the  desired  characteristics.  As  it  has  been  done  intentionally 
by  man  in  gardens  or  breeding  pens  it  has  been  termed  "  artificial 
selection."  A  similar  kind  of  variation  is  to  be  seen  in  all  living  ani- 


EVOLUTION,    VARIATION   AND   HEREDITY 


459 


mals  and  plants  under  natural  conditions.  No  two  members  of  the 
same  species  are  absolutely  alike  ;  they  vary  in  size,  shape,  colour, 
relative  size  of  parts,  and  so  on.  Not  only  is  this  true  of  the  external 
features,  but  it  applies  also  to  the  various  internal  organs.  All 
parts  of.  animals  vary  to  some  extent,  and  while  in  some  cases  it  is 
more  apparent  than  in  others,  it  is  almost  always  possible  to  measure 
this  variability.  This  statistical  investigation  of  the  phenomenon 
of  variation  was  started  in  England  by  Francis  Galton  and  continued 
by  Professors  Karl  Pearson  and  Weldon. 

Let  us  take  an  example  or  two. 

A  collection  of  a  large  number  of  beech  leaves  was  made  at 
random  and  the  number  of  main  veins  coming  off  from  the  mid 
rib  counted,  and  the  following  result  obtained  : — 


No.  of  veins 

10 

II   12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

No.  of  leaves 

i 

i 

7  34 

no 

3i8 

479 

596 

5i6 

3°7 

181 

36 

15 

I 

This  may  also  be  expressed  in  the  form  of  a  curve  with  the  number 
of  veins  on  the  abscissa  and  the  number  of  leaves  on  the  ordinate. 

A  similar  series  of  measurements  have  been  made  on  men,  i.e.  the 
height  of  4426  members  of  the  University  of  Cambridge  of  British 
extraction.  The  result  is  shown  in  the  accompanying  diagram, 
where  the  stature  in  inches  is  on  the  abscissa  and  the  number  of  indi- 
viduals on  the  ordinate.  The  diagram  also  shows  the  curve  that 
most  nearly  fits  the  points  obtained.  (Such  a  curve  is  known  as  a 
normal  curve,  and  is  expressed  mathematically  by  the  equation 


FIG.  164. — Curve  of  heights  of  undergraduates  of  Cambridge  University 
of  British  extraction. 


460  AN   INTRODUCTION   TO  ZOOLOGY 

The  line  from  the  base  to  the  highest  point  is  termed  the  mode, 
or  in  other  words  the  mode  is  the  point  at  which  the  largest  number 
of  individuals  occur,  and  often  the  curve  balances  on  each  side. 
Sometimes  the  curve  is  asymmetrical  because  the  variation  is  more 
limited  on  one  side  than  the  other.  This  is  termed  a  skew  curve. 

Heredity. — Heredity  is  the  second  great  factor.  At  the  com- 
mencement of  the  life  of  one  of  the  higher  animals  we  have  the 
fertilised  ovum.  This  is  derived  from  the  union  of  the  sperm,  a  part 
of  the  male  parent,  and  the  ovum,  a  part  of  the  female  parent.  As  we 
have  seen,  each  parent  contributes  an  equal  number  of  chromosomes 
to  the  nucleus  of  the  new  organism  and  these  it  is  that  are  the  impor- 
tant structures  in  controlling  the  course  of  development.  Because 
of  this  relation  to  its  parents  the  offspring  resembles  them,  and 
we  give  the  name  "  heredity  "  to  the  genetic  relation  between  parents 
and  offspring.  The  inheritance  of  an  animal,  then,  is  all  that  it 
possesses  in  virtue  of  its  relation  to  its  parents.  This  relationship 
has  long  been  recognised  and  is  expressed  in  a  number  of  popular 
sayings,  such  as  "  like  begets  like,"  "  a  chip  of  the  old  block."  That 
the  heritage  or  inheritance  is  also  generally  recognised  as  important 
is  expressed  by  the  old  adage,  "  You  cannot  make  a  silk  purse  out 
of  a  sow's  ear."  The  power  of  handing  on  characteristics  is  one  of 
the  necessary  conditions  of  evolution  ;  without  it  evolution  could 
not  have  taken  place. 

Francis  Galton  hi  1897  formulated  what  is  known  as  "  Gait  on 's 
law  of  ancestral  inheritance."  He  says,  "  the  two  parents  con- 
tribute between  them  on  the  average  J('5)  of  the  total  heritage  of 
the  offspring  ;  the  four  grandparents  £('25)  ;  the  eight  great-grand- 
parents ('125)  ;  and  so  on.  Thus  the  sum  of  the  ancestral  contribu- 
tions is  expressed  by  the  series  '5,  ('5)2,  (*5)3,  which  being  equal 
to  one  accounts  for  the  whole  heritage."  Professor  Pearson's  work 
leads  him  to  conclude  that  the  series  would  be  more  accurately 
expressed  by  '6244,  "1988,  '0630,  etc.,  but  this  does  not  affect  the 
validity  of  the  law  ;  it  only  indicates  that  the  actual  contribution  of 
the  immediate  parents  is  somewhat  higher  than  Galton  estimated. 
It  should  be  noted  that  Gait  on 's  law  is  a  statistical  law  giving  the 
average  result  of  the  analysis  of  large  groups,  and  does  not  neces- 
sarily apply  to  the  inheritance  in  any  one  individual  case. 

,  Selection. — We  have  seen  what  is  meant  by  Variation  and 
Heredity,  and  now  pass  on  to  consider  the  third  factor,  Selection, 
to  the  action  of  which  the  other  two  are  necessary  conditions.  This 
is  also  termed  the  Struggle  for  Existence,  and  was  arrived  at  firstly 
by  a  consideration  of  the  manner  in  which  animals  multiply. 

Darwin's  own  classical  example  is  that  of  the  Elephant,  one  of 
the  slowest  breeding  of  all  animals.  We  may  assume  as  fairly 


EVOLUTION,   VARIATION   AND   HEREDITY         461 

probable  that  the  elephant  does  not  breed  before  the  age  of  30  years, 
and  continues  to  do  so,  excluding  accidents,  till  90  years  of  age. 
Let  us  further  assume  that  during  this  period  six  young  are  born. 
You  will  see  that  this  by  no  means  over-estimates  the  possibility 
and  is  a  very  slow  rate  of  multiplication  ;  yet  in  740  or  750  years,  a 
mere  nothing  in  the  history  of  the  earth,  there  would  be  19  million 
living  descendants  of  the  first  pair.  Professor  Punnet  was  engaged 
in  breeding  Rotifers,  very  tiny  animals  not  much  larger  than 
Paramcecium,  and  found  that  each  female  produced  30  eggs  at  a 
time,  and  67  generations  were  obtained  in  less  than  a  year.  Had  he 
been  able  to  keep  all  the  animals  alive,  by  the  end  of  the  year  they 
would  have  formed  a  solid  mass  many  times  larger  than  the  earth. 
To  take  an  example  from  plants,  Professor  Buller  has  estimated  that 
the  giant  puff  ball  produces  7,000,000,000  spores,  so  that  in  two 
generations  there  would  be  a  mass  of  puff-balls  800  times  the 
volume  of  the  earth.  These  are,  of  course,  impossible  happenings, 
for  there  is  always  a  heavy  mortality,  but  they  serve  to  illustrate 
the  fact  that  living  organisms  reproduce  comparatively  rapidly,  and 
in  a  geometric  ratio,  so  that  in  time  their  number  would  be  infinite. 

A  rough  measure  of  the  amount  of  mortality  may  be  arrived  at 
in  the  following  way.  We  have  no  reason  to  suppose  that  the 
number  of  codfish  is  more  to-day  than  it  was,  say,  50  years  ago. 
Each  pair  of  cod  may  lay  9,000,000  eggs  in  a  year,  and  will  continue 
to  do  so  for  a  number  of  years — let  us  say  five,  although  actually  it 
is  more  than  that.  But  as  there  is  to  be  no  increase,  then  of  the 
45,000,000  possible  codfish  from  the  original  parents  only  two 
survive  to  replace  them. 

This  is  but  one  example,  and  it  is  paralleled  by  many  others. 
The  great  loss  of  life  incidental  to  any  great  war  is  nothing  to  the 
colossal  slaughter  that  is  carried  out  in  the  course  of  nature  every 
day.  Nature  is  absolutely  relentless  in  her  annihilation,  and  those 
who  talk  of  the  perfection  of  nature  do  not  always  realise  the  price 
that  is  paid  for  that  perfection. 

The  method  in  which  this  destruction  takes  place  can  easily 
be  conceived.  We  see  that  if  we  consider  an  imaginary  example 
of  animals  in  a  new  country,  say,  for  example,  an  island.  At  first 
all  will  be  well  and  they  will  flourish ;  before  long,  however,  the 
food,  supposing  the  supply  to  be  constant,  which  was  ample  for  the 
few,  is  only  just  sufficient  to  go  round.  This  stage  may  be  called 
one  of  Equilibrium,  and  after  it  has  been  passed  the  problem  of 
obtaining  food  becomes  serious.  Obviously,  if  there  is  only  food 
enough  for  100  animals,  then  all  above  that  number  must  be  starved 
out.  Which  ones  are  to  be  starved  ?  "  The  race  is  to  the  swift  and 
the  battle  to  the  strong."  Those  animals  that  cannot  run  so  well, 


462  AN   INTRODUCTION   TO  ZOOLOGY 

fight  so  well,  see  so  well,  are  not  so  cunning,  etc.,  will  be  exterminated 
ruthlessly.  The  more  animals  produced  the  more  fierce  this 
"  struggle  for  existence  "  and  the  larger  the  number  of  individuals 
blotted  out,  and  we  can  see  that  this  all  becomes  intensified  if, 
instead  of  the  simple  case  imagined,  we  have,  as  we  have  in  fact, 
large  numbers  of  different  animals  on  the  same  area. 

The  whole  of  the  external  surroundings  of  an  individual,  the 
earth  or  water,  the  other  animals,  the  plants,  the  climate,  the 
weather  and  all  the  many  factors  of  the  external  world  that  affect 
an  animal  even  in  the  remotest  way,  we  include  in  the  one  term 
"  environment. ' '  Thus  the  environment  of  an  animal  is  every  external 
influence  that  plays  upon  it  from  the  moment  it  starts  life  as  a 
fertilised  egg  until  its  death.  In  order  to  survive  in  the  struggle  for 
existence  the  animal  must  be  suited  to  its  surroundings  in  its  struc- 
ture and  habits.  Thus  it  would  be  useless  for  a  cat  to  develop  a  fin 
unless  it  at  the  same  tune  altered  its  mode  of  life,  and  so  on.  We 
put  this  in  another  way  by  saying  that  an  animal  must  be  "  adapted 
to  its  environment."  A  character  or  structure  is  called  adaptive 
when  it  is  of  obvious  use  to  the  possessor.  An  animal  that  is  well 
adapted  to  a  certain  environment  is  said  to  be  fitted  to  it.  Note 
the  scientific  use  of  the  word  "  fit,"  so  often  misunderstood  or 
misused  in  popular  writings.  When  we  say  an  animal  is  "fit  "  we 
imply  no  physical,  mental  or  moral  superiority  whatever  ;  we  simply 
mean  that  it  is  adapted  to  its  environment.  Should  the  environ- 
ment change,  the  animal  that  was  fit  probably  becomes  unfit.  We 
have  realised  that  in  the  struggle  for  existence  it  is  the  unfit  that 
are  eliminated,  and  so  indirectly  the  fittest  are  selected.  Hence 
the  philosopher  Herbert  Spencer,  who  arrived  at  much  the  same 
conclusions  as  Darwin  from  more  theoretical  reasoning,  termed 
natural  selection  "  the  survival  of  the  fittest." 

Thus  we  have  glanced  quite  briefly  at  the  three  main  factors 
that  Darwin  recognised,  Variation,  Heredity  and  Natural  Selection. 

The  struggle  for  existence  is  not  so  simple  a  process  as  we 
assumed  in  the  imaginary  example  of  the  animals  on  the  island,  and 
various  entirely  different  forms  have  an  enormous  indirect  influence 
and  dependence  upon  one  another.  To  illustrate  this  we  may  take 
two  well-known  examples.  It  is  not  at  first  sight  obvious  why  the 
crop  of  red  clover  (Trifolium  pratense)  should  be  in  any  way  de- 
pendent upon  cats,  yet  it  is  to  a  certain  extent.  Red  clover  is 
fertilised  almost  entirely  by  bees ;  the  number  of  bees  in  a  neighbour- 
hood (excluding  carefully  guarded  hives)  depends  on  the  mice, 
which  destroy  the  bees'  nests,  eggs  and  young  ;  the  number  of  mice 
is  largely  kept  down  by  cats  (owls  also  play  a  part). 

Many  years  ago  goats  were  introduced  into  St.  Helena,  which 


EVOLUTION,    VARIATION   AND   HEREDITY          463 

was  well  wooded.  They  destroyed  all  the  forests  by  eating  up  the 
seedlings  as  they  appeared.  With  the  forest  disappeared  also  the 
birds  and  insects  living  in  them,  insect-fertilised  flowers  were  also 
affected,  and,  indeed,  even  the  climate  was  changed. 

The  argument  set  out  in  the  Origin  of  Species  may  be 
briefly  summarised  by  what  is  known  as  Wallace's  chart  : 


Fact.  Consequence. 

A.  Rapid  increase  in  numbers    } 

B.  Total    number    of    species   V        .   Straggle  for  Existence. 

stationary  ) 


C.  Struggle  for  Existence         ?  Survival  of  the  Fittest 

D.  Variation  with  Heredity    f  •   h 

E.  Survival  of  the  Fittest      )  Structural  modification  and   differ- 

F.  Change  of  Environment    \    '          *       entiation  of  species. 

Darwin's  own  conclusions  on  the  matter  are  best  summarised 
in  his  own  words.  "  I  have  now  recapitulated  the  facts  and  con- 
siderations which  have  thoroughly  convinced  me  that  species  have 
been  modified,  during  a  long  course  of  descent.  This  has  been 
effected  chiefly  through  the  natural  selection  of  numerous  slight, 
favourable  variations  ;  aided  in  an  important  manner,  that  is,  in 
relation  to  adaptive  structures,  whether  past  or  present,  by  the 
direct  action  of  external  conditions,  and  by  variations  which  seem 
to  us  in  our  ignorance  to  arise  spontaneously.  It  appears  that  I 
formerly  underrated  the  frequency  and  value  of  these  latter  forms 
of  variations,  as  leading  to  permanent  modifications  of  structure 
independently  of  natural  selection." 

Here  then  we  have  a  statement  of  the  case  as  Darwin  conceived 
it,  and  in  it  we  have  the  phenomena  of  evolution  definitely  envisaged 
and  set  forth.  This  was  done  so  successfully  that  it  has  never  been 
seriously  doubted  since,  and  herein  lies  Darwin's  great  service  to 
Biology.  He  put  forward  an  enormous  amount  of  evidence  demon- 
strating evolution  in  such  a  cogent  manner  that  even  the  most 
sceptical  were  convinced.  With  it  he  also  proposed  a  theory  to 
account  for  the  causes  responsible  for  evolution,  which  is  quite 
logical  and  conceivable.  The  Biologist,  however,  is  not  entirely 
concerned  with  logical  possibilities,  and  he  desires  to  know  if  this  is 
actually  the  way  in  which  evolution  was  brought  about*  Herein, 
as  pointed  out  previously,  subsequent  workers  have  not  always 
agreed  with  Darwin. 

The  publication  of  the  "  Origin  of  Species  "  exerted  a  profound 
influence  upon  biological  work,  and  marked  a  complete  change  in  the 
scientific  outlook, 


464  AN   INTRODUCTION  TO  ZOOLOGY 

Post-Darwinian  Biology. 

Darwin  and  his  contemporaries  admitted  that  Lamarck's 
"  use  inheritance  "  probably  played  some  part  in  the  formation  of 
new  species,  though  in  the  main  this  was  due  to  the  action  of 
natural  selection.  Herbert  Spencer  upheld  this  view  strongly,  and 
perhaps  A.  R.  Wallace  alone  opposed  it. 

The  first  naturalist  to  oppose  it  in  a  very  thorough  way  was 
August  Weissmann,  who,  in  a  long  series  of  publications  starting  in 
1885,  denied  all  transmission  of  what  he  termed  "  acquired  cha- 
racters." Acquired  characters  he  defined  as  "  those  which  result 
from  external  influence  upon  the  organism,  in  contrast  to  such  as 
spring  from  the  constitution  of  the  germ."  In  other  words,  "  Modifi- 
cations which  are  wrought  upon  the  formed  body,  in  consequence  of 
external  influences,  must  remain  limited  to  the  organism  in  which 
they  arose.  No  such  modifications  of  the  same  can  be  transmitted 
to  the  germ  cells  from  which  the  next  generation  springs."  These 


Line   of  succession  of 

individuals. 


Line    of    here- 
ditary trans- 
mission. 


FIG.  165. — Diagram  to  illustrate  the  idea  of  continuity  of  the  germ  plasm. 

conclusions  were  reached,  firstly,  as  a  result  of  the  exhaustive  study 
of  all  the  instances  in  which  such  transmission  was  alleged  to  occur. 
He  was  able  to  show  that  none  of  these  cases  furnished  evidence  of  a 
definite  and  incontestable  character.  Secondly,  he  made  a  study 
of  the  mechanism  whereby  any  characters  could  be  transmitted. 
As  a  result  of  these  researches,  Weissmann  elaborated  a  series  of 
doctrines,  known  collectively  as  Weissmannism,  the  principle  of 
which  is  that  of  "  the  continuity  of  the  germ  plasm."  According 
to  this  theory,  we  must  regard  the  offspring  as  inheriting  from  the 
germ  cell  of  the  parent  and  not  from  the  body  of  the  parent.  In 
support  of  this,  it  is  urged  that  in  some  animals  one  of  the  very 
early  blastomeres  is  definitely  set  aside  as  the  producer  of  the  germ 
cell,  and  develops  more  or  less  independently  of  the  rest.  In  still 
more  cases  the  germ  mother  cell  is  recognisable  at  a -very  early  stage, 
as  we  have  seen  in  Obelia,  and,  finally,  in  almost  all  the  remaining 
animals  the  primitive  germ  cells  are  formed  before  the  appearance 
of  discrete  orga  ns.  So  that  the  body,  as  it  were,  acts  as  a  guardian 


EVOLUTION,    VARIATION   AND   HEREDITY         465 

and  trustee  for  the  germ  cells,  and  this  idea  may  be  expressed  roughly 
by  a  diagram.  Weissmann  points  out  that  no  satisfactory  way  has 
been  discovered  whereby  a  subsequent  alteration  in  such  organs 
could  conceivably  influence  the  germ  cells  from  which  they  have 
long  been  dissociated.  Many  modern  Biologists  are  in  general 
agreement  with  this  view,  which  they  claim  is  supported  by  the 
evidence  of  recent  cytological  and  embryological  investigations,  but 
there  are  others  who  disagree.  These  latter  do  not  indeed  hold  to 
the  crude  doctrine  of  "  use  inheritance  "  as  propounded  by  Lamarck, 
but  maintain  that  a  changed  environment  will  after  a  certain 
number  of  generations  impress  itself  on  the  germ  cells  and  bring 
forward  a  certain  amount  of  experimental  evidence  to  support 
this.  One  of  the  best  known  of  these  is  Tower's  work  on  breeding 
beetles  of  the  genus  Leptinotarsa  under  different  environmental 
conditions,  and  noting  the  effect  upon  the  offspring.  The  subject 
is  too  wide  to  be  pursued  further,  but  is  one  that  has  received  a 
great  deal  of  attention,  and  much  of  the  discussion  it  involved  has 
been  concerning  the  use  of  words  rather  than  the  import  of  the 
evidence  considered. 

All  these  works,  especially  that  of  Darwin,  caused  a  great  deal 
of  attention  to  be  paid  to  these  questions,  and,  in  particular,  led  to 
a  large  number  of  investigations  of  the  problems  of  heredity  along 
different  lines. 

One  of  the  most  striking  papers  was  that  of  Mendel,  which  was 
published  in  1866,  and  was  not  a  result  of  previous  activities.  It 
was,  however,  entirely  overlooked,  and  its  significance  not  appre- 
ciated, owing  to  the  discussion  aroused  by  the  "  Origin  of  Species." 
The  paper  was  not  heard  of  until  it  was  rediscovered  independently, 
and  practically  simultaneously  by  three  separate  workers,  Correns, 
Tschermak,  and  de  Vries,  in  different  countries  early  in  1900. 

Johann  Mendel  read  his  two  papers  to  the  Natural  History 
Society  of  Brunn,  where  he  was  a  priest,  and  soon  after,  unfortu- 
nately for  his  scientific  work,  he  was  made  Pralat  of  the  monastery 
of  Brunn.  Let  us  examine  some  of  his  actual  experiments.  He 
crossed  various  varieties  of  peas,  differing  in  certain  points,  and 
observed  numerically  the  result  of  breeding  from  these  crosses. 
Seven  pairs  of  characters  were  investigated,  but  it  is  only  necessary 
to  consider  two  here.  Firstly,  the  seeds  of  different  plants  were 
found  to  be  more  or  less  round  and  smooth,  or  irregular  and  deeply 
wrinkled.  Secondly,  the  seeds  were  noticed  to  be  either  yellow 
(or  orange)  or  green.  In  both  cases  the  result  was  the  same  as  that 
in  the  following  account  where  only  the  characters,  smooth  and 
wrinkled,  are  considered,  and  it  would  still  be  accurate  if  yellow  were 
substituted  for  smooth,  and  green  for  wrinkled. 

2  H 


466  AN   INTRODUCTION   TO  ZOOLOGY 

Plants  bearing  smooth  seeds  were  crossed  with  those  bearing 
wrinkled  seeds,  and  the  results  obtained  can  be  expressed  diagram- 
matically.  In  the  first  filial  generation  (F.  i),  all  the  peas  were 
smooth,  and  the  wrinkled  character  had  disappeared.  When 
members  of  this  generation  were  interbred,  however,  it  was  found 
that  in  their  offspring,  the  second  filial  generation  (F.  2),  wrinkled 


F.I.  ..(S), 

/ /\ 
•£*    F.  2.        S    (S)     (S)     W 

seeds  again  made  their  appearance,  and  in  the  proportion  of  i 
wrinkled  to  3  smooth.  It  is  interesting  to  recall  the  actual  results 
obtained  by  Mendel  to  see  how  closely  they  agree  with  the  pro- 
portions given  :  258  plants  yielded  8023  seeds,  6022  yellow  and 
2001  green  ;  their  ratio,  therefore,  was  3'0i  to  i  ;  253  plants  yielded 
7324  seeds,  5474  round  and  1850  wrinkled  ;  their  ratio  was,  therefore, 
2*96  to  i.  Further  investigation  showed  that  the  three  plants 
with  smooth  seeds  did  not  all  behave  in  the  same  way.  One  of  them 
was  similar  to  the  smooth-seeded  plants  with  which  we  started,  and 
if  inbred  would  never  produce  anything  but  smooth  seeds,  no  matter 
how  many  generations  were  tried  :  it  was,  as  we  say,  '*  pure."  The 
other  two  behaved  similarly  to  that  of  the  F.  i  generation,  and  of 
their  offspring  one- third  always  produced  wrinkled  seeds,  so  that 
they  were  impure.  The  true  ratio  then  of  the  plants  in  the  Fig.  i 
generation  is  i :  2  :  i.  For  a  character  that  we  can  examine  in  this 
sort  of  way  the  term  "  unit  character  "  is  employed,  and  when  we 
find  a  pair  of  such  characters  that  are  mutually  exclusive,  they  may 
be  termed  "  allelomorphs,"  as  they  were  called  by  Bateson.  Mendel, 
in  order  to  describe  the  masking  that  occurs  in  the  F.  i  generation, 

D    X    R 
F.  i. 
F.  2.        D     DR     DR     R 

proposed  the  term  dominant  for  that  character  that  appears  in  the 
F.  i  generation,  and  recessive  for  the  one  that  does  not,  so  that  we 
can  express  the  result  in  a  generalised  diagram.  This  result  is 
sometimes  referred  to  as  Mendel's  Law  of  Dominance,  although  it  is 
not,  strictly  speaking,  a  law,  since  it  is  not  of  general  application, 


EVOLUTION,    VARIATION   AND   HEREDITY          467 

and  in  many  crosses  we  get  a  condition  intermediate  between  the 
parent  forms. 

What  is  the  explanation  of  this  phenomenon  ?  In  the  first  place, 
it  is  obvious  that  the  germ  cell  of  the  parent  may  be  considered 
to  carry  something  which  we  term  a  factor,  let  us  say  D  or  R,  that 
determines  the  appearance  of  the  character  in  the  next  generation. 
If  there  were  a  complete  blending  of  these  characters  in  the  F.  i 
generation,  then  we  should  expect  all  the  F.  2  and  subsequent 
generations  to  remain  the  same,  but  they  do  not.  Suppose  we 
imagine  two  sorts  of  pollen  grain  to  be  produced  in  the  Fig.  i 
generation,  one  with  the  factor  D  and  the  other  with  the  factor  R, 
and  in  the  same  way  two  sorts  of  ovules.  We  can  easily  see  all  the 
chance  combinations  possible  and  the  result. 

D  may  fertilise  D  giving  DD 
D  „  „  R  „  DR 
R  „  „  D  „  RD 
R  „  „  R  „  RR 

This  is  the  proportion  that  we  actually  do  get,  and  so  it  seems 
the  reasonable  explanation.  That  is  to  say,  in  the  germ  cells  of  the 
F.  i  generation  the  unit  characters  are  not  blended,  but  segregated 
in  different  cells,  a  phenomenon  that  we  term  segregation.  It  can 
be  checked  by  the  consideration  of  other  cases,  e.g.  suppose  the 
F.  i  generation,  i.e.  DR,  be  back-crossed  with  the  parent  R,  then 
we  get  offspring  in  the  proportion  2DR  :  2RR,  exactly  what  we 
should  expect.  To  take  a  more  complicated  example,  suppose  we 
cross  a  green  smooth  pea  with  a  yellow  wrinkled  seed.  The  F.  i 
generation  is  smooth,  as  this  is  dominant  to  wrinkled,  and  it  is 
yellow,  as  this  is  dominant  to  green.  Thus  we  can  represent  this 
diagrammatically : 

Sg    X    wY 

^ 
F.  i.  SwYg 

Now  if  we  consider  the  possible  ways  in  which  these  factors  can 
be  segregated  in  the  gametes  of  the  F,  i  generation  we  shall  find 
it  is  four,  namely,  SY,  Sg,  wY,  and  wg.  The  possible  combinations 
of  these  gametes  can  be  seen  if  we  place  them  along  the  sides  of  a 
square  and  then  divide  the  square  up  into  16  smaller  squares.  In 
each  square  of  the  vertical  columns  place  the  symbols  above  the 
column,  and  in  each  square  of  the  horizontal  rows  place  the  symbols 
at  the  beginning  of  each  row.  The  squares  then  indicate  all  the 
possible  fertilisations  from  gametes  that  are  so  constituted.  It  will 
be  seen  that  in  9  squares  the  letters  S  and  Y  occur  ;  in  3  squares  the 


468 


AN   INTRODUCTION   TO  ZOOLOGY 


letters  Sg  ;  in  3  the  letters  Yw,  and  one  only  contains  nothing  but 
wg.  Since  smoothness  (S)  and  yellowness  (Y)  are  dominants,  the 
result  of  the  cross  will  be  9  plants  bearing  smooth  yellow  seeds, 
3  bearing  smooth  green  seeds,  3  bearing  wrinkled  yellow  seeds,  and 
i  bearing  wrinkled  green  seeds.  This  result  has  been  approximated 
to  quite  closely  in  experimental  results. 


vSY 


wY 


wg 


SY 


Sg 


wY        wg 


SY 

Sg          wY 

wg 

SY 

SY         SY 

SY 

SY 

Sg 

Sg 
Sg 

wY 

Sg 

wg 
Sg 

SY 

Sg 

wY 

wg 

wY 

wY 

wY 

wY 

SY 

Sg 

wY 

wg 

wg 

wg 

wg 

wg 

Two  other  terms  are  used  to  indicate  certain  kinds  of  individuals. 
The  gametes  or  germ  cells  are  always  pure  for  certain  factors,  but 
the  zygote  or  individual  is  not.  It  may  be  pure  with  regard  to  a 
certain  character,  say  D,  when  we  say  it  is  a  Homozygote.  On  the 
other  hand,  it  ma}^  be  impure  as  in  the  F.  I  generation,  in  which  case 
we  say  it  is  a  Heterozygote  with  regard  to  that  particular  pair  of 
allelomorphs.  A  Homozygote  produces  gametes  all  bearing  the 
same  factor,  a  Heterozygote  produces  gametes  bearing  different 
factors.  Lastly,  because  of  the  way  in  which  the  recessive  character 
disappears  in .  the  F.  I  generation  in  many  of  the  crosses  made  and 
then  reappears  again  in  the  F.  2  generation,  this  particular  type 
of  inheritance  is  called  alternate,  but  as  rioted  above  this  pheno- 
menon is  by  no  means  always  manifested  in  crossing. 

The  great  value  of  Mendel's  work  and  of  its  rediscovery  was  not 
merely  in  the  actual  facts  it  brought  to  light,  but  in  the  stimulating 
effect  it  had  upon  the  experimental  investigation  of  the  problems 


EVOLUTION,   VARIATION   AND   HEREDITY         469 

of  heredity  ;  suggesting  the  lines  of  work  and  also  the  technical 
methods  to  be  employed.  Numerous  investigations  have  been  made 
along  these  and  similar  lines,  and  many  useful  results  achieved  ;  to 
some  of  them  we  shall  return  later.  It  cannot  yet  be  said,  however, 
that  Mendelism  has  supplied  the  key  to  all  problems  in  heredity,  and 
it  would  appear  that  certain  characters  so  far  have  not  been  analysed 
in  Mendelian  terms.  Before  considering  the  direct  extension  of 
Mendel's  work  we  may  turn  aside  to  consider  two  other  discoveries. 
An  important  contribution  to  our  knowledge  of  heredity 
was  made  by  de  Vries,  one  of  the  rediscoverers  of  Mendelism,  whose 
important  book,  "  The  Mutation  Theory,"  was  published  in  1903. 
This  author  found  wild  in  a  potato  field  hundreds  of  specimens  of 
the  evening  primrose  (Enothcra  lamarckiana,  which  seemed  to 
exhibit  more  than  ordinary  variability.  These  he  removed  to  the 
gardens  at  Amsterdam  and  bred  carefully.  In  a  few  years  he  found 
he  had  produced  seven  distinct  types,  "  elementary  species,"  as  he 
termed  them,  all  so  different  from  the  original  that  each  would  have 
been  described  as  a  new  species  had  it  been  found  in  Nature.  More- 
over, he  found  that  each  of  these  would  breed  true  to  type  generation 
after  generation.  The  most  remarkable  point  about  them  was  that 
they  appeared  suddenly  and  without  warning  and  then  persisted. 
They  were  so  different  from  the  original  that  they  would  find  no 
place  in  the  normal  curve  of  the  species.  For  these  suddenly  arising 
but  persistent  forms  de  Vries  proposed  the  name  mutant,  and  the 
process  was  described  as  mutation.  The  cause  of  these  mutations  is 
practically  unknown,  and  although  there  has  been  much  theoretical 
speculation  concerning  them  we  have  not  time  to  enter  into  it  here. 

It  is  quite  clear  from  this,  however,  that  we  shall  have  to  re- 
consider what  we  mean  by  variation.  We  have  seen  that  ordinary 
variations  may  be  expressed  by  a  normal  curve,  but  these  are  varia- 
tions about  a  mean,  and  no  amount  of  breeding  from  among  them 
would  produce  forms  outside  the  limits  of  the  curve.  For  such 
variations,  which  may  be  described  as  "  normal  variations,"  the 
more  strict  term  "  fluctuations  "  has  been  proposed.  Against  these 
we  may  set  the  mutations  of  de  Vries.  The  term  "  variation,"  then, 
as  it  was  used,  and  as  we  first  used  it,  included  both  fluctuations 
and  mutations,  and,  according  to  de  Vries,  it  is  only  the  selection 
of  the  latter  that  can  have  led  to  the  appearance  of  new  species. 

Further  interesting  observations  were  made  by  Professor 
Johannsen,  who  investigated  the  phenomenon  known  as  the  "  pure 
line."  He  found  in  beans,  where  self-fertilisation  is  possible,  that 
all  the  descendants  of  a  single  plant  showed  normal  variation  about 
a  certain  type.  This  type  need  not  be  the  same  as  the  type  for  the 
general  population  of  all  beans  of  the  same  species.  Moreover,  if 


470  AN  INTRODUCTION  TO  ZOOLOGY 

one  of  the  extreme  variants  of  this  subsidiary  type  were  taken,  its 
offspring  showed  regression  to  the  type  of  the  line  not  the  type  of 
the  species.  He  denned  as  a  "  pure  line,"  therefore,  a  group  of 
individuals  which  has  a  normal  variability  of  its  own,  and  the  off- 
spring of  which  by  self-fertilisation  breed  true  to  the  type  of  their 
own  particular  group.  A  result  of  this  is  to  show  that  selection  in 
a  population  will  come  to  an  end  when  you  have  been  able  to 
separate  out  the  most  eccentric  line.  Further  progress  must  be 
made  by  utilising  mutations  or  dealing  with  the  characters  in  a 
Mendelian  manner.  This  phenomenon  of  a  pure  line  is  of  consider- 
able practical  importance  to  the  breeder  of  plants. 

To  return  again  to  Mendel's  work,  we  find  that  we  can  deduce 
from  his  and  similar  experiments  two  general  principles.     The  first 
is,  that  whatever  unit  factors  may  be  introduced  into  the  zygote  at 
fertilisation,  such  factors  are  not  completely  blended,  but  are  sorted 
out  again  in  the  gametes  it  produces.     This  is  termed  the  law  of 
segregation.     The  second  is,  as  we  have  seen  when  considering  the 
behaviour  of  two  pairs  of  unit  characters,  that  the  inheritance  of  each 
pair  of  characters  is  independent  of  the  other.     This  is  known  as  the 
law  of  independent  assortment.    As  was  noted  above,  the  rediscovery 
of  Mendel's  work  served  as  an  inspiration  to  further  investigation 
along  similar  lines,  and  many  fruitful  fields  of  research  were  opened 
up.     One  of  these  that  is  particularly  associated  with  the  names  of 
Morgan  and  his  fellow- workers  is  of  interest,  sinee  it  is  in  the  main 
a  direct  extension  of  Mendel's  own  experimentation,  and  has  yielded 
additional  laws.     In  the  first  place,  it  has  been  found  that  the  second 
law  given  above  is  not  of  universal  application,  and  that,  while  it  is 
true  in  quite  a  number  of  cases,  an  ever-increasing  number  of  examples 
are  found  in  which  there  is  a  marked  tendency  for  groups  of  cha- 
racters to  be  inherited  together  from  generation  to  generation.     This 
tendency  to  keep  together,  so  that  it  is  really  the  groups  that  are 
independently  assorted  rather  than  the  individual  characters,  is 
termed  linkage,  and  it  is  obvious  that  this  limits  the  law  of  inde- 
pendent assortment.     There  are  certain  theoretical  points  that  can 
be   advanced  to   explain   this  apparent   contradiction.     We  have 
reason  for  thinking  that  the  chromosomes  are  the  actual  bearers  of 
the  factors  whose  presence  in  the  zygote  is  necessary  for  the  appear- 
ance of  certain  characters  in  the  adult.     Moreover,  the  chromosomes 
retain  their  identity  from  division  to  division,  and  the  factors  are 
related  to  them  in  a  very  intimate  and  individual  manner,  and  not 
in  a  loose  general  way.     We  should  expect,  therefore,  that  the 
character  groups  correspond  with  the  chromosomes.     In  support 
of  this,  we  find  that  in  Drosophila  melanogaster ,  the  vinegar  fly, 
there  are  as  many  groups  as  chromosomes,  the  three  largest  con- 


EVOLUTION,   VARIATION   AND   HEREDITY         471 

sisting  of  at  any  rate  100,  75,  and  60  factors,  and  one  small  group  is 
intimately  associated  with  sex  inheritance  and  depends  upon  the 
presence  of  a  certain  small  "  sex  chromosome."  Furthermore,  up 
to  the  present  no  species  has  been  found  in  which  the  number  of 
linked  groups  exceeds  the  number  of  chromosomes.  When  this  is 
taken  into  consideration,  therefore,  we  find  that  the  second  law 
should  read  that  groups  of  characters  tend  to  keep  together — this  is 
linkage — but  that  the  pairs  of  groups  may  be  assorted  independently. 
Thus  the  explanation  of  Mendel's  apparently  contradictory  result  is 
that  he  happened  to  choose  factors  that  belonged  to  separate  groups. 
The  linkage  of  these  large  groups  is  not  absolute,  and  while  on 
the  whole  the  members  of  one  group  tend  to  keep  together,  yet  in  a 
certain  percentage  of  cases  in  some  of  the  crosses  it  is  clear  that  a 
small  group  from  each  one  of  a  pair  of  large  groups  interchanges. 
This  can  be  represented  diagrammatically  as  follows  :  suppose 
the  allelomorphic  factors  of  the  original  large  linked  group  to  be 
represented  by  letters : 

A,   B,   C,  D,  E,  F,   G,   H,   I,   J,   K,   L, 

a,   b,     c,   d,    e,    f,    g,    h,    i,    j,    k,      1, 

Then  in  a  certain  percentage  we  shall  find  that  in  the  formation  of 
the  germ  cell,  equivalent  groups  from  each  have  interchanged  (as 
evidenced  by  the  appearance  of  the  corresponding  factors  in  the 
zygote)  thus : 

a,  b,   c,  d,   E,   F,    G,   H,   I,   J,   K,  L, 
A,  B,  C,  D,  e,   f,      g,    h,    i,    j,    k,     1, 

As  a  rule,  the  percentage  that  do  this  is  small,  though  sometimes 
it  may  reach  as  high  as  33  per  cent. ;  in  such  cases  we  say  that  the 
two  groups  at  the  beginning  were  loosely  linked.  It  should  be  borne 
in  mind,  however,  that  the  exchange  takes  place  between  allelo- 
morphic factors  in  homologous  large  groups — the  exchange  is  not 
haphazard,  but  exact ;  and  also  that  the  percentage  of  cases  in  which 
it  occurs  is  definite  in  all  crosses  for  the  same  group  of  factors.  This 
phenomenon  is  termed  "  crossing  over,"  and  is,  as  it  were,  an 
addendum  to  the  law  of  linkage,  which  can  be  correspondingly 
modified  to  state  that  while  groups  tend  to  remain  linked  in  in- 
heritance, yet  in  some  cases  a  definite  amount  of  crossing  over  may 
occur.  Without  entering  into  a  discussion  of  the  subject,  it  may  be 
noted  in  passing  that  there  is  a  possibility  of  such  an  exchange 
taking  place  during  the  synapsis  stage  of  maturation  when  the  two 
halves  of  a  bivalent  chromosome  may  be  twisted  around  one  another 
to  a  greater  or  less  extent. 

When  the  numerical  results  of  a  series  of  matings  in  which 


472  AN   INTRODUCTION  TO  ZOOLOGY 

crossing  over  occurs  are  investigated,  it  is  found  that  there  is  a  very 
definite  relation  between  the  percentage  of  crossing  over  in  groups 
containing  a  certain  number  of  common  factors.  Let  us  consider 
an  imaginary  example.  If  a  is  a  factor  that  is  usually  linked  with  a 
group,  but  sometimes  crosses  over,  then  we  might  find  in  a  cross 
that  a  would  remain  linked  with  the  group  in  95  per  cent,  of  the  cases, 
while  it  would  cross  over  in  5  per  cent.  Thus  we  could  term  5  the 
cross-over  value  and  95  the  linkage  value.  Suppose,  further,  that 
in  crossing  over  a  is  sometimes  accompanied  by  other  factors, 
b  and  c,  then  of  course  there  would  be  a  cross-over  value  for  each 
combination.  Suppose  the  cross-over  value  of  the  pair  ab  is  5 
and  of  be  is  10,  both  pairs  containing  a  common  factor  b,  then  we 
shall  find  that  the  cross-over  value  of  a-c  is  either  a  function  of  the 
difference  between  the  values  of  ab  and  be,  i.e.  5,  or  a  function  of 
their  sum,  i.e.  15.  This  is  so  unless  the  results  are  interfered  with 
by  some  other  phenomenon,  and  it  is  a  condition  that,  as  may  be 
seen,  would  follow  if  the  factors  were  regarded  as  three  points  on  a 
straight  line.  If  we  take  four  factors  or  five  factors,  and  so  on,  this 
inter-relationship  still  holds  good,  and,  again,  this  is  readily  compre- 
hensible if  they  are  arranged  in  a  straight  line.  Thus  we  can  deduce 
a  further  generalisation  of  the  inheritance  of  factors,  namely,  that 
such  factors,  responsible  for  the  production  of  certain  characters, 
are  arranged  in  a  linear  order.  This  is  sometimes  referred  to  as  the 
law  of  the  "  Linear  order  of  the  factors.' '  It  should  be  noted  that  this 
result  follows  from  an  analysis  of  the  mathematical  results  obtained 
by  experiments,  and  is  independent  of  any  consideration  of  chromo- 
somes, etc.,  but  nevertheless  it  is  obvious  that  the  chromosomes 
and  their  behaviour  in  maturation  and  fertilisation  provide  a  good 
mechanism  by  means  of  which  we  can  conceive  that  such  a  linear 
arrangement  of  the  factors  with  its  attendant  phenomena  could 
easily  be  carried  out. 

We  may  here  leave  the  question  of  hereditary  relationship 
between  parent  and  offspring  and  return  to  the  more  general 
question  of  the  manner  of  evolution  of  the  various  forms  of  life  by 
considering  the  outlines  of  the  past  history  of  one  group  of  animals 
as  far  as  it  has  been  revealed  by  Palaeontology.  For  an  example  we 
shall  take  the  Class  REPTILIA,  since  they  are  fairly  well  known,  and 
from  them  have  sprung  the  higher  vertebrates  living  in  the  world 
to-day. 

Somewhere  near  the  middle  of  the  Devonian  period  the  primitive 
pentadactyl  Vertebrates  had  already  appeared  on  the  Earth.  Various 
authorities  have  calculated  how  long  ago  this  was,  and  arrived  at  very 
different  estimates,  but  even  low  ones  give  it  as  over  20,000,000 


EVOLUTION,   VARIATION   AND   HEREDITY 


473 


474  AN   INTRODUCTION  TO  ZOOLOGY 

years  ago,  and  some  think  very  much  longer  than  this.  These  first 
quadrupeds  were  the  AMPHIBIA,  swamp-dwelling  creatures  some- 
what resembling  the  newts  and  salamanders  of  to-day,  and  from 
which  the  tailless  forms  like  frogs  and  toads  arose  later.  Towards 
the  end  of  the  Devonian  period  appeared  a  specialised  group,  the 
Stegocephalia,  found  all  over  Europe  and  North  America.  To  this 
belonged  a  form  termed  Eryops,  which,  while  appearing  at  a  some- 
what later  time,  is  interesting,  since  it  is  but  little  specialised,  and 
*from  similar  forms  the  early  reptiles  could  have  arisen.  In  appear- 
ance it  is  not  unlike  a  large  newt,  although  its  skeleton  and,  in 
particular,  its  skull  is  of  a  more  advanced  type.  The  first  true 
Reptiles  made  their  appearance  in  the  Carboniferous  period,  and 
by  the  end  of  it  and  the  beginning  of  the  Permian,  we  find  them 
committed  to  at  any  rate  two  different  lines  of  development,  which 
we  shall  follow  separately.  The  differences  between  these  early 
Reptiles  and  the  Amphibia  are  very  slight,  so  that  the  line  of 
demarcation  between  them  is  difficult  to  draw. 

The  first  reptiles  to  notice  are  a  group,  the  Pelycosauria,  of  world- 


FIG.  167. — Restoration  of  the  Permian  reptile,  Limnoscelis  paludis,  from 
New  Mexico. — -After  a  model  by  Lull. 

wide  distribution,  and  of  which  Limnocelis  will  serve  as  a  little- 
specialised  type.  It  is  slightly  more  lizard-like  than  Eryops,  and 
more  adapted  for  life  on  dry  land.  As  a  rule,  the  members  of  this 
group  remained  little  modified,  but  certain  forms  like  Edaphosaums 
and  its  allies  had  the  neural  spines  of  the  vertebrae  enormously 
elongated  and  a  large  web-like  extension  of  the  skin  between  them. 
These  strange  forms  rarely  exceeded  a  length  of  5  feet,  and  soon 
died  out.  From  the  Pelycosauria  were  derived  a  second  group, 
the  Theriodontia,  or  reptiles  with  mammal-like  teeth,  first  appearing 
in  Upper  Permian  beds  in  South  Africa.  They  are  far  more  adapted 
to  life  on  land,  and  progressed  fairly  rapidly,  holding  their  bodies 
well  up  off  the  ground  and  running  like  a  mammal  instead  of  crawling 
like  a  lizard.  From  this  group  in  the  Triassic  period  arose,  the 
primitive  Mammalia,  whose  remains  have  been  found  in  the  upper 
strata  of  this  series  in  South  Africa.  Thus  we  have  derived  from  the 
one  specialisation  of  the  early  Reptiles,  the  Mammals,  a  group 
destined  to  spread  over  the  whole  world,  to  become  adapted  to 


EVOLUTION,   VARIATION   AND   HEREDITY 


475 


various  conditions  of  life,  and,  finally,  to  become  a  dominant  class 
culminating  in  Man  himself.  Lack  of  space  prohibits  more  than 
this  brief  indication  of  their  line  of  origin. 

To  pass  back  to  the  second  group  of  primitive  reptiles,  we 
find  in  early  Permian  times  a  form  termed  Seymouria.  This  is  a 
slightly  stouter  animal  than  Eryops,  exhibiting  many  Stegocephalian 
features,  but  showing  a  certain  approach  towards  the  higher 
Reptilia.  It  may  be  considered  as  a  lowly  representative  of  a 
diversified  group  of  primitive  Reptiles,  the  Cotylosauria,  from  which 


FIG.  1 68. — Ship-lizard,  Edaphosaurus  cruciber,  Per  mo-Carboniferous, 
North  America. — -After  Case. 


the  higher  forms  originated,  and  which  was  duly  established  by  the 
end  of  the  lower  Permian.  At  any  rate,  three,  or  perhaps  four, 
distinct  lines  of  development  from  this  group  can  be  traced.  The 
Cotylosaurs  were  land  dwellers,  but  from  time  to  time  some  of  their 
descendants  returned  again  to  the  sea.  The  first  line  we  shall 
notice  is  but  little  known,  and  culminated  in  the  Turtles  and  Tortoises, 
or  Chelonia,  as  they  are  collectively  termed.  They  first  make  their 
appearance  in  early  Triassic  times  as  quite  highly  specialised  forms, 
easily  recognisable  as  similar  to  their  descendants  living  to-day. 
They  are  characterised  by  the  broadening  and  flattening  of  their 
ribs  and  the  development  of  a  characteristic  box-like  skeleton. 
Of  their  previous  history  but  little  is  known,  save  for  an  interesting 
form,  Eunotosaurus ,  from  the  late  middle  Permian  beds,  an  animal 
with  broad  expanded  ribs  and  a  peculiar  skull  that  is  regarded  by 
some  authorities  as  a  form  linking  the  Chelonia  with  the  Cotylosaurs. 
The  second  line  is  the  Plesiosauria,  large  aquatic  lizards  that  make 
their  appearance  in  Mid  Triassic  times.  Their  remains  are  probably 
the  most  widely  spread  and  common  of  all  fossils,  being  recorded 
from  all  parts  of  the  world.  Of  the  hundreds  of  skeletons  known 


476  AN   INTRODUCTION  TO  ZOOLOGY 

some  are  almost  complete  and  beautifully  preserved.  They  varied 
considerably  in  size,  but  were  practically  all  large,  ranging  from 
about  10  up  to  50  feet  in  length.  The  body  was  stout  and  the 
limbs  transformed  into  two  parts  of  large  powerful  paddles  ;  the 
tail  was  fairly  short,  the  neck  long,  and  the  mouth  armed  with 
formidable  teeth.  In  the  case  of  Elasmosaurus,  the  neck  reached  a 


FIG.  169. — Plesiosaur,  Cryptocleidus,  restored  by  Knight. 

length  of  23  feet  out  of  a  total  of  41  feet.  The  third  line,  the 
Icthyosauria,  also  appearing  in  Mid  Triassic  strata,  were  also  aquatic, 
large  in  size,  widespread,  and  again,  have  left  some  rem'arkably  well- 
preserved  skeletons.  These  were  more  like  fish  or,  rather,  dolphins 
in  body  form  than  the  previous  group,  and  swam  vigorously  by 
means  of  a  powerful  tail.  The  limbs  are  not  so  large,  and  apparently 


FIG.  170. — Ichthyosaur.     Restoration  by  Knight. — From  Schnchert's 
"  Historical  Geology." 

acted  in  the  manner  of  fins.  The  neck  is  quite  short  and  the  head 
pointed.  They  varied  in  size  from  2  feet  up  to  30  feet,  and  possessed 
very  large  eyes.  Often  the  remains  of  an  adult  are  found  accom- 
panied by  those  of  several  young,  so  that  the  offspring  apparently 
remained  with  the  parents  for  some  time.  As  noted  above,  the 
Plesiosaurs  and  Icthyosaurs  were  widely  spread  over  the  globe,  and 
both  groups  died  out  in  Cretaceous  times  ;  the  latter  disappearing 
in  the  middle  strata  and  the  former  lasting  on  into  the  upper  beds. 
The  fourth  line  of  evolution  is  a  very  important  one,  since  it  led 


EVOLUTION,    VARIATION   AND   HEREDITY         477 

• 

to  the  groups  ancestral  to  all  modern  Reptilia,  except  the  Chelonia, 
and  to  Birds,  as  well  as  to  the  most  extraordinary  forms  of  Verte- 
brates that  have  lived  on  the  Earth.     They  all  probably  originated 
from  a  group,  the  Thecodontia,  first  known  from  the  Lower  Trias. 
These  were  comparatively  small,  little  specialised  forms,  showing 
certain  advances  over  the  Cotylosauria  in  the  structure  of  the  skull 
and  shoulder  girdle.     From  them  sprang  the  Squamata,  the  Dino- 
sauria,  and,  perhaps,  also  the  Rhynchoeephalia.    The  last  group 
probably  appeared  in  Mid  Triassic  times,  and  while  resembling  true 
Lizards  in  external  appearance,  they  nevertheless  present  important 
points  of  difference  from  them  in  the  characters  of  their  skeleton, 
notably  of  the  skull.    They  were  apparently  never  a  very  extensive 
group,  but  one  solitary  member  of  it,  the  New  Zealand  Tuatara, 
Sphenodon  punclalus,  has  persisted  to  the  present  day.     It  is  now 
confined  to  certain  islands  off  the  coast  of  New  Zealand,  and  is  in 
grave  danger  of  extinction  at  no  very  distant  date.     The  Squamata 
also  appear  in  the  Mid  Tiias,  and  constitute  the  true  Lizards  and 
other  allied  forms  derived  from  them.    The  term  Lizard,  as  applied 
to  living  forms,  is  restricted  to  the  Order  Lacertilia,  examples  of 
which  are  first  found  in  late  Triassic  times.    They  have  persisted 
until  to-day,  and  now  form  a  fairly  widespread  group,  the  Lizards, 
Iguanas,  Monitors,  Blind-worms,  etc.,  living  in  the  temperate,  sub- 
tropical, and  tropical  zones.     On  the  whole,  they  are  adapted  for  a 
life  on  dry  land,  many  of  them  even  to  the  desert,  but  some  are 
burrowing  forms  and  a  few  partly  aquatic.     From  the  Squamata, 
perhaps  during  the  Upper  1  rias,  were  derived  the  snakes,  or  Ophidia, 
which  still  persist,  although  we  find  no  record  of  them  until  the 
Upper  Cretaceous.     They  also  gave  rise  presumably  about  the  same 
time  to  the  Mosasauria,  which  were  a  group  of  aquatic  forms  having 
but  a  relatively  short  existence,  appearing  first  in  the  Upper  Cre- 
taceous and  becoming  extinct  in  the  same  period.     In  spite  of  their 
limited  duration  they  were  highly  specialised,  large  in  size,  varying 
trom  8-50  feet  in  length,  and  were  widely  scattered  over  the  globe. 
They  left  well-preserved  fossil  remains,  including  in  some    cases 
pieces  of  skin,  and  from  the  fact  that  they  had  a  powerful  set  of 
teeth,  and  their  bones  often  bear  healed  scars,  we  may  assume  they 
were  fierce  fighters.     In  form  they  were  somewhat  eel-shaped,  with 
limbs  reduced  to  small  fin-like  structures  and  a  powerful  tail,  which 
was  not  of  the  same  fish-tail  shape  as  in  the  Ichthyosaurs. 

The  Dinosauria  are  a  large  diverse  group  that  were  ap- 
parently the  dominant  land-dwelling  Vertebrates  throughout  a 
large  part  of  the  Mesozoic  era.  They  first  appear  in  the  Middle 
Trias,  and  show  two  distinct  lines  of  development :  the  Saurischia, 
in  which  the  bones  of  the  pelvic  girdle  resemble  those  in  Lizards,  and 


478 


AN   INTRODUCTION   TO  ZOOLOGY 


the  Ornithischia,  in  which  the  girdle  approaches  more  closely  that 
of  the  birds.  The  Order  Crocodilia — Crocodiles,  Alligators,  and 
Ga  vials — probably  arose  from  the  Saurischia  in  Mid  Triassic  times, 
but  their  remains  are  not  found  until  the  early  layers  of  the 


FIG.  171. — 'Restoration  of  the  sauropod  dinosaur,  Diplodocus,  based  upon  the 
mounted  specimen  in  the  Carnegie  Museum.  Length,  87  feet.  Coman- 
chian,  Wyoming. — After  Lull,  from  Schuchert's  "Historical  Geology." 

Comanchian,  when  they  are  fully  differentiated  and  resemble 
modern  forms.  Undoubted  Saurischian  Dinosaurs  appear  in  the 
Upper  Trias,  and  -continue  on  until  the  end  of  the  Cretaceous. 
They  become  specialised  along  two  lines,  the  one  herbivorous  and 


FIG.  172. — Restoration  of  Brachiosaurus,  the  most  ponderous  sauropod. 
Length,  about  80  feet.  Comanchian,  North  America  and  East  Africa. — 
Modified  from  Matthew. 

the  other  carnivorous.  The  herbivorous  forms  reached  their  zenith 
in  the  Comanchian  times,  when  they  were  represented  by  three 
gigantic  forms,  Diplodocus,  Brontosaurus,  and  Brachiosaurus, 
undoubtedly  the  largest  land  animals  of  which  we  have  any  know- 
ledge. They  were  about  87  feet,  66  feet,  and  80  feet  long  respectively, 


EVOLUTION,   VARIATION   AND   HEREDITY         479 

but,  in  spite  of  its  greater  length,  Diplodocus  was  not  so  huge  an 
animal  as  Brachiosaurus,  for  while  the  former  weighed  about  30  tons, 
the  latter  was  over  40,  and  Brontosaurus  was  intermediate  between 
them.  They  were  all  apparently  slow-moving  swamp-dwellers, 
walking  on  all  fours.  They  had  long  necks  and  tails,  and  very  small 
heads  compared  with  the  bulk  of  the  body.  The  carnivorous  types 
culminated  in  Tyrjnnosaurus,  which  reached  a  length  of  47  feet. 
It  ran  on  its  strong  hind  legs,  and  in  this  position  its  height  was 
from  18-20  feet.  Its  head  is  about  4  feet  long,  very  massive,  and 


FIG.  173. — Restoration  of  Tyrannosaurus,  based  upon  a  specimen  in  the  Ameri- 
can Museum  of  Natural  History.  Length,  47  feet.  Cretaceous,  western 
North  America. — After  Lull,  from  Schuchert's  "  Historical  Geology." 

provided  with  powerful  teeth,  so  that  altogether  it  was  well  fitted  to 
prey  on  the  large  herbivorous  forms. 

The  Ornithischia  probably  arose  about  the  same  time,  or  perhaps 
a  little  later,  than  the  Saurischia,  and  they  show  three  distinct 
lines  of  modification,  and  are  remarkable  not  so  much  for  their  size 
as  for  the  extraordinary  and  often  bizarre  form  assumed  by  their- 
armour.  The  first  line  led  to  swift -running  forms,  which  were  bird- 
footed  and  bipedal ;  the  second  line  produced  forms  with  a  great 
development  of  armour  in  the  form  of  plates  and  spines ;  and  the 
third  line  is  characterised  by  the  development  of  horns  and  of  a 
large  bony  neck  frill.  The  two  last  groups  were  quadrupedal.  The 
bird-like  forms  are  represented  by  a  number  of  animals  like  Iguanodon, 
from  the  Cretaceous  beds  of  Belgium  and  the  Isle  of  Wight.  This 
was  a  large  creature  about  34  feet  long  with  very  powerful  hind 
limbs,  and  the  hand  bore  a  very  well-marked  spine-like  thumb. 
The  most  striking  of  the  armoured  Dinosaurs  is  perhaps  Stegosaurus, 
which  reached  a  length  of  about  20  feet.  Along  each  side  of  its 


480  AN   INTRODUCTION  TO  ZOOLOGY 

back  was  a  row  of  great  upstanding  plates,  and  on  the  tail  four  pairs 


FIG.  174. — Restoration  of  Iguanodon.     Cretaceous  of  Belgium  and  the 
Isle  of  Wight. — After  Heilmann. 

of  powerful  spines  2  feet  or  more  in  length.     It  is  also  noteworthy 


FIG.  175. — Restoration  of  the  armoured  dinosaur,  Stegosaurus,  based  upon  the 
mounted  skeleton  in  the  Yale  University  Museum.  Length,  about  20  feet. 
Comanchian,  Wyoming  and  Colorado. — After  Lull,  from  Schnchert's 
"Historical  Geology." 

in  that  its  brain  was  extremely  small,  and  it  had  a  big  swelling  in 


EVOLUTION,   VARIATION   AND   HEREDITY 


481 


the  lumbar  region  of  the  spinal  cord  many  times  larger  than  the 
brain,  and  presumably  to  look  after  the  heavy  hind  limbs  and  tail. 
The  horned  forms  are  represented  by  such  an  animal  as  Triceratops, 
in  form  somewhat  like  a  Rhinoceros,  and  reaching  a  length  of  20- 


FIG.  176. — Restoration  of  the  horned  dinosaur  Triceratops.  Length,  20-25 
feet.  Upper  Cretaceous  (Lance),  western  North  America. — After  Lull, 
from  Schuchert's  "Historical  Geology." 

25  feet.  It  had  a  sort  of  short  beak,  two  large  horns  over  the  eyes, 
one  over  the  nasal  region,  and  a  large  frill-like  collar  of  bone  extending 
back  over  the  neck. 

In  early  Jurassic  times  a  remarkable  group  of  flying  lizards, 
the  Pterodactyla,   arose   probably  from   the  same  group   as   the 


FIG.  177. — Pterodactyl,  Rhamphorhynchus  phyllurus. — After  Lull. 

Omithischia,  and  persisted  until  the  end  of  the  Cretaceous.  They 
were  provided  with  wings,  not  produced,  as  in  birds,  by  the  develop- 
ment of  feathers  on  the  fore  limb,  but  by  the  growth  of  a  membrane 
stretching  from  the  tips  of  the  enormously  elongated  fingers  back  to 
the  toes,  and  so  somewhat  similar  to  the  bats  of  to-day.  Their  heads 

2  I 


482 


AN   INTRODUCTION   TO  ZOOLOGY 


were  fairly  large,  and  the  strong  jaws  were  well  armed  with  teeth. 
While  some  of  them  were  no  larger  than  a  sparrow,  others  reached  a 
large  size,  even  as  much  as  27  feet  across  the  wings,  and  so  they  must 
have  presented  a  truly  terrific  aspect  well  justifying  their  popular 
name  of  "  flying  dragons." 

Again,  the  common  Ornithischian  stock  probably  gave  rise  to 
the  true  birds.     The  forerunner  of  the  Class  Aves  was  a  remarkable 


FIG.  178. — Reptilian  bird,  Arch&opteryx  (A),  compared  with  pigeon,  Columba 
livia  (B). — After  Lull. 

creature  that  is  found  in  early  Jurassic  times  and  known  as  Archce- 
opteryx.  Two  fairly  complete  skeletons  of  this  animal  are  known, 
but  we  have  no  indication  of  its  exact  derivation.  It  was  bird-like 
in  many  respects,  such  as  the  general  shape  of  the  body,  the  feet,  and 
the  possession  of  quite  typical  feathers  and  true  wings.  On  the 
other  hand,  it  had  also  distinct  Dinosaur  affinities,  which  mark  it  off 
sharply  from  modern  birds.  Thus  it  had  a  long  pointed  tail,  short, 


EVOLUTION,  VARIATION  AND  HEREDITY          483 

clawed  fingers  on  the  front  of  the  wing,  and  jaws  with  a  number  of 
pointed  teeth.  Later,  in  Comanchian  times,  we  encounter  another 
bird  group  exemplified  by  Ichthyornis  and  Hesperornis.  These  were 
fairly  typical  water  birds  in  general  appearance,  had  shortish  tails, 
and  were  covered  with  feathers.  Their  heads  have  certain  reptilian 
resemblances,  the  jaws  bore  teeth,  and  the  breast  bone  was  not  keeled. 
True  birds  appear  in  the  Tertiary,  and  are  closely  related  to  living 
forms.  In  spite  or  the  breaks  in  the  record,  however,  there  is  little 
doubt  about  the  general  relationship  of  true  birds  with  their  precursors. 
We  have  thus  in  a  very  superficial  manner  touched  on  some 
of  the  main  types  of  the  Reptilia,  indicated  the  important  lines  of 
evolution  they  followed,  and  shown  their  relation  to  the  animals 
living  to-day.  Only  a  few  of  the  striking  forms  have  been  mentioned 
and  many  more  are  known.  As  a  class  they  spread  widely  over  the 
surface  of  the  earth,  and  were  so  plentiful  and  diverse  in  the  Mesozoic 
times  that  we  often  refer  to  this  period  as  the  "  Age  of  Reptiles." 
As  to  the  reasons  for  the  decline  of  the  Reptiles,  the  times  in  which 
they  lived  are  so  remote  that  they  become  matters  of  conjecture, 
but  certain  points  seem  to  stand  out  fairly  clearly.  The  greater 
part  of  the  Mesozoic  was  a  period  of  relative  stability,  or  of  only 
very  gradual  change,  so  that  many  groups  of  the  Reptiles  were  able 
to  branch  out  and  become  highly  adapted  to  certain  environments. 
The  close  of  the  era  was  marked  by  very  far-reaching  and  profound 
changes  in  the  configuration  of  the  land  surface.  These  were 
probably  accompanied  also  by  considerable  climatic  alterations. 
The  result  was,  that  the  highly  modified  forms  had,  as  the  very 
outcome  of  this  specialisation,  apparently  lost  the  power  to  become 
re-adapted  to  altered  environmental  conditions.  Thus  during  this 
transition  time  the  Ichthyosaurs,  Plesiosaurs,  Mosasaurs,  Ptero- 
dactyls, and  both  groups  of  the  Dinosaurs,  died  out  in  the  Middle 
and  Upper  Cretaceous  times.  This  left  a  number  of  forms  which 
were  either  little  specialised  as  the  early  Mammals,  Crocodiles,  and 
Lacertilia,  and  capable  of  giving  rise  to  new  diversity,  or  else,  like 
the  Aves  and  Chelonia,  capable  of  becoming  adapted  to  the  changed 
conditions.  It  seems  probable,  then,  that  the  end  of  the  dominance  of 
the  Reptiles  was  the  result,  direct  or  indirect,  of  changed  environment. 

The  history  of  the  Reptiles  serves  to  illustrate  the  general 
course  of  evolution  in  the  animal  kingdom  in  general.  In  the  first 
place,  we  have  seen  lowly  little  specialised  forms  branching  out  into 
various  modes  of  living,  and  becoming  fitted  for  life  in  various 
environments.  That  is  to  say,  the  generalised  type  spreads  out 
into  a  series  of  adaptive  specialisations  that  produce  a  diversity  of 
forms,  and  we  may  say  that,  on  the  whole,  groups  of  animals  have 


484  AN   INTRODUCTION  TO  ZOOLOGY 

followed  this  process  of  "  Divergent  Evolution."  They  tend  to 
diverge  more  and  more  from  one  another  and  from  the  ancestral 
stock  as  evolution  proceeds,  and  we  can  express  their  inter-relations 
schematically  in  a  branched  fan-like  diagram. 

Looked  at  from  another  point  of  view,  we  may  regard  it  as  an 
illustration  of  what  has  been  termed  "  Adaptive  Radiation.""  That 
is  to  say,  when  a  form  or  type  of  animal  organisation  has  been 
established,  its  descendants  proceed  to  spread  out  and  become 
adapted  to  a  number  of  different  environments  :  the  air,  the  sea, 
the  swamp,  the  forest,  the  arid  plain,  and  so  on.  Any  line  may 
become  highly  adaptive,  possessing  remarkable  peculiarities,  yet 
each  group  retains  certain  characters,  not  the  same  in  each  case,  of 
course,  reminiscent  of  the  ancestral  form. 

Another  phenomenon  not  nearly  so  widespread  as  divergence,  but, 
nevertheless,  of  considerable  importance,  may  also  be  noted.  It 
follows  as  a  result  from  this  adaptive  radiation  that  takes  place  in 
various  groups.  Members  of  various  and  quite  different  groups 
may  take  to  living  in  almost  identical  environments,  say,  for  example, 
the  air  or  the  sea.  If  it  is  an  environment  that  demands  a  high 
degree  of  specialisation  in  order  to  bring  about  satisfactory  adapta- 
tion, as  both  of  these  do,  then  we  sometimes  find  animals  of  quite 
different  groups  assuming  forms  with  a  greater  or  less  amount  of 
superficial  resemblance.  Thus,  for  example,  we  find  birds,  bats, 
and  pterodactyls,  while  not  at  all  closely  related,  live  in  the  same 
environment  and  exhibit  obvious  similarities,  particularly  the  last 
two.  Again,  there  is  a  strong  resemblance  between  Fish,  Ichthyo- 
saurs,  Mosasaurs,  Dolphins,  and  Whales,  and  in  the  case  of  the  limb- 
less lizards,  e.g.  Blind-worms  and  Snakes,  the  approach  is  very 
striking.  To  this  we  apply  the  term  "  Convergent  Evolution  "  or 
"  Parallelism  in  Evolution."  In  classifying  animals  it  is  obvious 
that  this  convergence  must  be  taken  into  account,  or  otherwise  we 
should  be  liable  to  group  together  erroneously  animals  whose 
similarities  are  due  to  their  being  adapted  to  a  similar  environment, 
and  not  to  a  community  of  descent. 

Here  the  above  outline  of  some  of  the  main  problems  that 
arise  from  a  consideration  of  Evolution,  Variation,  and  Heredity 
may  be  brought  to  a  conclusion.  It  is  not  intended  to  imply  that 
there  are  no  further  problems  to  be  studied,  but  this  chapter  does 
not  aim  at  being  even  a  statement  of  all  the  various  biological 
theories,  which  would  in  itself  require  a  volume  many  times  larger 
than  this.  Rather  has  it  attempted  to  indicate  certain  outstanding 
points  and  to  provide  an  introduction  to  further  and  wider  reading 
in  these  matters  in  which  many  excellent  texts  are  now  available. 


INDEX 


Abdomen  268 

Abdominal  pore  211 

Aboral  150 

Accessory  structures  of  sense  organs 

100 

Accommodation  97 
Accretion  7,  122 
Acetabulum  32 
Achromatic  figure  364 
Achromatin  302 
Acromegaly  104 
Adaptation  in 
Adaptive  radiation  484 
Adradii  164 
Adipose  tissue  39 
Adrenal  bodies  103 
Adult  8 
Etiology  3 
Afferent  fibres  90 
Afferent  vessels  176 
Agametes  147 
Agamobium  169 
Age  of  Reptiles  457,  483 
Alimentary  canal  18 
Alimentary       System,      Rana       46, 

Scyllium  228 
Allanto-chorion,    Chick    421,    Lepus 

438 

Allantois,  Chick  420,  Lepus  437 
Alternation  of  generations,  Protozoa 

139,  Tasnia  202 

Alveolus,  Rana  43,  51,  Lepus  307 
Amitosis  363 

Amnion,  Chick  418,  419,  Lepus  434 
Amoeba  118 
Amphiaster  364 
Amphibia  12,  474 
Amphioxus  378 
Ampulla  99 
Amylase  55 
Anabolism  6,  in 
Anaphase  363,  366 
Anatomy  2,  4,  human  4 
Animals  and  Plants  108 
Animal  increase  461 
Animate  4 
Anisogametes  145 
Anisogamy  145 
Anisotropic  42 

485 


Annelida  170,  171 

Annulata  171 

Arnmlus  tympanicus  150 

Ante-brachium  15 

Anterior  14 

Anterior  intestinal  portal  414 

Antibodies  70 

Antitoxin  70 

Anus  172    • 

Apex  cordis  314 

Aponeurosis  41,  plantaris  41 

Appendix  vermiform  305 

Aqueductus  vestibuli  219 

Aqueous  humour  94 

Arachnoid  membrane"  327 

Archceopteryx  482 

Archenteron,   Amphioxus   381,   Rana 

389 

Arches,  branchial  223,  Hyoid  223, 
422,  second-sixth  414,  visceral  422 

Archoplasm  362 

Area  of  cohesion  42 

Area  pellucida  399,  opaca  399, 
vasculosa  401 

Areolar  tissue  37 

Arterial  system,  Rana  61,  Scy Ilium 
235,  Lepus  308 

Arteries  18 

ARTERIES:  Afferent  branchial  235; 
anterior  carotid  236 ;  anterior 
mesenteric,  Scyllium2^8,  Lepus  310 ; 
Aorta  cardiac  235  ;  Aorta,  dorsal, 
Rana20, 62,  Scyllium236,  Lepus  308, 
Chick  413;  Aorta,  ventral,  Rana  6 1 , 
Scyllium  235,  236,  Chick  414  ; 
Aortic  arches,  Scyllium  239,  Lepus 
308  ;  Brachial  309  ;  Carotid  60  ; 
Carotid  Arch  61  ;  Carotid,  dorsal 
237  ;  Carotid,  external,  Rana  61, 
Scyllium  237,  Lepus  309;  Carotid, 
internal,  Rana  61,  Scyllium  237, 
Lepus  309;  Carotid,  common, 
Scyllium  237,  Lepus  308,  309 ; 
Carotid,  posterior  236 ;  Cerebral 
62  ;  Coeliac,  Rana  62,  Scyllium 
238,  Lepus  310 ;  Coeliaco-mesen- 
teric  62  ;  Coronary  308 ;  Cu- 
taneous 63  ;  Ductus  arteriosus 
308 ;  Efferent  branchial  236  ; 
Epibranchial  236  ;  Epigastric  63  ; 
Epigastric,  anterior  309 ;  Epigastric, 


486 


INDEX 


posterior  310  ;  Femoral,  Rana  63, 
Lepus  310  ;  Gastric  62  ;  Genital, 
Rana  63,  Lepus  310  ;  Hepatic  62  ; 
Hyoid  236 ;  Iliac,  Rana  63, 
Scyllium  239  ;  Iliac,  common  310  ; 
Iliac  external  and  internal  310 ; 
Ilio-lumbar  310 ;  Innominate, 
Scyllium  235,  Lepus  308  ;  Inter- 
costal 309,  310  ;  Laryngeal  62  ; 
Lieno-gastric,  Scyllium  238,  Lepus 
310  ;  Ligamentum  arteriosum  308  ; 
Lingual  61  ;  Mandibular  236 ; 
Mandibular  Arch  414 ;  Median 
sacral  310  ;  Mesenteric  62  ;  Mesen- 
teric  posterior,  Rana  63,  Scyllium 
238,  Lepus  310  ;  Occipito- vertebral 
62 ;  CEsophageal  62  ;  Omphalo- 
mesenteric  436 ;  Ovarian,  Rana 
63,  Lepus  310 ;  Palatine  62  ; 
Parietal  239  ;  Pelvic  239  ;  Pulmo- 
cutaneous  60,  63  ;  Pulmonary, 
Rana  63,  Lepus  308 ;  Recto- 
vesicular  63  ;  Renal,  Rana  63, 
Scyllium  239,  Lepus  310  ;  Sciatic 
63 ;  Spermatic,  Rana  63,  Lepus 
310 ;  Splenic  62  ;  Sub-clavian, 
Rana  62,  Scyllium  238,  Lepus  308, 
3°9 1  Systemic  60,  62  ;  Truncus 
arteriosus  60 ;  Umbilical  438 ; 
Vertebral,  Rana  62,  Lepus  309  ; 
Vesicular  310  ;  Vitelline  413 

Arterioles  58 

Ascidians  449 

Aspiration  57 

Assimilation  6,  Rana  113 

Association  neurons  187 

Aster  364 

Astral  rays  364 

Atlanto-occipital  membrane  28 

Atrio- ventricular  orifice  316 

Atrium,  Rana  60,  Scyllium  234,  Lepus 
308 

Attraction  sphere  362 

Auditory  organ  98 

Auricle,  Rana  17,  59,  Scyllium  234, 
Lepus  315 

Auriculo-ventricular  aperture  60 

Automatism  165 

Autostylic  skull  222,  Lepus  267 

Aves  482 

Avian  malaria  143 

Axon  86 


Bacteriolytic  substances  70 
Basal  disc,  Hydra  148,  Obelia  168 
Basal  granule  126 
Basis  cordis  314       ^ 
Bateson,  Professor  466 


Bay  head-fold  403 

Bile  1 8 

Binomial  nomenclature  115 

Biogenesis  448 

Biology  i,  Post -Darwinian  464 

Bionomics  3 

Birds,  wingless  450 

Bladder  181 

Blastocoel,  Rana  106,  389,  Hydra  159, 

Obelia  168,  Amphioxus  381,  Chick 

398 

Blastoderm  398,  extra-embryonal  403 
Blastodisc  398 
Blastomeres  377,  Chick  398 
Blastopore,    Amphioxus    381,    Rana 

387 

Blastostyle  161 

Blastula,  Rana  106,  387,  Hydra  159, 
Obelia  168,  Amphioxus  381 

Blind  spot  94 

Boa  constrictor  448 

Bone  24,  cancellous  35,  cartilage  24, 
cells  of  36,  compact  35,  corpuscles 
36,  endochondral  37,  membrane 
24,  perichondral  37 

Botany  i 

Bowman's  capsule  74 

Brachiosaurus  478,  479 

Brachium  14 

Brain  21 

BRAIN  :  Anterior  Pillars  of  fornix 
347  ;  Aqueduct  of  Sylvius,  Rana 
79,  Scyllium  253,  Sheep  351  ; 
Arbor  vitae  353  ;  Archipallium 
349 ;  Area  acustica  353  ; 
Brachium  conjunctivum  352  ; 
Brachium  pontis  352  ;  Capsule, 
external  346 ;  Capsule,  internal 
346  ;  Cephalic  flexure  255  ;  Cere- 
bellar  peduncle  254,  anterior  352, 
inferior  353,  middle  352 ;  Cere- 
bellum, Rana  78,  Scyllium  253, 
Lepus  329,  Chick  429  ;  Cerebrum, 
Scyllium  251,  Lepus  328  ;  Choroid 
plexus  345,  anterior  77,  posterior 
78,  of  4th  Ventricle  352  ;  Claus- 
trum  346 ;  Colliculi  351  ;  Com- 
missure, anterior  347,  cerebral  347, 
habenular  350,  hippocampal  347, 
posterior,  Scyllium  253,  Sheep  350, 
superior,  Scyllium  252,  Sheep  350 ; 
Corona  radiata  345  ;  Corpora 
bigemina,,  Rana  78,  Scyllium  253 ; 
Corpus  callosum,  Lepus  329,  Sheep 
340,  346  ;  Corpus  mammilare  343  ; 
Corpora  quadrigemina  269,  329, 
351  ;  Corpora  restiformia,  Scyllium 
254,  Sheep  353  ;  Corpora  striata, 
Scyllium  251,  Sheep  345,  346, 
Chick  428  ;  Corpus  trapezoideum, 


INDEX 


487 


Lepus  331,  Sheep  352  ;  Cnira 
cerebri,  Rana  78,  Scyllium  253,  257, 
Lepus  329,  Sheep  351,  Chick  428  ; 
Divisions  of  257  ;  Epiphysis  cerebrir 
Rana  78,  Scyllium  252,  Lepus  329, 
Sheep  350,  Chick  428 ;  Fimbriae 
hippocampi  347  ;  Fissura  cruciata 
340,  rhinalis  349,  suprasylvia  340  ; 
Floccular  lobes  329 ;  Flocculus 
352  ;  Foramen  interventriculare 
345,  of  Munro,  Rana  79,  Scyllium 

253,  Sheep  345,  Chick  428  ;    Fore- 
brain,    Scyllium    255,    Sheep    340, 
Chick  407  ;    Fornix  347  ;     Fourth 
ventricle  352  ;    Frontal  lobe  329  ; 
Geniculate  bodies  350  ;    Gyri  340  ; 
Habenular  ganglia,   Scyllium  252, 
Sheep   350  ;     Hemisphaerium  cere- 
belli    351  ;     Hemispheres    cerebral 
427 ;     Hind   brain,    Scyllium   255, 
Lepus    351,    Chick    407 ;     Hippo- 
campus 345,  347,  Horns  of  ventricle 
345 ;  Hypophysis  cerebri,  Rana  78, 
Scyllium    253,    Lepus    329,    Sheep 
343  ;       Infundibulum,     Rana     78, 
Scyllium    253,    Lepus    329,    Sheep 
343,    Chick    428  ;      Isthmus    428  ; 
Iter,  Rana  79,  Scyllium  253,  Sheep 
351,  Chick  428  ;  Lamina  terminalis, 
Rana  77,  Scyllium  253,  Sheep  347, 
Chick  407,  427  ;  Lobe,  frontal  340, 
parietal    329,    340,    pyriform    340, 
olfactory,  Scyllium  251,  Lepus  329, 
Sheep    342,    temporal    340 ;     Lobi 
inferiores     253  ;     Lobule,     hippo- 
campal  340  ;    Lobas,  linea  lateralis 

254,  visceralis  254  ;    Massa  inter- 
media   350 ;     Medulla    oblongata, 
Rana  78,  Scyllium  254,  Lepus  330, 
Sheep,      352,      353,     Chick     430 ; 
Medullary     velum,    anterior     and 
posterior      352  ;       Mesencephalon, 
Rana  78,  Scyllium  255,  Lepus  329, 
Chick   407,   428  ;     Metaccele   254  ; 
Metencephalon,    Rana    78,    Lepus 
329,       Chick      428  ;        Mid-brain, 
Scyllium    253,     255,     Sheep    351, 
Chick  407  ;    Myelencephalon,  Rana 
78,  Scyllium  254,  Lepus  330,  Chick 
428 ;     Neo-pallium    349 ;     Neuro- 
pore  255  ;    Neuroporic  recess  255  ; 
Nucleus,  caudatus  and  lenticularis 
346 ;     Olfactory    peduncles     251  ; 
Optic    chiasma,    Rana    78,    Lepus 
329,  Sheep  343,  Chick  428  ;    Optic 
lobes,  Rana  78,  Scyllium  253,  Lepus 
329,  Chick  428  ;    Optic  stalk  256, 
427  ;    Optic  thalami,  Scyllium  251, 
Sheep     350,     Chick     428  ;      Optic 
tract   343,    350 ;     Optic   ventricles 


253  ;  Optic  vesicle  256,  primary, 
Scyllium  255,  Chick  427  ;  Pallium 
348  ;  Pallium,  olfactory  349  ;  Para- 
flocculus,  Lepus  330,  Sheep  352  ; 
Pedunculi  cerebri  351  ;  Perforated 
spot,  anterior  343  ;  Pineal  body, 
Rana  78,  Scyllium  252,  Lepus  329, 
Sheep  341,  350  ;  Pineal  stalk  252  ; 
Pituitary  body,  Rana  78,  Scyllium 

253,  Lepus  329,  Sheep  343  ;    Pons 
varolii,     Lepus    330,     Sheep    352, 
Chick    429 ;     Posterior    pillars    of 
fornix      347 ;       Pre-optic     recess, 
Scyllium'fett,    Sheep    351  ;    Pros- 
encephalon,     Rana     77,    Scyllium 
255,  Chick  407  ;    Psalterium  347  ; 
Pulvinar     350 ;       Pyramids     331, 
352  ;       Restiform      bodies      254 ; 
Rhinencephalon  77,  257  ;    Rhomb- 
encephalon      78,      257 ;       Saccus 
vasculosus    253 ;     Sagittal    fissure, 
Rana    77,    Lepus    329 ;      Septum 
lucidum     345  ;       Splenium     347 ; 
Sulci,     Lepus     329,     Sheep     340 ; 
Tectum    opticum    253 ;     Telence- 
phalon,    Rana    77,    Scyllium    2-51, 
257,  Lepus  329,  Chick  427;   Tha- 
lamencephalon,  Rana  77,  Scyllium 
251,    257,    Chick   428 ;     Trigonum 
habenula  350 ;    Tuber,   acusticum 

254,  cinereum,  Rana  78,  Sheep  343  ; 
Tubercle,    olfactory    343  ;    Tuber- 
culum    posterius    428 ;     Valve    of 
Vieussens   352  ;     Velum  interposi- 
tum    349,    transversum,    Scyllium 

5^252,  Chick  427  ;  Ventricles,  lateral 
427,  third,  Rana  78,  Scyllium  253, 
Lepus  329,  Sheep  349,  Chick  428  ; 
Vermis,  Lepus  329,  Sheep  351 

Branchial  filament  210 

Brassica  oleracea  458 

Breathing,  costal    and    diaphragmal 

3°7 

Broad  ligament  324 
Bronchi  307 
Bronchioles  307 
Brontosaurus  478,  479 
Buccal  cavity,  Rana  15,  Lumbricus  173 
Buffon  445 

Bulbus  arteriosus  414,  cordis  60 
Buller,  Professor  461 


Calcar  32 

Calcareous  bodies  99 

CANAL  :     Bidder's   75  ;     Central  21  ; 

Circular     164  ;       Collecting     127  ; 

Gastro- vascular    160 ;      Haversian 


488 


INDEX 


35  ;  Hyomandibular  263  ;  Infra- 
orbital  263  ;  Inguinal  321  ;  Inter- 
orbital  220;  Lateral  line  211; 
Mucous  211  ;  Naso-palatine  302  ; 
Neural,  Rana  20,  Chick  406 ; 
Neurenteric,  Amphioxus  382,  Rana 
392  ;  Notochordal  412  ;  Peri- 
cardio-peritoneal  234 ;  Per-radial 
164  ;  Radial  164  ;  Supra- orbital 
263 

Canalis  centralis,  Rana  79,  Scyllium 
206,  258,  Lepus  336 

Canthus  270 

Capillaries  58 

Capsule,  auditory,  Rana  27,  28, 
Scyllium  218;  olfactory,  Rana  27, 
28,  optic  27 

Caput  epididymis  322 

Carbohydrates  9 

Cardia  304 

Carpus  15 

Cartilage  16,  24,  articular  24,  calci- 
fied, 34,  hyaline  34 

Cauda  epididymis  322 

CAVITY  :  amniotic  419,  amniotic 
primitive  411,  pericardial  234, 
peritoneal  268,  pleural  258,  pleuro- 
peritoneal  17,  21,  pulp  301,  seg- 
mentation, Frog  1 06,  Hydra  159, 
Amphioxus  381  ;  Tympanic  TOO 

Cavum  aorticum  61,  pulmo-cutaneum 
61 

CELLS:  2,  n,  22,  central  398, 
cleavage  377,  cone  95,  ganglion  85, 
granular  37,  lamellar  37,  marginal 
398,  mucous  212,  muscle  fibre  167, 
neuroglia  80,  87,  nerve  85,  pole 
192,  primary  mesoderm  192,  primi- 
tive germ  368,  primordial  germ  167, 
rod  95,  somatic  168,  sperm  mother 
368,  vacuolated  37 

Cell  mass,  intermediate  406,  internal 
409 

Cell  sap  359,  theory  355 

Cement  301 

Centriole  362 

Centrosome  362 

Centrosphere  362 

Chaetae  172 

Chalaza  396 

Chamber,  posterior  and  vitreous  430 

Chambers,  Robert  448 

Characters,  generic  114,  specific  114 

Chelonia  475 

Chloragogen  cells  175 

Chlorella  vulgaris  156 

Chondrin  34 

Chondrosomes  361 

Chorda  dorsalis,  Rana  116,  Scyllium 
205,  217,  Amphioxus  383 


Chordae  tendinae,  Rana  60,  Lepus  317 
Chordal  sheath,  primary  and  second- 
ary 217 

Chordata  116,  204 
Chorion,  Lepus   434,    Chick   419 ;    of 

Insects  374 
Choroid  coat  93 
Chromatin  119,  361,  362 
Chromosomes  364,  Paramaecium  129, 

bivalent  369,  univalent  370 
Chyle  55 
Chyme  54 
Cilia  48 

Ciliary  process  94 
Ciliated  rosette  183,  tube  180 
Cingulum  172 
CIRCULATION:       body      wall      178; 

branchial,  Scyllium  244,  Lepus  317  ; 

complete  double  268  ;   dermal  178  ; 

incomplete   double,    Scyllium   245, 

Lepus  318  ;   intestinal  178  ;  nephri- 

dial  178  ;  single  244,  317 
Circulatory  system,  Rana  58,  Scyllium 

233,  Lepus  307 

Circum-pharyngeal  connectives  185 
Cirripedia  449 
Cirrus  197 
Clasper  209 
Class  115 
Classification    3,    37,     114,     117,    of 

sedimentary  rocks  451,  456 
Cleavage  377 
Clefts,  hyoidean  2 2 3,  internal  branchial 

229,  visceral  422 
Clitellum  172 
Clitoris  270,  324 

Cloaca,  Rana  14,  71,  Scyllium  211 
Cloacal  papillae,  and  pits  211 
Cnidoblasts  152 
Cnidocil  152 

Cochlea,  Rana  99,  Lepus  269 
Cocoon  185 
Ccecum  305 
Coelenterata  116,  148 
Ccelenteron  149 
Ccelom,  Rana  17,  116,  Lumbricus  170, 

190 

Coelomata,  Rana  116,  Lumbricus  170 
Coelomic  fluid  175 
Ccenosarc  160 
Collaterals  86 
Colloids  54 
Colon  304 
Colony  1 60 
Columba  livia  458 
Commissural  neurons  187 
Commissure  dorsal,  and  ventral  336 
Cone  of  reception  145 
Conjugants  131 
Conjugation  130 


INDEX 


489 


Conjunctiva,  Rana  93,  Chick  432 
Connector  neuron  91 
Continuity  of  germ  plasm  464 
Conus  arteriosus,  Rana  59,  Scyllium 

235 

Coracoid,     foramen    31,     portion    of 

shoulder  girdle  31 
Cords,  vocal,  Rana  56,  Lepus  306 
Corium  271 
Corned  92 

Cornu,  dorsal  80,  ventral  80 
Coronary  sulcus,  Rana  60,  Lepus  314 
Corpora,  adiposa  71,  spongiosa  322 
Corpus  cavernosum  322 
Corpuscles  21,  22,  connective  tissue 

37,  red  22,  white  22    • 
Cortex  126,  of  Kidney,  Lepus  320 
Cotylosauria  475 
Craniata  204 

Crests,  neural,  Rana  391,  Chick  408 
Cretinism  103 
Cristae  acusticas  99 
Crocodilia  478  — 
Crop  173 

Cross-infection  147 
Crossing  over  471 
Crus  15 
Crystalloid  53 
Culex  pipiens  143 
Cuticle,   Monocystis   126,    136,   Lum- 

bricus  172 
Cuvier  444 
Cyclosis  128 
Cyst  137 
Cysticercus    199,    C.    cellulose    200  ; 

C.  pisiformis  200 
Cytology  2,  355,  358 
Cytopharynx  126 
Cytophore,     Monocystis     137,     Lum- 

bricus  184 
Cytoplasm  22,  359 
Cytoproct  127 
Cytostome,  126 


D 


Darwin,  Charles  457,  Erasmus  445 

Death  8 

Defaecation  122 

Degeneration  146 

Delamination,  multipolar  159,  uni- 
polar 1 68 

Delimitation  of  embryo  416 

Dentinal  tubules  213 

Dentition,  diphyodont  228,  267, 
heterodont  267,  homodont  228, 
polyphyodont  228 

Dentive,  Scyllium  213,  Mammal  301 

Dendrite  86 


Dendrons  87 

Dermal  denticles  212 

Dermatome  423 

Dermis,  Rana  43,  Lepus  271 

Dermo-myotome  423 

Dermotrichia  214,  225 

Deutoplasm,-  374 

Deutoplasts  157 

Development  8 

de  Vries  465,  469 

Dialysis  53 

Diastema  280 

Diastole,  Amoeba  120,  Lepus  318 

Dibothriocephalus  latus  200 

Differentiation  of  structure  134 

Digestion  6,  53,  intercellular  122,  156, 
intracellular  122,  156 

Digestive  system,  Rana  46,  Scyllium 
228,  Lepus  302 

Digits  15 

Dinosauria  477 

Dioecious  155 

Diploblastic  151 

Diplodocus  478,  479 

Diplosome  362 

Dipylidium  caninum  201 

Disc,  germinal  375,  Chick  396 

Distribution  3,  geographical  3,  geo- 
logical 3 

Division,  direct  363,  equatorial  3.79, 
heterotype  370,  holoblastic  379, 
meridional  379,  mitotic  363,  re- 
ducing 370 

Division  of  physiological  labour  134 

Dominance,  Law  of  466 

Dorsal  surface  14 

Drosophila  melanogaster  470 

DUCT  :  43  ;  bile  47  ;  common  bile, 
Scyllium  232,  Lepus  306 ;  cystic, 
Rana  47,  Scyllium  232,  Lepus  306  ; 
endolymphatic,  Scyllium  219, 
Chick  432  ;  hepatic,  Rana  47, 
Scyllium  232,  Lepus  305  ;  muscular 
181  ;  Stenonian  303  ;  Wolffian 
425  ;  Wharton's  303 

Ductus,  Botalli  63,  caroticus  63, 
choledocus  47,  endolymphaticus 

99 

Duodenum,  Rana  18,  51,  Lepus  304 
Dura  mater,  Rana  76,  Scyllium  251, 

Lepus  327 


Ear,   Rana  98,  Scyllium  264,  Lepus 

239 

Ear,  development  of  432 
Earthworm  170 
Echidna  450 


490 


INDEX 


Ecology  3 

Ectoderm  357,    358,   Rana   116,  389, 
Hydra  150,  Amphioxus  381,  Chick 
349 ;     embryonal,    formative    and 
trophoblastic,  Lepus  410 
Ectoplasm  119,  136 
Edaphosaurus  474,  475 
Efferent  vessels,  176,  dermal  178 
Egestion  6,  122 
Egg,     centrolecithal,     heterolecithal, 

homolecithal  374  ;   telolecithal  385, 

Rana  375 
Elasmosaurus  476 
Elastin  38 
Embryo,     Rana     107,     Lepus     323, 

Amphioxus  381 
Embryology  2,  376 
Enamel,  Scyttium  213,  Mammal  301 
Enchylema  363 
End  buds  263 
Endocardium  414 
Endocyst  137 
Endolymph  99 
Endomysium  41 
Endomyxis  130 
Endoneurium  89 
Endoparasites  135 
Endoplasm  119,  136 
Endothelium  89 

Energy,  kinetic  and  potential  9,  in 
Enteric  canal  18 
Enterocoel  383 

Enteron,  Hydra  149,  Amphioxus  383 
Entoderm  357,  358,  Rana  116,  389, 

Hydra  150,  Amphioxus  381,  Chick 

399,  Lepus  411 
Environment  5,  462,  adaptation    to 

462 

Enzymes  50 
Eohippus  452 

Ependyma,  Scyttium  252,  Chick  430 
Epicyst  137 
Epidermis  43 

Epididymis,  Scyttium  248,  Lepus  322 
Epiglottis  303 
Epineurium  88 
Epiphyses  30,  37 
Epithelio- muscular  cells  151 
EPITHELIUM  :        355 ;        Rana      43  ; 

ciliated  356,  Rana  49  ;    columnar 

356  ;  compound  356  ;   cubical  356  ; 

enamel     213  ;      ependymal     336 ; 

glandular  44  ;    neural  80  ;    simple 

356  ;      squamous    356,    Rana    48  ; 

stratified  356  ;    transitional  75 
Equus  455,  E.  Scotti  455 
Ereptase  55 
Eryops  474,  475 

Erythrocytes,  Rana  68,  Chick  413 
Essential  parts  of  sense  organs  100 


Euglenoid  136 

Eunotosaurus  475 

Eustachian  tubes,  Rana  16,  Lepus 
303,  Chick  422 

Eutheria  438 

Evolution  2,  442,  443,  446,  con- 
vergent 484,  divergent  484 

Evolution,  evidence  for,  anatomical 
446,  embryological  448,  geographi- 
cal 449,  palaeontological  450 

Ex-con  jugant  132 

Excreta  53 

Excretion  7,  23 

Exoskeleton,  Rana  14,  Scyttium  211 

Expiration  57 

External  features,  Scyttium  208 

Exumbrella  163 

Eye,  Rana  92-96,  Scyttium  263, 
Lepus  338 

Eye,  development  of  430 


Factor  467 

Faeces  305 

Falciform  young  138 

Fallopian  tube  323 

Falx  cerebri  327 

Family  115 

Fascia  16 

Fasciculi  41,  88 

Fat  body  20 

Features,  external,  Lepus  269 

Feeding,     holophytic     no,    holozoic 

no 

FelidcB,  F.  leo,  F.  tigris,  etc.  447 
Femur  15 

Fenestra  ovalis  28,  100 
Fenestrated  membrane  of  Henle  67 
Fertilisation  7,  376,  Rana  105 
Fibres,  interzonal   366,    Muller's   96, 

white  38,  yellow  elastic  38 
Fibrillae  41 
Fibrin  68 
Fibrinogen  68 
Fibro-cartilage  39 
Filial  generations  466 
Fin,    Anterior    dorsal    208 ;     Caudal 

208 ;     Diphycercal    209 ;     Hetero- 

cercal     209 ;       Homocercal     209 ; 

Ventral  208 

Fission,  binary  123,  multiple  139 
Fissure,  choroid  431  ;   dorsal,  Rana, 

80,     Scyttium     258,     Lepus     336 ; 

ventral,  Rana  78,  80,  Scyttium  258, 

Lepus  336 
Fixation  360 

Flexure,  cervical  418,  cranial  416 
Fluid,  amniotic  419 


INDEX 


491 


Foetus  323 

Fold,  head  403  ;    lateral  neural  391  ; 

medullary,     Scy  Ilium    206,     Chick 

402  ;  neural,  Amphioxus  381,  Chick 

402 ;     primitive    401  ;     transverse 

neural  391 
Follicles,  lymph  231 
Fontanelle,    anterior     28,    pituitary 

221,  posterior  28 
Foramen,      internal     carotid      218 ; 

magnum,  Rana  27,  Scyllium  218  ; 

optic  220  ;    orbit o-nasal  220 
Fossa  ovalis  316 
Fundus  49 


Gall  1 8 

Gall-bladder,  Rana  18,  47,  Lepus  305 

Galton,  Francis  459,  460 

Gamete,  Rana  105,  Monocystis  137, 
maturation  of  372,  373 

Gametocytes,  Monocystis  137,  Plas- 
modium  142 

Gamobium  169 

GANGLION  :  Anterior  cervical  338  ; 
Anterior  mesenteric  338  ;  Bidder's 
89  ;  Cerebral  185  ;  Cceliac,  Rana 
89,  Lepus  338  ;  Dorsal  root  336  ; 
Gasserian  332  ;  Geniculate  333  ; 
Median  cervical  338 ;  Posterior 
mesenteric  338 ;  Remak's  89 ; 
Sphenopalatine  332  ;  Sub-pharyn- 
geal  185  ;  Supra-pharyngeal  185  ; 
Vagus  334 

Gastric  juice  50 

Gastrula  381 

Gastrulation,  Lumbricus  193,  Rana 
387,  Amphioxus  381,  Chick  399, 
Lepus  411 

Gelatin  34,  38 

Genital  pore  197,  sinus  197 

Genus  114 

Geological  record  450,  451 

Germ,  layers  381,  cell  7,  wall  399 

Gill  slits  206 

Gizzard  174 

GLANDS  :  Bartolini  324  ;  Capsulogen 
172 ;  Carotid  61  ;  Compound 
saccular  356  ;  Compound  tubular 
356 ;  Cowper's  322 ;  Cutaneous  43 ; 
Ductless,  Rana  101,  Lepus  324 ; 
JFlask  45,  356  ;  Gastric  49  ;  Infra- 
orbital  303  ;  Mammary  266,  273  ; 
Milk  266 ;  (Esophageal  173  ; 
Parotid  303  ;  Periganglion  99  ; 
Perineal  267,  270,  322  ;  Prostate 
322 ;  Racemose  356,  Rana  52  ; 
Rectal,  Scyllium  .231,  Lepus  322  ; 


Saccular  356  ;  Salivary  303  ;  Se- 
baceous 273  ;  Simple  tubular  49  ; 
Sub-lingual  303  ;  Sub-maxillary 
303  ;  Sweat  273  ;  Thymus,  Rana 
102,  Chick  422,  Lepus  324  ;  Thy- 
roid, Rana  102,  Chick  422,  Lepus 
325  ;  Tubular  356  ;  Vitelline  198 

Glans  clitoridis  324 

Glenoid  Cavity  29,  31 

Glomerulus  74 

Glottis,  Rana  16,  Lepus  303 

Glycogen  52 

Goethe  445 

Gonad  105 

Gonangium  163 

Gono-nucleus  133 

Gonotheca  in 

Graffian  follicles  323 

Granules,  metaplasmic  361,  Nissl's 
85,  yolk  74,  zymogen  50 

Groove,  Dorsal  402  ;  Laryngeo- 
tracheal  422  ;  Medullary,  Scyllium 
206,  Chick  403  ;  Naso-buccal  210  ; 
Neural  391  ;  Peristome  126  ;  Post- 
orbital  220  ;  Primitive,  Rana  392, 
Chick  400 

Growth  7,  109 

Gubernaculum  322 

Gut,  Development  of  421  ;  Definitive 
383;  Fore,  Lumbricus  173,  Rana 
393,  Chick  403  ;  Hind  and  Mid 
393  ;  Primitive  381 

Gyri  269 


H 


Habitat  5 
Haemal  canal 


iXCClUCLl     V^CHACH     JiJby 

Haemoglobin  69 

Hair,  cuticle  272,  follicle  271,  papilk 

272 


272 

Hallux  15,  32 

Haversian  system  36 

Head  14 

Heads,  Lumbricus  176,  Scyllium  233 

Heat  409 

Hemibranch  229 

Henson's  line  42 

Hepatic    portal    system,    Rana    63, 

Scyllium  207 
Hermaphrodite,  Hydra  155,  Lumbricus 

182 

Hesperornis  483 
Heterozygote  468 
Hexacanth  embryo  199 
Hibernation  13 
Hilus  320 

Histology  2,  4,  355 
Hotobranch  229 


492 


INDEX 


Homologous  26 

Homozyote  468 

Hormones  102 

Horse,  geological,  history  of  452 

Host   135,    principal    140,    secondary 

140 

Hyaline  layer  272,  membrane  94 
Hyaloplasm  359 
Hydra  148 
Hydranth  160,  165 
Hydrocauli  160 
Hydrocope  162 
Hydrolysis  54 
Hydrorhiza  160 
Hydrotheca  160 
Hydrula  168 
Hymen  249 
Hyostylic  skull  222 
Hypohippus  453 
Hypophyseal  ingrowth  393 
Hypostome  148 


Ichthyornis  483 

Ichthyosauria  476 

Idiochromatin  133 

Iguanodon  429,  480 

Ileum,  Rana  18,  51,  Lepus  304 

Immigration,  multipolar  159,  unipolar 

1 68 

Immunity  69,  acquired  70,  natural  70 
Implantation,  central   434,  eccentric 

438,  interstitial  434 
Inanimate  4 
Incisors  267 
Incubation  399 
Independent  assortment  470 
Ingestion  6,  121 
Inheritance,  alternate  468,  of  acquired 

characters  446 
Inspiration  57 
Integument,   Rana  39,   43,   Scyllium 

208,  Lepus  271 
Inter-auricular  septum  60 
Inter-cellular  parasites  135 
Internal  secretion  102 
Inter-radii  164 
Interstitial  cells  152 
Intervertebral  capsular  ligament  24 
Intestine  174 

Intra-cellular  parasites  135 
Intussusception  7,  123 
Invertebrata  116 
Iris  14,  94 
Irritability  4,  109 
Isogametes  140 
Isogamy  140 
Isotropic  42 


Jaws  27,  lower  16,  29,  upper  15,  29 
Johannsen,  Professor  469 


K 


Karyokinesis  363 

Karyosomes  363,  Monocystis  136 

Katabolism  7,  112 

Kidneys,    Rana    20,    Scyllium    246, 

Lepus  320 
Knot,  Henson's  primitive  401 


Lacertilia  477 

Lacteals  55,  67 

Lacunae  36 

Lagena  99 

Lamarck  445 

Lamarckism  446 

Lamella,  entodermal  164 

Lamellae,  circumferential  36,  Haver- 
sian  35,  interstitial  36,  peri- 
medullary  36,  peripheral  36 

Lamina  24 

Larva,  Rana  108,  Obelia  168 

Laryngo-tracheal  chamber  56 

Larynx,  Rana  16,  Lepus  303,  306 

Latebra  396 

Layer,  chalaziferous  396,  epidermal 
387,  nervous  387,  Rauber's  411, 
sub-zonal  409 

Lens  capsule  94 

Leptinotarsa  465 

Lepus  cuniculus  266,  development  of 
408 

Leucocytes  22,  68 

Life  4,  cycle  8 

Life-history  8,  digenetic  139,  Frog 
104,  Monocystis  137,  Monogenetic 

139 
Ligament,      Ethmo-palatine       222  ; 

Intervertebral    215  ;      Suspensory, 

Scyllium   231,    Lepus   305  ;     Sym- 

plectic  223 

Ligamentum  nuchae  38 
Limbs  14 

Limnoscelis  palustris  474 
Linea  alba  16 
Linear  order  of  factors  472 
Linin  362 
Linkage  470 
Linnaeus  444 
Lipase  55 
Liver,  Rana  18,  47,  52,  Scyllium  231, 

Lepus  305 


INDEX 


493 


Lobe,  Spigelian  305 

Lumen  51 

Lungs i 8 

Lymph  16,  66 

Lymphatics  66 

Lymph  sacs,  abdominal  67,  sub- 
cutaneous 66 

Lymph  sinus,  sub-cutaneous  16, 
sub-vertebral  20,  67 

Lymph  space,  sub-dural  327 


M 


Macrogamete  143 

Macrogametocyte  142 

Macromeres  191 

Macronucleus  128 

Macula  lutea  94 

Maculae  acusticae  100 

Malarial  parasite -140 

Malpighian    Body,    Rana    74,    Lepus 

320  ;   Layer,  Rana  43,  Lepus  271 
Mammas  267,  271 

Mammalian  heart  313  t 

Mammalia,  origin  of  474 
Mandibular  symphysis  222 
Mantle  fibres  364 
Manubrium  163 
Manus  15 
Marsupials  450 
Mastigophora  136 
Matrix  33 
Maturation,  6,  108,  Rana  74,  Plasmo- 

dium  143 

Means  of  dispersal  146 
Meatus,  auditory  433 
Medulla  126,  of  kidney,  320 
Medusa  161,  163 
Meganucleus  128 
Meiosis  370 
Melanin  141 
Membrana     elastica     externa      217, 

interna  217 

Membrana  granulosa  408 
Membrane,  egg,  primary,    secondary, 

tertiary  374 
Membrane,  follicular  374  ;    Krause's 

42  ;    nictitating  14  ;    nuclear  362  ; 

tympanic  14  ;    vitelline  374,  Rana 

74,  I05 

Membranes,  foetal  418 
Membranous  labyrinth  98 
Mendel,  Johann  465 
Meninges,    Rana    76,    Scy ilium    251, 

Lepus  327 
Merozoites  142 
Merychippus  454 
Mesenchyme    357,    358,    Rana    395, 

Chick  404 


Mesenteries  20,  47 

Mesenteron,  Scyllium  228,  Amphioxus 

383 

Mesentery,  dorsal  232 
Mesoderm,  357,  358,  Rana  116,  389, 

Lumbricus    170,    190,    Amphioxus 

383 
Mesoderm,     later    history    of    423  ; 

parietal,    somatic    383  ;      visceral, 

splanchnic  384 
Mesodermal  somites  193 
Mesogaster  232 
Mesoglea  150 
Mesohippus  453 

Mesometrium,  Rana  7-1,  Lepus  323 
Mesonephros,    Scyllium    245,    Lepus 

320,  Chick  426,  427  ;    caudal  246  ; 

cranial  246 
Mesorchia  73 
Mesorectum  232 

Mesothelium  357,  358,  Chick  405 
Mesovaria  71 

Metabolism  5,  7,  Rana  108,  112 
Metagenesis  139 
Metameres  171 
Metamerically  segmented  171 
Metamorphosis  107 
Metanephroi,   Lepus  268,  320,  Chick 

427 

Metaphase  363,  366 
Metazoa  n,  Rana  116 
Microgamete  144 
Microgametocyte  142 
Micromeres  191 
Micronucleus  128 
Micropyle  374 
Microsomes  361 
Middle  tube  180 
Mid-gut  228 
Milk  teeth  267 
Miohippus  453 
Mitochondria  361 
Mitosis  363 
Mode  400 
Molars  267,  280 
Monocystis  135 

Monoecious,  Lumbricus  155,  182 
Morgan,  Professor  470 
Morphology  I,  gross  2 
Morula  409 
Mosasauria  477 
Motor  neurons  186 
Mouth  148 
Movement,  automatic  121  ;    induced 

121  ;    spontaneous  121  ;    voluntary 

12 

Mucosa  47 

Mucous  membrane  47 
Multicellular  animals  n 
Multiplicative  147 


494 


INDEX 


Multiplication  7 

MUSCLES  :  Biceps  40  ;  Ciliary  94  ; 
Cutaneous  273  ;  Diaphragm  268, 
273 ;  Epiaxial  215 ;  Gastro- 
cnemius  40  ;  Hypaxial  215  ;  Inter- 
costal, external  273  ;  internal  273  ; 
Mylo-hyoideus  16 ;  Obliquus 
externus  17,  inferior  98,  internus 
17,  superior  98  ;  Pectoralis  16 ; 
Recti  abdominis  16 ;  Retractor 
bulbi  98 ;  Rectus  anterior  97, 
externus  97,  inferior  97,  internus 
97,  posterior  97,  superior  97  ; 
Transversalis  273 

Muscle  segments  214,  processes  151 

Muscles,  involuntary  40  ;  non-striate 
40  ;  striate  40 ;  voluntary  40 

Muscular  system,  Rana  39,  Scyllium 
214,  Lepus  271 

Muscularis  mucosae  50 

Musculi  papillaris  317,  pectinati  316 

Mutant  469 

Mutation  469 

Myoccel,  Amphioxus  384,  Rana  395, 
Chick  406 

Myocomata  215 

Myofibrillae  151 

Myoneme  nbrillae  127,  136 

Myosepta  215 

Myotomes,  Scyllium  214,  Amphioxus 
384,  Chick  423 

Myx  oedema  103 


N 


Narial  canals  302,  passage  15 

Naris,  external,  Rana  14,  Chick  433 ; 
internal,  Rana  14,  Chick  433 

Narrow  tube  180 

Natural  Selection  458 

Navel  441 

Nematocyst  152 

Nee-pallium  269 

Nephridia  178 

Nephridiopores  172 

Nephrostomes,  Rana  75,  Lumbricus 
178,  Scyllium  245 

Nephrotome,  Chick  406,  425 

NERVES  :  Abducens,  Rana  81, 
Scyllium  259,  Lepus  333,  Sheep 
354  ;  Accessorius  354  ;  Acustico- 
lateralis  262  ;  Afferent  80  ;  An- 
terior laryngeal  334 ;  Auditory, 
Rana  82,  Scyllium  261,  Lepus  333, 
Sheep  554  ;  Brachial  plexus  85  ; 
Branchiomeric  262  ;  Buccal  260  ; 
Cauda  equina  85  ;  Chorda  tympani 
333  ;  Cochlear,  Lepus  333,  Sheep 
354  ;  Cranial,  Rana  76,  Scyllium 


259,  Lepus  331  ;  Cranial,  origin  of 
258  ;  Depressor  334  ;  Efferent  80  ; 
External  mandibular  261  ;  Facial, 
Rana  82,  Scyllium  259,  Lepus  333, 
Sheep  354  ;  Filum  terminale  79  ; 
Glosso-pharyngeal,  Rana  83, 
Scyllium  261,  Lepus  333,  Sheep 
354  ;  Great  auricular  337  ;  Hyo- 
mandibular,  Rana  82,  Lepus  333  ; 
Hypoglossal,  Rana  83,  Lepus  335, 
Sheep  354  ;  Inferior  maxillary  333  ; 
Infra-orbital  332  ;  Mandibular, 
Rana 82,  Scylliumz^g  ;  Mandibulo- 
maxillary  333 ;  Maxillary,  Rana 
82,  Scyllium  259,  Lepus  332  ; 
Maxillo-mandibular  82  ;  Mentalis 
333  ;  Mixed  261  ;  Motor  80,  90  ; 
Motor  oculi  81  ;  Myomeric  261  ; 
Naso-ciliaris  332  ;  Oculomotorius, 
Rana  81,  Scyllium  259,  Lepus  332, 
Sheep  353  ;  Olfactory,  Rana  80, 
Scyllium  259,  Lepus  331  ;  Ophthal- 
mic, Rana  82,  Lepus  352,  Scyllium 
259,  260  ;  Optic,  Rana  78,  Scyllium 
259,  Lepus  332,  Sheep  353  ;  Pala- 
tine, Rana  81,  82,  Scyllium  260  ; 
Palatine  anterior  333,  posterior 
333 ;  Patheticus,  Scyllium  259, 
Lepus  332,  Sheep  353 ;  Petrosus 
superficial  major  333  ;  Phrenic 
337  ;  Pneumogastric  83  ;  Posterior 
dental  332  ;  Posterior  laryngeal 
334 ;  Post-trematic  262  ;  Pre- 
trematic  262  ;  Ramus  communi- 
cans  89 ;  Ramus  dorsalis  336 ; 
Ramus  palatinus  333  ;  Ramus 
ven tralis  336  ;  Recurrent  laryn- 
geal 334 ;  Sciatic  plexus  85 ; 
Sensory  80  ;  Septalis,  Lepus  331, 
332,  Sheep  353 ;  Sphenopalatine 
332 ;  Spinal,  Rana  76,  83,  Scyllium 
262,  Lepus  336  ;  Spinal  accessory 
334 ;  Sympathetic  262 ;  Termi- 
nalis,  Scyllium  258,  Lepus  331, 
Sheep  353  ;  Trigeminal,  Rana  81, 
Scyllium  259,  Lepus  332,  Sheep 
354  ;  Trochlear,  Lepus  332,  Sheep 
353  ;  Vagus,  Rana  83,  Scyllium 
261,  Lepus  334,  Sheep  354  ;  Vesti- 
bular,  Lepus  333,  Sheep  354 ; 
Visceralis  261  ;  Vomero-nasalis, 
Lepus  331,  332,  Sheep  353 

Nervous  System,  Rana  76,  Scyllium 
251  ;  Central  76  ;  Development  of 
427  ;  Involuntary  76,  89  ;  Minute 
structure  of  85  ;  Peripheral,  Rana 
80,  Lumbricus  180  ;  Sympathetic, 
Rana  76,  89,  Lepus  337 

Neuraxis  86 

Neurilemma  87 


INDEX 


495 


Neuro-epithelium  356 

Neuro-fibrillae  85 

Neuroglia  258 

Neuromasts  263 

Neuromeres  407 

Neuron  87 

Neuropore  406 

Nodes  of  Ranvier  88 

Nostrils.     See  Naris 

Notochord,  Rana  116,  390,  Scyllium 

205,  217,  Amphioxus  383 
Nuclear  division,  indirect  130 
Nucleo-centrosome  129 
Nucleolus  363 
Nucleoplasm  359 
Nucleus     u,     22 ;      Cleavage     377 ; 

Conjugation     131  ;       Fertilisation 

377,  Plasmodium  145  ;    Migratory 

131  ;    Stationary  131 
Number,  diploid    367,    haploid    368, 

reduced  368,  somatic  367 


Obelia  159 

Ocellus  164 

Odontoblast  213 

(Enothera  lamarckiana  469 

(Esophagus,  Rana  18,  Lumbricus  173 

CEstrus  409 

Oken  445 

Olfactory  organ,  Rana  15,  91,  Lepus 
338  ;  Development  of  433 

Omenta  47 

Omentum,  duodenal-hepatic  46, 
gastro-duodenal  46,  gastro-hepatic, 
Rana  47,  Scyllium  233 ;  gastro- 
intestinal 233,  gastro-splenic, 
Scyllium  233,  Lepus  324,  306 

Omphalopleure  436 

Onchosphere  199 

Oocyst  145 

Oocyte  157,  primary  371,  secondary 

373 

Oogenesis  371 

Oogonia  368,  371,  Hydra  157,  Lum- 
bricus 184 

Ookinete  145 

Oospore  105 

Operculum  107 

Ophidia  477 

Opsonic  index  70 

Ora  serrata  94 

Oral  cone  148,  evagination  393, 
surface  150,  valve  228 

Orbits  15,  27 

Order  115 


Organs   23,    355,    of    Jacobson    303, 

segmental  178 
"  Origin  of  Species  "457 
Ornithischia  478,  479 
Ornithorhynchus  450 
Orohippus  452,  453 
Osmosis  53 

Ossicles,  auditory  267 
Ossa  uteri  324 
Ossification  36 
Osteoblasts  36 
Osteoclasts  37 

Ova  20,  ovarian  371,  primordial  184 
Ovaries,    Rana   20,    T.    solium    197, 

Lepus 323 
Oviducal  funnel,  Rana  71,  Lumbricus 

184,  Scyllium  249 
Oviducal  gland  249 
Oviduct,  Rana  20,  71,  Lepus  323 
Oviparous  205 
Ovisac,    Rana    71,    Lumbricus    184, 

Scyllium  249 
Ovo-viviparous  205 
Ovulation  374 

Ovum  73,  313,  fertilised  105 
Oxidation  22 
Oxyhaemoglobin  69 
Oxyntic  cells  50 


Palate  267,  302,  hard  267,  soft  267 
Pancreas,  Rana  18,  47,  51,  Lepus  306 
Papillae,  circumvallate    303,    Dermal 

213,  foliatae  302,  taste  302,  urinary 

211,  urogenital2ii 
Parahippus  453 
Parallelism  in  Evolution  484 
Paramcecium  125 
Parasites  135 
Pars,   ciliaris  retinas  94,   glandularis 

325,     intermedium    325,     nervalis 

325 

Parthenogenesis  368,  artificial  378 
Pearson,  Karl  459,  460 
Pectoral  girdle  17 
Pelvis  of  Kidney  320 
Pelycosauria  474 
Penis  270,  322 
Periblast  396,  central  398,  marginal 

398 

Pericardial  cavity  17,  space  315 
Pericardium,  Rana  17,  Lepus  314 
Perilymph,  Rana  99,  Chick  433 
Perimysium  41 
Perineal  pouches  270 
Perineum  267 
Perineurium  89 
Periosteum  36 


496 


INDEX 


Perisarc  160 

Peristalsis,   Rana  40,   51,  Lumbricus 

175 

Peristomium  171 

Peritoneum  20 

Perradii  164 

Peyer's  patches  304 

Phagocytes  69,  Phagocytosis  69 

Pharynx,  Rana  16,  Lumbricus  173 

Phase  contraction  368 

Photo-synthesis  113 

Physiology  i,  4,  chemical  3,  experi- 
mental 3 

Pia  mater,  Rana  76,  Scyllium  251, 
Lepus  327 

Pinnae  267 

Pit,  primitive  401,  auditory  432,  433 

Pituitary  body,  Rana  104,  Lepus  325 

Placenta  268,  323,  438,  deciduate 
441,  omphalopleural  438,  yolk-sac 

438 

Placoid  scales  212 

Planula  168 

Plasma,  Rana  21,  Scyllium  207 

Plasmodium  140 

Plastids  361 

Plastin  363,  Monocystis  136 

Plate,    Auditory    432  ;     Basal    212 
Brain  406  ;    Cutis  423  ;    End  42 
Equatorial  364,  Paramcecium  130 
Hyoid    1 6 ;     Lateral,    Amphioxus 
384,   Rana  395,   Chick  404 ;    Me- 
dullary, Scyllium  206,  Amphioxus 
381,  Rana  391,  Chick  402  ;    Muscle 
423  ;      Neural     381  ;      Oral    421  ; 
Polar  129 ;    Segmental   395  ;    Ver- 
tebral 404 

Plesiosauria  475,  476 

Pleura  307 

Plexus  89,  cardiac  89,  solar  89 

Pliohippus  454 

Plug,  yolk  388 

Polar  body  373,  Plasmodium  143, 
Hydra  157 

Polymorphism  161 

Polyp  1 60,  165 

Pores,  dorsal  172,  neural  381,  ovi- 
ducal  173,  spermathecal  173,  sper- 
miducal  172 

Portal  system  65 

Post-axial,  Rana  31,  Lepus  290 

Posterior  14 

Post-septal  178 

Pouches,  branchial  421,  cesophageal 
173,  visceral  421 

Pre-axial,  Rana  31,  Lepus  290 

Pre-molars  267,  280 

Prepuce  270,  322 

Pre-septal  portion  of  nephridium  178 

Pro-amnion  401 


Process,   external  nasal  433,   fronto- 

nasal  433,  head  402,   notochordal 

402 

Proctodceum  382 
Proglottids  195 
Pronation  290 
Pronephros,  Scyllium  245,  Lepus  320, 

Chick  425 
Pro-nuclei,    male    and    female    374, 

Rana  105 
Prophase  363,  364 
Proscolex  199 
Prostomium  171 
Proteins  9 
Protista  109 
Protohippus  454 
Protoplasm  8,  108 
Protozoa  n,  116,  parasitic  135 
Proventriculus  173 
Proximate  principles  53 
Pseudobranch  210 
Pseudonavicella  137 
Pseudop6dia  119 
Psychology,  experimental  3 
Pterodactyla  481 
Pterygiophores  225 
Ptyalin  54 
Pulmonary  58 
Punnet,  Professor  461 
Pupil  14,  94 
Pure  line  469,  470 
Pyloric  sphincter,  Scyllium  229,  Lepus 

3°4 

Pylorus  304,  Rana  18,  46,  51,  Scyllium 
229 


Rabbit  266 

Radial  symmetry  150 

Rana,  development  of  385,  R.  escu- 

lenta  12,  56 
Ray,  John  444 
Recapitulation  theory  448 
Receptaculum  ovorum  184 
Rectum,  Rana  18,  51,  Lepus  305 
Reflex  Arc,  Rana  90,  Lumbricus  188 
Regeneration,  Hydra  157 
Region,  myotomal  395 
Renal  portal  system  65 
Rennin  54 
Reproduction    7,     asexual     124,    of 

Hydra  156 
Reptilia,    geological   history   of  472, 

473 

Reserve  material  123 

Respiration  6,  23,  buccal  58, 
cutaneous  58,  pharyngeal  58 

Respiratory  system,  Rana  56,  Scyl- 
lium 233,  Lepus  306 


INDEX 


497 


Rete  mirabile  65,  mucosum  271 

Retina  94 

Rhombencephalon  407 

Rhynchocephalia  477 

Ridges,  placental  439 

Rima  glottidis  56 

Ring,  formative  399 

Rodent  278 

Root,  Dorsal  of  Nerve  336  ;    of  tooth 
301  ;    Sheath,  inner  272  ;    Sheath,    j 
outer  272,  Ventral  of  nerve  336 

Rosette  142 

Rostellum  195 

Rugae  49 


Sac,     auditory    432,     endolymphatic 

432,  Vocal  15,  16,  56 
Sacculus,     Rana     99,     Chick     432  ; 

Endolymphaticus,  Rana  99,  Lepus 

339,  Rotundus  304 
Salt  solution  21 
Sap,  nuclear  363 
Sarcolemma  41 
Sarcomere  42 
Sarcoplasm  41 
Sarcostyle  41 
Saurischia  477,   478 
Scapular  portion  of  shoulder  girdle 

3i 

Schizoccel  395 
Schizogony  141 
Schizont  141 

Sciences,  Biological  i,  Physical  i 
Sclerotic  92 
Sclerotome  425 
Scolex  195 
Scrotal  sac,  270,  321 
Scyllium  canicula  204 
Section,  transverse  of,  Lumbricus  188 
Segmentation     377,     Rana     106,     of 

Tcenia  201,  Holoblastic  191,  Mero- 

blastic  398 

Segments,  Lumbricus  171,  Rana  396 
Segregation  467,  470 
Selection  460 
Self-infection  147 
Semen  250 

Semicircular  canals  99 
Semilunar  valves  60 
Seminal  vesicle  198 
Senescence   109 
Sense  organs,  Rana  76,  91,  Scyllium 

251,  263,  Lepus  338  ;   Development 

of  430 

Sensitivity  5,  109 
Sensory  fibres  90 
Septa  connective  tissue  16 


Septum,  atriorum  315;  Inter  Atrial, 
Lepus  268,  Chick  414  ;  Inter- 
branchial  229 ;  Interventricular 
414  ;  Medium  61  ;  Pericardio- 
peritoneal  234  ;  Ventriculorum 
268,  316 

Serially  homologous  171 

Sero-amniotic  connection  419 

Serum  68 

Setae  172,  penial  172 

Setigerous  sac  172 

Sex-chromosome  471 

Sexual  reproduction  167 

Seymouria  475 

Sheath,  primitive  87,  medullary  88 

Shell  gland  198 

Shield,  embryonal,  Chick  399,  Lepus 
411 

Sinu-auricular  valves  59 

Sinus,  Sagittalis  340  ;  Terminalis  413  ; 
Transversus  340  ;  Urinary  248  ; 
Urogenital  248  ;  Venosus,  Rana 
1 8,  59,  Scyllium  234,  Chick  414 

Sinusoids,  Rana  65,  Scyllium  250, 
Chick  423 

Skein  364 

Skeletogenous  sheath  217 

Skeleton  appendicular,  Scyllium  225, 
Lepus  283  ;  Endoskeleton  215 

SKELETON,  APERTURES,  ETC.  :  Aceta- 
bulum  286 ;  Alisphenoid  canal 
293  ;  Aquasductus  Fallopii  282  ; 
Auditory  meatus,  external  296  ; 
Canal,  Infra-orbital  298  ;  Canal, 
Vertebrarterial  275  ;  Cranial  fonta- 
nelle,  anterior  218 ;  Eustachian 
canal  296  ;  Fenestra  ovalis,  Lepus 
282,  Cahis  297  ;  Fenestra  rotunda, 
Lepus  282,  Canis  297  ;  Floccular 
fossa  282 

Foramen  caroticum  296 ;  In- 
ferior dental,  Lepus  283,  Canis  300  ; 
Infra-orbital  297  ;  Inter  vertebral, 
Rana  26,  Lepus  274  ;  Lacerum 
anterius,  Lepus  281,  Canis  293  ; 
Lacerum  medium,  Lepus  282, 
Canis  296  ;  Lacerum  posterius, 
Lepus  283,  Canis  296  ;  Lachrymal 
296  ;  Magnum  292  ;  Mental  300  ; 
Obturator  286  ;  Optic  294  ;  Ovale 
293  ;  Palatine,  anterior  299 ; 
Palatine,  posterior  297  ;  Pituitary, 
Lepus  281,  Canis  293  ;  Post-glenoid 
299 ;  Rotundum  293  ;  Stylo- 
mastoid  282,  296 

Gluteal  fossa  286 ;  Iliac  fossa 
286  ;  Intercondylar  notch  287  ; 
Inter  vertebral  notch  274  ;  Meatus 
auditorius  internus  282  ;  Orbital 
fossa  294  ;  Sella  turcica,  Lepus 

2    K 


498 


INDEX 


281,  Cams  293  ;    Sphenoidal  fissure 
281  ;     Supra  trochlear  fossa  286  ; 
Temporal  fossa  294 
SKELETON,  BONES,  ETC.  :    Acromion 
288  ;    Alisphenoid,  Lepus  280,  281, 
Canis     293  ;      Anapophysis     276 ;    | 
Angle  of  Jaw  283  ;    Angular  pro- 
cess    299 ;      Angulo-splenial     29  ;    ! 
Astragalus    32  ;     Atlas,    Rana   26,    I 
Lepus  267  ;    Axis  267  ;    Basi-hyal.  i 
300  ;      Basi-occipital,    Lepus    281, 
283,     Canis    291  ;      Basi-sphenoid,    j 
Lepus    281,     Canis    293  ;      Bulla,    | 
Lepus  282,  Canis  296  ;   Calcaneum,    i 
Rana    32,    Lepus    288  ;      Capitate 
287  ;    Carpalia  30  ;    Centrale  287  ; 
Centrum    24  ;      Cerato-hyal     301  ;    ' 
Clavicle,     Rana    31,     Lepus    285  ;    | 
Columella    22  ;    Condyle,    occipital    i 

27  ;   Condyle  of  Jaw  299  ;   Coracoid    i 
31  ;  Coracoid  process  285  ;  Cornua    ! 
of    hyoid,    Rana   29,    Lepus    283  ; 
Coronoid  process,  Lepus  283,  Canis 
299  ;       Cotyloid     286 ;      Cranium, 
Rana   27,    Canis   291  ;     Cribriform 
plates,     Lepus    281,     Canis     294  ;    | 
Cuboid  288  ;    Dentary  29  ;     Ecto- 
cuneiform    288  ;     Epicoracoid    32  ; 
Epi-hyal  301  ;    Epi-otic  282  ;    Epi- 
sternum    32  ;     Epistropheus     267, 
275  ;    Ethmo-turbinals,  Lepus  279, 
Canis    296  ;     Ethmoid    294  ;     Ex- 
occipital,     Rana    27,     Lepus    283,    j 
Canis  293  ;    Fabellae  288  ;    Femur, 
Rana  32,  Lepus  287  ;    Fibula  288  ; 
Fore-limb    286 ;      Frontal,    Lepus 

280,  Canis    294  ;     Fronto-parietal    j 

28  ;      Greater     multangular     287 ; 
Haemal  process  216  ;    Haemal  spine 
217  ;      Hamate    287  ;      Hind-limb 
287  ;     Humerus,    Rana   30,    Lepus 
286  ;   Hyoid,  Lepus  283,  Canis  300  ; 
Hypapophysis   276 ;     Ilium,    Rana 
33,  Lepus  286  ;    Incus,  Lepus  282, 
Canis     297  ;       Intermedium     30  ; 
Interparietal,  Lepus  280,  Canis  293  ;    | 
Intervertebral  discs  274  ;    Ischium,    J 
Rana  33,  Lepus  286  ;  Jugal,  Lepus 

281,  Canis  299  ;    Lachrymal,  Lepus 
280,    Canis    296  ;    Lamina,    dorsal    ! 
274  ;      Lesser     multangular     287  ;    | 
Lunate    287 ;     Malar,    Lepus    281,    j 
Canis     299 ;      Malar     arch     294  ;    j 
Malleus,    Lepus    282,    Canis    297  ; 
Mandible,    Rana    29,    Lepus    283,    i 
Canis  299  ;    Mandibular  symphysis    i 
299 ;     Manubrium    278 ;     Mastoid 
process    297  ;     Maxilla,    Rana    29,. 
Lepus  279,  280,  Canis  298  ;  Maxillo- 
turbinals,   Lepus   279,    Canis  296 ; 


Mento-Meckelian  29  ;    Mesethmoid 

279,  281  ;      Mesethmoidal     plate 
294  ;    Meso-cuneiform  288  ;    Meso- 
sternum    32  ;     Metacromion    285  ; 
Metapophysis  276  ;   Metatarsal  32  ; 
Nasal,  Rana  28,  Lepus  278,  Canis 
294  ;     Nasal    process,    Lepus    279, 
Canis     294,     298  ;      Naso-turbinal 

296  ;    Navicular;  287,  288  ;    Neural 
arch  24  ;    Neural  spine,  Rana  25, 
Lepus    274  ;     Occipital     condyles, 
Lepus  283,    Canis  293  ;     Occipital 
segment    282  ;     Odontoid    process 
275  ;    Olecranon  process,  Rana  30, 
Lepus     287 ;       Omosternum     32  ; 
Orbital      process      280 ;       Orbito- 
sphenoid,   Lepus  280,    Canis  294  ; 
Os  innominatum  285  ;    Os  orbicu- 
lare,  Lepus  282,  Canis  297  ;    Pala- 
tine,   Rana  29,   Lepus  280,    Canis 

297  ;     Palatine  process   279,    280  ; 
Parasphenoid  28  ;    Parietal,  Lepus 

280,  Canis  293  ;    Paroccipital  pro- 
cess 283  ;     Patella  288  ;     Pectoral 
girdle    284  ;      Pedicle,     Rana    24, 
Lepus    274  ;      Pelvic    girdle    285  ; 
Pentadactyl    limb    289  ;     Periotic, 
Lepus  281,  Canis  296  ;    Phalanges 
30  ;     Pisiform     287  ;     Post- orbital 
process  294,  299  ;   Pre-coracoid  31  ; 
Pre-maxilla,  Rana  29,  Lepus  278, 
Canis    298  ;     Pre-sphenoid,   Lepus 

281,  Canis  294  ;    Process,  external 
pterygoid  281  ;  Process,  pre-orbital, 
220  ;   Pro-otic,  Rana  28,  Lepus  282  ; 
Pterygoid,    Rana    27,    Lepus    281, 
Canis  297  ;    Pubic  symphysis  286  ; 
Pubis,      Rana     33,     Lepus     286 ; 
Quadrato-jugal   29  ;     Radiale    30  ; 
Radio-ulnar     30 ;       Radius     286  ; 
Rami  of  mandible  299  ;    Rib  267, 
277,  capitulum  277,  sternal  portion 

277,  vertebral  portion  277,  tuber- 
culum   277  ;    Ridges,    condylar   30, 
deltoid  30,  infra-orbital  219,  supra- 
orbital  219,    Sacrum  26  ;    Sagittal 
crest  291  ;    Sagittal  suture,  Lepus 
280,  Canis  293  ;    Scapula,  Rana  31, 
Lepus    284  ;     Septo-maxillary    28  ; 
Septum,  internasal  220,  interorbital 

278,  narium  279,   294  ;     Sesamoid 
274  ;    Skull,  Lepus  278,  Canis  291  ; 
Sphenethmoid      28  ;       Squamosal, 
Rana  29,  Lepus  280,  281,  Canis  299  ; 
Stapes,  Rana  28,   100,  Lepus  282, 
Canis  297  ;    Sternebrae  278  ;    Ster- 
num, Rana  16,  32,  Lepus  277,  278  ; 
Stylo-hyal    301  ;      Supra-occipital, 
Lepus    283,     Canis    293  ;      Supra- 
orbital      process       280  ;        Supra- 


INDEX 


499 


scapular  31  ;  Suspensorium  29  ; 
Sutures  278 ;  Symphysis  menti 
283  ;  Talus  288 ;  Thyro-hyal, 
Lepus  283,  Canis  300  ;  Tibia  288  ; 
Tibio-fibula  32  ;  Transverse  pro- 
cess 25,  Trochanter,  Rana  30, 
Lepus  287  ;  Turbinals  279  ;  Tym- 
panic, Lepus  281,  Canis  296 ; 
Tympano-hyal  300  ;  Ulna  286  ; 
Ulnare  30  ;  Ungual  phalanges  289  ; 
Urostyle  24,  26 ;  Vertebrae  24, 
amphicoelous,  Rana  26,  Scy Ilium 
215,  proccelous  24  ;  Visceral  222, 

224  ;    Vomer,  Rana  28,  Canis  294  ; 
Zygapophysis,   Rana  25,  Post-  26, 
Pre-  26;   Zygoma,  Lepus  281,  Canis 
299  ',    Zygomatic  arch  298  ;    Zygo- 
matic  process,  280  ;    Xiphisternum, 
Rana  32,  Lepus  278 

SKELETON,  CARTILAGES,  ETC.  :  Aryte- 
noids,  Rana  5,  6,  Lepus  306 ; 
Basalia  225  ;  Basipterygium  226  ; 
Branchial  rays  223  ;  Capsule,  optic 
219,  otic  218 ;  Cerato-branchial 
223  ;  Cerato-hyal  223  ;  Chondro- 
cranium  218 ;  Cricoid,  Rana  56, 
Lepus  306  ;  Hypo-branchial  223  ; 
Epi-branchial  223 ;  Extra-bran- 
chial 224  ;  Hyoid  29 ;  Meckel's, 
Rana  29,  Scyllium  222  ;  Meseth- 
moidal  plate,  220  ;  Neural  plate, 
intervertebral  215,  vertebral  215  ; 
Orbital  222  ;  Palatine  29  ;  Palato- 
pterygo-quadrate  222  ;  Pharyngeo- 
branchial  223  ;  Pre-arytenoid  56  ; 
Pre-spiracular223  ;  Process,  lateral 
ethmoidal  220 ;  Thyroid  306 ; 
Meso-pterygium  225  ;  Meta- 
pterygium  225  ;  Parachordal  221  ; 
Plate,  basi -branchial  223,  basi-hyal 
223,  trabecular  221  ;  Propterygium 

225  ;     Quadrate    29 ;      Radialium 
225;  Rib  216;  Trabeculaecranii22i 

Skull  27 

Somactidia  225 

Somatopleure,  Lumbricus  193,  Chick 

420 
Somites,  Lumbricus   171,   Rana  396, 

Chick  404,  405 
Space,  sub-arachnoid  336 
Spawn  13 
Species  114,  443 
Spencer,  Herbert  462,  464 
Sperm  morulae  184  ;    reservoirs  183  ; 

sacs,  Lumbricus  135,  Scyllium  248 
Spermathecae  184 
Spermatic  cord  321 
Spermatids  370,  Lumbricus  184 
Spermatocytes    157 ;     primary    368, 

secondary  368 


Spermatogenesis  368 
Spermatogonia  368,   Lumbricus   183, 

Hydra  157 
Spermatozoon    370,    373  ;     cap    371, 

head   370,    middle   piece   370,    tail 

37° 

Sphenodon  punctatus  477 

Sphincter  46,  51 

Spinal  cord,  Rana  21,  Scyllium  258, 

Lepus  335 

Spindle  364,  Paramcecium  129 
Spireme  364 
Splanchnoccel  385 
Splanchnopleure  193 
Spleen,  Rana  18,  102,  Lepus  324 
Spongeoplasm  359 
Spores  135 
Sporoblast,    Monocystis    137,    Plas- 

modium  145 
Sporocyst  137 
Sporozoa  135 
Sporozoites,    Monocystis    138,    Plas- 

modium  141 
Sporulation  135 
Squamata  477 
Stalk,  allantoic  420,  optic  430,  somitic 

406,  yolk  420 
Statocysts  164 
Stegocephalia  474 
Stegosaurus  478,  480 
Stimulus  5,  39 
Stomach,    Rana    18,    Scyllium    229, 

Lepus  304 
Stomodceum,    Lumbricus    173,    Rana 

393,  Chick  421 
Stratum  corneum  271 
Streak,   primitive,   Rana   392,   Chick 

399 

Strobilla  195 

Struggle  for  Existence  462 
Sub-classes  115 
Sub-cutaneous  tissue  37 
Sub-epithelial  cells  152 
Sub-mucosa  50 
Sub-umbrella  163 
Sulci  269,  coronary  315,  limiting  416, 

longitudinal     dorsalis    315,    longi- 
tudinal ventralis  315 
Supination  290 
Supporting  lamella  150 
Supra-renal    body,     Rana    71,     103, 

Lepus  325 

Suspensory  ligament  94 
Symbiosis  156 
Symphyses  33 
Synapsis  369 

Syncytium,-  Rana  42,  Chick  398 
Synizesis  368 
Synkaryon  145 
Synovial  cavity  24 

2    K   2 


5oo 


INDEX 


System  355,  Rana  23 

Systole,  Amoeba  120,  Lepus  318 


Tapetum  264 

Tarsus  15 

Taste  98,  buds  263 

Taxonomy  3,  Rana  114 

Teats  267 

Technique  i 

TEETH  :  Canines  267 ;  Crown  of 
301  ;  Incisor  278  ;  Maxillary  15  ; 
Permanent  267 ;  Premolar  267- 
280  ;  Vomerine  15 

Teloblasts  192 

Telophase  363,  366 

Temperature  optimum  121 

Tendo  Achillis  41 

Tentacles  148 

Tentorium  327 

Testis,  Rana  20,  T.  solium  197, 
Scyllium  249,  Lepus  270,  321 

Tetrad  370 

Thecodontia  477 

Theriodontia  474 

Thorax  268 

Tigroid  substance  85 

TISSUE  :  355,  Rana  23,  Areolar  con- 
nective 37,  271,  Connective  356, 
Muscular  357,  Nervous  357,  Sub- 
cutaneous 271 

Tcenia  echinococcus  200,  T.  saginata 
200,  T.  serrata  200,  T.  solium  194 

Tongue  16 

Tonsils  302,  324 

Touch  98 

Tower  465 

Toxins  70 

Trabeculae  carneae  317 

Trachea  306 

Tract,  optic  431 

Triceratops  481 

Trichocysts  126 

Trifolium  pratense  462 

Trigger  process  152 

Trimorphism  161 

Trophoblast  410 

Trophochromatin  133 

Trophoderm  434 
Tropho-nucleus  133 

Trophozoite,  Monocystis  136,  Plas- 
modium  141 

Truncus  arteriosus  18 
Trunk  14 

Tuberculum  intervenosum  316 
Trypsin  55 

Tube,  neural,  Amphioxus  382,  Chick 
406 


Tubule,  collecting  75,  seminiferous  74, 

uriniferous  74 
Tunica    adventitia    67,    interna    67, 

media  67 

Tunica  skeletogena  217 
Tympanum  100 
Typhlosole  175 
Tyrannosaurus  479 


U 


Umbilical  cord  436 

Umbilicus  420 

Undulating  membrane  128 

Unicellular  animals  n 

Unit  character  466 

Urea  52 

Ureter,  Rana  20,  71,  Lepus  320 

Urethra  322 

Urinary  bladder,  Rana  18,  Lepus  320 

Urogenital  System,  Rana  71,  Scy  Ilium 

245,  Lepus  319 
Uterus,    T.   solium    198,    Lepus   268, 

323  ,  masculinus  322 
Utriculus,  Rana  99,  Chick  432 


V 


Vacuoles  361,  contractile  120,  food 
1 20,  pulsating  127,  water  120 

Vagina,  T.  solium  198,  Lepus  324 

Valve,  bicuspid  316,  mitral  316,  spiral 
231,  semilunar  308,  317,  tricuspid 
3i6 

Variation  458 

Vas  deferens,  Rana  73,  Lumbricus 
183,  Scyllium  248,  Lepus  322 

Vasa  efferentia,  Rana  73,  Lumbricus 
183,  Scyllium  249 

Vascular  System  development  413 

Veins  18 

VEINS  :  Afferent  renal  242  ;  Anterior 
abdominal  16,  66 ;  Anterior  cere- 
bral 240  ;  Anterior  mesenteric  311  ; 
Azygos  cardinal  311  ;  Azygos  311  ; 
Brachial,  Rana  64,  Lepus  311  ; 
Brachial  sinus  242  ;  Cardiac  66  ; 
Cardinal  sinus  anterior,  Scyllium 
240,  Chick  415 ;  Caudal  240 ; 
Caval  59  ;  Dorsal  •  anterior  gastric 
243 ;  Dorso-lumbar  66 ;  Ductus 
Cuvieri,  Scyllium  243,  Chick  416  ; 
Ductus  venosus  416 ;  Duodenal 
311  ;  Efferent  hepatic  243 ; 
Efferent  renal  242  ;  Epigastric, 
right  anterior  311  ;  External 
jugular  64  ;  External  jugular,  right 
311  ;  Facial,  anterior  311;  Facial, 


INDEX 


posterior  311  ;  Femoral,  Rana  65, 
Lepus  313  ;  Gastric  66  ;  Gastro- 
intestinal 243  ;  Genital,  Rana  65, 
Lepus  313  ;  Hepatic,  Rana  65, 
Lepus  313;  Hepatic  portal, 
Scyllium  243,  Lepus  311  ;  Hepatic 
portal  system,  Scyllium  242,  Lepus 
311;  Hepatic  sinus  243  ;  Hyoidean 
sinus  240  ;  Iliac  242  ;  Iliac,  external 
313 ;  Iliac,  internal  313  ;  Ilio- 
lumbar  313  ;  Inferior  jugular  sinus 
240  ;  Innominate  64  ;  Intercostal, 
right  anterior  311  ;  Inter  orbital 
240 ;  Intestinal  66 ;  Intra-intes- 
tinal  243  ;  Jugular,  common  312  ; 
Jugular,  internal,  Rana  64,  Lepus 
312  ;  Lateral  abdominal  242  ; 
Lateral  cutaneous  242  ;  Lieno- 
gastric  311  ;  Lingual  64  ;  Mandi- 
bular  64  ;  Musculo-cutaneous  16, 
64  ;  Nasal  sinus  239  ;  Omphalo- 
mesenteric  436 ;  Orbital  sinus 
240  ;  Orbito-nasal  239  ;  Ovarian, 
Rana  65,  Lepus  313  ;  Parietal  66  ; 
Pelvic  66  ;  Phrenic  313  ;  Phrenic, 
anterior  311  ;  Post-caval,  Rana 
2t>,  65,  Lepus  312  ;  Post-mesen- 
teric  311  ;  Posterior  cardinal 
sinuses  242 ;  Posterior  cardinal 
vein  426  ;  Posterior  cerebral  240  ; 
Posterior  intestinal  242  ;  Posterior 
lieno  -  gastric  243  ;  Post  -  orbital 
sinus  240 ;  Pre-caval  65  ;  Pre- 
caval,  left  312  ;  Pre-caval,  right 
311  ;  Pulmonary,  Rana  60,  63, 
Lepus  310,  Ramus  communicans 
iliacus  67  ;  Renal  portal,  Rana  66, 
Scyllium  240 ;  Renal  veins,  Rana 
65,  Lepus  313  ;  Sciatic  65  ;  Sper- 
matic, Rana  65,  Lepus  313  ;  Splenic 
66  ;  Sub-clavian,  Rana  65,  Scyllium 
242,  Lepus  311  ;  Sub-scapular 
sinus  242  ;  Umbilical  438  ;  Vena 
cava  anterior  65  ;  Vena  cava 
anterior  dextra  311  ;  Vena  cava 
anterior  sinistra  312  ;  Vena  cava 
posteria,  Rana  65,  Lepus  312  ; 
Venae  cavae  59 ;  Venae  renales 
revehentes  65 ;  Venae  renales 
advehentes  66 ;  Ventral  anterior 
gastric  243  ;  Vesicular,  Rana  66, 
Lepus  313  ;  Vitelline  anterior  413  ; 
Vitelline  lateral  413 

Venous   System,   Rana  63,   Scyllium 
239,  Lepus  310 

Vent  211 

Ventral  surface  14 

Ventricle,  Rana  17,  21,  59,  Scyllium, 

235 
Venules  58 


Vertebral  column  20,  24 

Vertebrata,  Rana  116,  Scyllium  204 

Vertebrate  animals  21 

Verumontanum  322 

VESICLES  :  Amnio  -  cardiac  413, 
auditory  221,  blastodermic  409, 
brain,  fore,  mid,  hind  393,  caudal 
!99>  germinal  375,  olfactory  222, 
secondary  optic  430 

Vesicula  seminalis,  Rana  20,  73, 
Lumbricus  183,  Scyllium  248 

VESSELS  :  afferent  dermal,  178, 
afferent  intestinal  178,  afferent 
nephridial  178,  commissural  176, 
dorsal  176,  efferent  nephridial, 
efferent  intestinal  178,  lateral 
neural  176,  lateral  cesophageal  178, 
sub-intestinal  176,  sub-neural  176, 
|  supra-intestinal  176,  typhlosolar 
176,  ventral  176 

Vestibule,  Rana  98,  Lepus  324 

Vestigial  structures  448 

Vibrissae  270 

Villi  304,  placental  440 

Viscera  17 

Vital  phenomena  108 

Vitreous  humour  94 

Vitrodentine  213 

Viviparous  205 

Volutin  362 

Vulva  270,  324 


W 


Wallace,  Alfred  Russel  458 
Wallace's  Chart  463 
Web  15 

Weissmann  464 
Weldon  459 
Wide  tube  181 

Wolffian  body,  Scyllium  245,  Lepus 
320 


Yolk  105,  sac,  Lepus  436,  Chick  420; 
spheres  374,  Hydra  157 


Zona  pellucida,  radiata  409 

Zone,  paraxial,    segmental,    parietal 

404 

Zoochlorellae  156 
Zooid  1 60,  nutritive  161 
Zoology  i 
Zygote,  Rana  105,  Monocystis  137 


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WILLIAM    CLOWES   AND    SONS,    LIMITED, 

LONDON    AND    EBCCLES. 


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T 


a  1933 


FEB  2 


MA  1940 

APR  1  2  1843 

2  4  1943 


AfR   28  1948 


LD  21-50m-l,'< 


51526 


UBRARY 

a 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


